SNOQUALMIE RIVER HYDROLOGIC STUDY

EVALUATION OF FLOODING TRENDS AND CURRENT CONDITIONS

Department of Natural Resources and Parks Water and Land Resources Division

SNOQUALMIE RIVER HYDROLOGIC STUDY

EVALUATION OF FLOODING TRENDS AND CURRENT CONDITIONS

Prepared for

Department of Natural Resources and Parks Water and Land Resources Division Seattle, Washington 98104

Prepared by

Watershed Science & Engineering 506 Second Avenue, Suite 2700 Seattle, Washington 98104 and

Herrera Environmental Consultants, Inc. 2200 Sixth Avenue, Suite 1100 Seattle, Washington 98121 Telephone: 206-441-9080

July 13, 2018

Prepared by:

The analyses documented in this report were conducted by Watershed Science & Engineering (WSE) on behalf of the King County Department of Natural Resources and Parks, Water and Land Resources Division (King County). This work was performed in close coordination with King County to obtain available information and strategize the best technical approaches to answer flooding-related questions and concerns voiced to King County in recent years by many residents and landowners in the Snoqualmie River valley. While King County staff provided guidance and review of the contents presented in this report to ensure the work would meet the study’s objectives, WSE undertook all of the technical work. Herrera Environmental Consultants assisted in managing the study, facilitating independent technical review of it, and compiling the report contents with editorial support from McAuliffe Technical Editing Services.

An independent technical reviewer representing the interests of Snoqualmie River valley residents and stakeholders, Ed McCarthy, PhD, was retained by King County. He provided input on the information sources and methods of analysis, and reviewed analysis results and documentation contained in this report to confirm that the study is technically sound. Dr. McCarthy, an expert water resources engineer, had complete autonomy in reviewing products and developing comments and recommendations on interim and final products. His role in the project and his conclusions offered on a draft of this report are documented in a separate letter.

For comments or questions contact: Chris Ewing, King County, 206.477.3027 or Chase Barton, King County, 206.477.4854

Alternate Formats Available. Call 206.477.4800 or TTY 711

CONTENTS

Acronyms and Abbreviations ...... vii Executive Summary ...... i 1. Introduction...... 1 2. Review of Snoqualmie River Gages and Data ...... 3 2.1. Available Information ...... 5 2.2. Overview of USGS Gaging Program ...... 5 2.3. Gage Issues and Uncertainty ...... 6 2.4. Summary of Snoqualmie River Basin Gage Records...... 8 2.4.1. Middle Fork Snoqualmie Near Tanner, WA (12141300) ...... 8 2.4.2. Middle Fork Snoqualmie Near North Bend, WA (12141500) – Discontinued ...... 8 2.4.3. North Fork Snoqualmie River Near Snoqualmie Falls, WA (12142000) ...... 9 2.4.4. North Fork Snoqualmie River Near North Bend, WA (12143000) – Discontinued ...... 9 2.4.5. South Fork Snoqualmie River Above Alice Creek Near Garcia, WA (12143400) ...... 9 2.4.6. Sum of Snoqualmie River Forks (Sum of Gages 12141300, 12142000, 12143400) ...... 9 2.4.7. South Fork Snoqualmie River at Edgewick, WA (12143600) ...... 10 2.4.8. South Fork Snoqualmie River at North Bend, WA (12144000) ...... 10 2.4.9. Snoqualmie River Near Snoqualmie, WA (12144500) ...... 10 2.4.10. Snoqualmie River Near Carnation, WA, (12149000)...... 11 2.4.11. Snoqualmie River at Duvall, WA (12150400) ...... 11 2.5. Noted Anomalies in Gage Records ...... 11 2.5.1. Middle Fork Snoqualmie Near Tanner ...... 12 2.5.2. South Fork Snoqualmie Near North Bend ...... 13 2.5.3. Sum of the Forks ...... 14 2.5.4. Snoqualmie River Near Snoqualmie 1990 Peak...... 15 2.5.5. Snoqualmie River Near Snoqualmie Rating 7 ...... 17 2.5.6. Snoqualmie Near Carnation ...... 19 2.5.7. Shifts in Carnation Gage Stage-Discharge Relationship ...... 21 2.5.8. Relationship Between Snoqualmie and Carnation Gages ...... 22 2.6. Recommendations for Improving Discharge Records and Streamflow Gaging ...... 23

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County i 2.7. Recommendations for King County Use of Gage Data for Flood Warning ...... 25 3. Hydrologic Trends and Potential Future Conditions in the Snoqualmie River ...... 27 3.1. Summary of Findings ...... 27 3.2. Background ...... 28 3.2.1. Historical Peak Flows at Key Streamflow Gages ...... 30 3.3. Trends in Magnitude and Timing of Flood Flows at Key Streamflow Gages...... 33 3.3.1. Results ...... 35 3.3.2. Discussion ...... 37 3.4. Flood Arrival Time ...... 42 3.5. Potential Causes of Hydrologic Change ...... 44 3.5.1. Precipitation ...... 45 3.5.2. Land Development ...... 48 3.5.3. Timber Harvest ...... 50 3.5.4. Sediment ...... 52 3.5.5. Large Capital Projects ...... 54 3.6. Summary and Conclusions ...... 56 4. Anatomy of A Flood ...... 59 4.1. Summary of Findings ...... 59 4.2. Background ...... 60 4.3. Flood Event Descriptions ...... 61 4.3.1. January 2009 ...... 62 4.3.2. January 2015 ...... 64 4.3.3. December 2015 ...... 65 4.4. Storm Comparison ...... 65 4.5. Comparison of Flood Characteristics ...... 68 4.6. Road Closures ...... 75 4.7. Summary ...... 77 5. High Water Mark Evaluation ...... 79 5.1. High Water Mark Survey ...... 79 5.2. Comparison of HWMs to Carnation Stage ...... 83 5.3. Comparison of HWMs to FEMA FIS Flood Model Predictions ...... 85 5.4. Discussion ...... 89 5.5. High Water Mark Profiles ...... 90 6. References...... 95

July 2018 ii Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

APPENDICES

Appendix A USGS Gage Summaries, Tributary Gages Appendix B Summary of Discussion with CIG, USGS, and NWS Appendix C Trends for Water Year Discharge, 1962–2015 Appendix D Trends for Water Year Precipitation, 1962–2015

TABLES

Table 1. USGS Gages in the Snoqualmie River Basin...... 3 Table 2. Peak Flow Comparison at Key Gages...... 16 Table 3. Ten Highest Observed Annual Peak Flows at Key Snoqualmie River Gages...... 30 Table 4. Available Data Record at Key Snoqualmie River Gages...... 31 Table 5. Flow Frequency for Key USGS Gages in the Snoqualmie River Basin...... 35 Table 6. Land Cover in the Snoqualmie River Basin...... 49 Table 7. Ten Highest Observed Annual Peak Flows at Key Snoqualmie River Gages...... 62 Table 8. 48-Hour Precipitation and Snow Accumulation Related to the January 2009, January 2015, and December 2015 Flood Events...... 67 Table 9. Flood Characteristics at Snoqualmie and Carnation Gage Locations...... 70 Table 10. Snoqualmie River Flood Phases at the Sum of the Forks...... 74 Table 11. High Water Mark Survey Data...... 80 Table 12. Comparison of HWMs to FEMA FIS Flood Model Predictions...... 86

FIGURES

Figure 1. USGS Gaging Stations Located Within the Snohomish River Basin, Including the Snoqualmie River and Tributaries...... 4 Figure 2. Example Rating Curve (Rating Curve 8) for the Snoqualmie River Gage Near Snoqualmie...... 6 Figure 3. Relationship of Peak Annual Flows at the Middle Fork Tanner Gage and Snoqualmie Gage...... 12 Figure 4. Difference in Peak Annual Flow Between the North Bend Gage and the Edgewick Gage...... 14 Figure 5. Difference in Peak Annual Flow Between the Sum of the Forks and the Snoqualmie Gage Near Snoqualmie Since 1987...... 15 Figure 6. Recent Snoqualmie Gage Ratings Versus Measured Discharge...... 18

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County iii Figure 7. Gage Height (Stage) Over Time at the Snoqualmie Gage...... 19 Photo 1. Viewing Upstream to the Northeast Carnation Farm Road Bridge During the November 1995 Flood. Significant Flow is Bypassing the Carnation Gage, Which was Located just Downstream of the Bridge on the Left Bank...... 20 Figure 8. Gage Height (Stage) Over Time at the Carnation Gage...... 21 Figure 9. Difference Between Peak Annual Flows at the Snoqualmie River Gages Near Snoqualmie and Carnation...... 23 Figure 10. Observed Peak Annual Floods at Snoqualmie River Near Carnation Gage (1929–2015)...... 32 Figure 11. Observed Peak Annual Floods at Snoqualmie River Near Snoqualmie Gage (1958–2015)...... 33 Figure 12. Flow Trends Analysis Results for Six Snoqualmie River Gages...... 36 Figure 13. Number of Days Exceeding 80 Percent of the 2-Year (20,100 cfs), the 2-Year (25,200 cfs), and the 5-Year (36,500 cfs) Mean Daily Flow at the Carnation Gage (1962–2015)...... 38 Figure 14. Number of Days Exceeding 80 Percent of the 2-Year (17,600 cfs), the 2-Year (22,100 cfs), and the 5-Year (32,300 cfs) Mean Daily Flow at the Snoqualmie Gage (1962–2015)...... 39 Figure 15. Timing of Annual (Water Year) Maximum Flow for the Period 1962–2008 at the Key Snoqualmie River Gages...... 41 Figure 16. Example Start and Peak of a Flood Hydrograph Used for the Rate-of-Rise Analysis...... 42 Figure 17. Rate of Rise of Annual Peak Flood Events at Snoqualmie and Carnation Gages Since 1988...... 43 Figure 18. Time Between Peaks at Snoqualmie and Carnation Gages for Annual Peak Flood Events at the Snoqualmie Gage Since 1988...... 44 Figure 19. Projected Changes in Seasonal Timing of Streamflow in the Snohomish River Basin...... 46 Figure 20. Precipitation Trends Analysis Results for Eight Regional Gages with Long-Term Records...... 47 Figure 21. Reported Timber Harvest in King County 1965–2014...... 51 Figure 22 Example Cross-Section Adjustment to Account for 2 Feet of Sediment Deposition...... 53 Figure 23 Average Bed Elevations of Snoqualmie River Near Snoqualmie Gage Based on River Surveys in 1965, 1997, 2004, and 2011...... 54 Figure 24. Snoqualmie River Basin, Sub-basins, and Key USGS Gaging Locations...... 61 Figure 25. Observed Hydrographs at Key Gages for January 2009 Flood Event...... 63 Figure 26. Observed Hydrographs at Key USGS Gages for January 2015 Flood Event...... 64

July 2018 iv Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Figure 27. Observed Hydrographs at Key USGS Gages for December 2015 Flood Event...... 65 Figure 28. 48-Hour Quantitative Precipitation Estimates in the Snoqualmie River Basin for January 2009, January 2015, and December 2015 Flood Events...... 66 Figure 29. Comparison of Flood Hydrographs at Snoqualmie River Near Snoqualmie Gage Location...... 69 Figure 30. Comparison of Flood Hydrographs at Snoqualmie River Near Carnation Gage Location...... 70 Figure 31. Difference Between HWM Elevations from January 2009 Flood and the Simulated 100-Year Flood Event...... 71 Figure 32. Difference Between HWM Elevations from January 2015 Flood and the Simulated 10-Year Flood Event...... 72 Figure 33. Difference Between HWM Elevations from December 2015 Flood and the Simulated 10-Year Flood Event...... 73 Figure 34. Duration Between Flood Phases at the Snoqualmie River Gage at Snoqualmie...... 75 Figure 35. Road Closures During January 2009, January 2015, and December 2015 Flood Events...... 76 Figure 36. Location of Lower Snoqualmie Highwater Marks...... 81 Figure 37. Seed Line on a Building (Left), and Water Surface Run-up and Drawdown, Which Impact HWM Interpretation (Right)...... 82 Figure 38. Relative Change in Stage at HWM Locations Compared to Change in Stage at the Carnation Gage During the January 2009 Flood...... 84 Figure 39. Difference Between HWM Elevations from the Three Largest Snoqualmie River Floods and the 50-Year Flood Event Simulated with the FIS Model...... 87 Figure 40. Difference Between HWM Elevations from the November 1995, February 1996, and November 2008 Floods and the Simulated 10-Year Flood Event...... 88 Figure 41. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 2006, January 2009, and November 1990 Flood Events, Downstream of Carnation Gage...... 91 Figure 42. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 2006, January 2009, and November 1990 Flood Events, Upstream of Carnation Gage...... 92 Figure 43. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 1995, February 1996, and November 2008 Flood Events, Downstream of Carnation Gage...... 93 Figure 44. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 1995, February 1996, and November 2008 Flood Events, Upstream of Carnation Gage...... 94

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County v

ACRONYMS AND ABBREVIATIONS 205 Project US Army Corps of Engineers Snoqualmie Flood Damage Reduction Project C-CAP Coastal Change Analysis Program CIG Climate Impacts Group (University of Washington) cfs cubic feet per second CoCoRaHS Community Collaborative Rain Hail and Snow Network County King County Ecology Washington State Department of Ecology FEMA Federal Emergency Management Agency FIS flood insurance study HEC-RAS Hydrologic Engineering Center River Analysis System HWM high water mark mi2 square miles NAVD 88 North American Vertical Datum of 1988 NGVD 29 National Geodetic Vertical Datum of 1929 NHC Northwest Hydraulic Consultants NOAA National Oceanic and Atmospheric Administration NWRFC Northwest River Forecast Center NWS National Weather Service PSE Puget Sound Energy RM river mile Sno-Tel Snow Telemetry SR State Route SWE snow water equivalent SVPA Snoqualmie Valley Preservation Alliance USGS US Geological Survey WDNR Washington State Department of Natural Resources WSE Watershed Science & Engineering

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County vii

EXECUTIVE SUMMARY

Watershed Science & Engineering (WSE) conducted analyses to evaluate changes in hydrology and the resultant effects on flooding in the Snoqualmie River basin. The intent of this investigation was twofold: to determine the extent to which flood hydrology has changed over time and to investigate contributing factors at a level of detail sufficient for King County, in consultation with stakeholders, to prioritize further analysis and basin-related actions. In addition, the analyses provide a foundation for future planning efforts in the basin.

The investigation consisted primarily of three separate tasks: 1) a review of the US Geological Survey (USGS) gaging program in the Snoqualmie River basin, which is the primary source of data used to assess hydrologic trends in this study; 2) an evaluation of hydrologic trends and potential factors influencing flood hydrology; and 3) an “anatomy of a flood” discussion to examine the development, progression, and impacts of three recent flood events within the lower Snoqualmie River valley (Snoqualmie Valley). The intent of the third task was to explore the variability and uniqueness of recent flood events to illustrate some of the challenges and limitations inherent in making a generalized assessment of flooding patterns and trends. This report summarizes the work conducted and findings related to the three tasks. A fourth task to evaluate historical high water mark (HWM) data was subsequently completed in response to public input on a draft version of this report.

Key findings of this investigation include evidence of increasing annual, fall, and spring peak flows at all USGS gaging locations; however, most trends were not found to be statistically significant. There is also evidence of increased frequency of flooding, particularly at the Carnation gage, but in most locations it has not occurred to an extent that is statistically significant. The investigation found a statistically significant trend toward increasing peak annual water levels (stage) at the Carnation gage. This trend is likely due to a combination of increasing peak annual flows and localized changes in river channel geometry, including those due to sedimentation. Review of the USGS gaging program found that flood flow reporting can occasionally be inaccurate in real time, especially at the Carnation gage where numerous factors are known to complicate the flow gaging process. High flow estimates are associated with the greatest uncertainty, but reported stage data (i.e., water levels) are generally quite reliable.

The remainder of this Executive Summary is presented in a “Frequently Asked Questions” format, addressing common questions and concerns relating to flooding in the lower Snoqualmie Valley. Summary responses are included below, with cross-references to sections of the report where further information can be found.

Are floods getting bigger? Despite the occurrence of a number of large floods in the 1990s and 2000s, investigations conducted for this project did not yield compelling evidence of a long-term trend towards larger

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County ES-i floods in the Snoqualmie basin. Section 3 details the analysis of peak annual and maximum daily flow records (1962 to 2015) at several Snoqualmie River basin gages.1 While all locations show a slight increase in peak annual flow and annual maximum mean daily flow, the results do not meet a statistical definition of significance (see Figure 12 later in this report).2 This may indicate that the frequency and magnitude of recent floods are within the bounds of natural flood variability, or that flooding is changing, but more time (and data) is required for trends to emerge as statistically significant.

Although the peak flow data do not support a conclusion that flows have gotten significantly larger over time, it is possible that flooding or impacts of flooding may be getting worse in some locations. The investigations documented in this report did not examine localized effects, such as flooding changes at a specific property or river segment, which may result from localized changes to the river channel or overbank topography. Investigation of localized impacts in the floodplain is limited by the lack of available long-term flow and water level records.

Are floods more frequent? Increasing frequency of flooding is somewhat supported based on analysis of historical gaging records, detailed in Section 3.3. Flooding frequency was assessed by counting the number of days per year on which the mean daily flow at key gages exceeded a threshold flow associated with minor flooding. A weak increasing trend was found at the Carnation gage. Small increases were found at all other gages, though these increases are not statistically significant. Following community meetings, the analysis of flooding frequency was expanded to include the 2-year and 5-year recurrence mean daily flows3. For these thresholds, increases were found at all gages, with statistically significant trends at the Snoqualmie Falls gage for the 2-year flow, and at the Middle Fork Snoqualmie at Tanner gage and Carnation gage for the 5-year flow.

Are floods coming faster? Reports that floods are rising faster cannot be confirmed using currently available data. As discussed in Section 3.4, the “lag time” for annual flood peaks to travel from the Snoqualmie (upstream) gage to the Carnation (downstream) gage has increased slightly on average since 1988, when the data used to assess lag time (15-minute flow data) first became available. In other words, the peak flood stage appears to be taking more time to pass from the Snoqualmie gage to the Carnation gage than historically. There is also considerable variability in this lag time

1 North Fork Snoqualmie River near Snoqualmie Falls (USGS gage 12142000; North Fork or NF Snoqualmie gage), Middle Fork Snoqualmie River near Tanner (12141300; Middle Fork or MF Tanner gage), South Fork Snoqualmie River above Alice Springs near Garcia (12143400; South Fork or SF Garcia gage), Snoqualmie River near Snoqualmie (12144500; Snoqualmie gage), Snoqualmie River near Carnation (12149000; Carnation gage). 2 Sufficient evidence of a trend was defined as statistically significant to the 90 percent significance level. 3 A 2-year flow event (or mean daily flow) has a 50 percent chance of occurring, on average, in any given year. A 5-year flow event has a 20 percent chance of occurring, on average, in any given year.

July 2018 ES-ii Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

ranging from around 4 hours to over 18 hours among historical flood events, illustrating the complexity of flood dynamics in the Snoqualmie Valley.

As discussed later in the executive summary, recent projects at Snoqualmie Falls (Falls) have been shown to slightly decrease flood travel time between the Falls and Carnation, which would tend to cause flood peaks to occur approximately 20 minutes earlier at Carnation than they would have in pre-project conditions. This change is difficult to detect given the natural variability in lag time (4 hours to 18 hours) in the flood record (see Figure 18).

Why do we have less time to prepare for a flood? The data described in Section 3.4 do not indicate that floods are rising faster than they did historically. Many other elements, however, influence the character of a flood or the information involved in flood preparedness. For example, existing flood warning systems rely on forecasts issued by the Northwest River Forecast Center (NWRFC) to predict flood peak timing and magnitude and are therefore limited by the accuracy of these forecasts, as discussed in Section 2.7. The spatial and temporal patterns of rainfall and the timing of tributary inflows (among other factors) can also affect flood characteristics and impacts. As shown in Section 4, unique issues like the late spike of high flow on the Tolt River during the January 2009 flood (Figure 25) and the rapid rise and fall of the January 2015 flood (Figure 26) can greatly influence flooding impacts and are difficult to forecast. Because each flood is unique in many respects, past experiences cannot always be used to successfully predict the timing or extent of any given flood (see Section 4).

Why are there more fall and spring floods? Spring and fall floods appear to be increasing in magnitude and frequency, judging from upward trends in monthly maximum flows in March, April, May, October, and November recorded at several gages in the Snoqualmie River basin; however, most of these trends are not statistically significant. Additional discussion of this issue is provided in Section 3.3.

The increase in spring and fall floods is consistent with climate change projections for the Puget Sound area, discussed in Section 3.5.1. These projections suggest that an overall shift in flood timing will result in lower flows in summer, higher annual peak flows (which typically occur in winter), and higher flows in fall and spring.

How reliable are the flow gages? The USGS gaging program generally provides consistent and high-quality data that constitute the best source of current and historical flow information in the Snoqualmie Basin. A review of the program, presented in Section 2, highlights several factors that affect the accuracy and reliability of reported gage data during Snoqualmie River flood events. For example:

 Gage ratings, which define how much flow is passing a specific gage site at a given river stage (surface water elevation), are developed by the USGS based on physical streamflow calibration measurements taken several times a year. Extreme flows are both infrequent

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County ES-iii and more difficult to measure, making estimates of high flow subject to greater uncertainty than is the case with lower flows.

 Malfunctioning equipment can hinder flood warning efforts, especially at upstream gages. For instance, the Middle Fork Snoqualmie at Tanner gage failed during two recent floods (January 2015 and February 2016), causing it to miss recording the flood peak. The USGS replaced much of the equipment at this gage to address those failures.

 Flow reporting at the Carnation gage is known to be problematic at high flows due to several complicating factors discussed in the following section and in Section 2.5.5.

 Flows during large floods at the Snoqualmie gage were likely underestimated between 1998 and 2011 because the stage-discharge rating curve used by the USGS during that period was inaccurate. Actual peak flows during the 2006 and 2009 floods may therefore have been 5 to 10 percent larger than reported by the USGS.

Historical gage records show that the comparisons between peak stages measured at various gages along the Snoqualmie River vary considerably from event to event. This is possibly due in part to the challenges noted above but is also strongly influenced by variability in precipitation distribution, timing of tributary inflows to the main stem river, and differences in how water is stored in the floodplain. Flood stage at a particular gage cannot therefore be used to confidently predict the flood stage that will be seen elsewhere in the basin (although the two are related). Section 2 includes recommendations for improving the gaging program, such as including additional gaging locations between Snoqualmie and Carnation to provide better detail on floods as they progress down the river and to support improved flood warning.

Why doesn’t the Carnation gage work like it used to? Flow reporting at the Carnation gage is known to be problematic at high flows because of flood storage effects (i.e., hysteresis4) and flow that bypasses the gage location. The effects of floodplain storage can cause the gage to underpredict flows on the rising limb of a flood hydrograph, and overpredict them on the falling limb (as a flood recedes). The USGS is currently developing more accurate stage-discharge estimates for the Carnation gage to account for this effect. However, the corrected rating, which was first implemented in 2015, must be applied manually during the rising limb of a flood and is based on the forecasted peak flow at the gage so it is influenced by the accuracy of the river forecast. Final flow values are often revised by the

4 When a flood wave moves down a river channel and through a given cross section of the channel, the effect of the wave when moving from upstream of the cross section is to increase the flow velocity at the cross section. When the flood peak passes, the rear of the wave increases the backwater conditions and so reduces the velocity at a given discharge at the cross section. The result is that, for the same stage, the discharge is higher during rising stage than during falling stage. This phenomenon is called hysteresis. For some rivers, especially those with low channel gradient and complex floodplain geomorphology such as at the Carnation gage, these effects will be manifest as distinctive loops in the stage–discharge relationship.

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USGS after the flood event. Flood event flows reported in real time will therefore be deemed “poor quality” at this gage, meaning they may be off by more than 10 percent.

These data quality issues are not new, although the extent of these issues was only recently recognized by the USGS. Flow reporting at the Carnation gage has likely never been particularly accurate or reliable for real-time flood warning, and since historical data have not been corrected for hysteresis effects or bypass flows, use of the historical record is problematic. (See additional discussion in Section 2.5.5.)

While real-time stage reporting (as opposed to flow reporting) at the Carnation gage is believed to be accurate, the relationship of stage and flow has varied over time, and the same flow may produce a different stage from event to event, or on the rising and falling limbs of a single flood event. In other words, a particular stage at Carnation may not consistently predict flood impacts elsewhere. A trend towards increasing stage at the Carnation gage is discussed in Section 2.5.7, and the impacts of sedimentation at this gaging site are discussed in Section 3.5.4.

Why are the river forecasts inaccurate? Accurate forecasting of future weather conditions, with subsequent conversion into a forecast of future flows, is a very complex and challenging undertaking. Despite the National Weather Service River Forecast Center’s (NWRFC’s) attempts to produce accurate forecasts of river flows and stages, actual flows can vary greatly from those predicted, and forecasts can be highly sensitive to assumptions such as storm progression and snow level. Nonetheless, these forecasts still provide insight for predicting potential flooding. NWRFC forecasts at the Snoqualmie River gages are currently retransmitted via the NWRFC website and are reproduced for flood warning on the King County flood warning website and the privately hosted Floodzilla website. These data represent the best currently available information for predicting potential flood magnitude and duration, and can be used to help valley residents and commuters anticipate the possible development of flood conditions.

A discussion of the NWRFC forecasting model is included in Appendix B, and recommendations for use of the data are included in Section 2.7.

Aren’t floods worse because of … Forest Practices? A direct link between timber harvest and increased flooding was not found through this investigation. In general, conclusive evidence that extreme floods are significantly affected by forest practices is not found in the literature (Perry et. al. 2016). Although specific timber harvest records for the Snoqualmie River basin were not available for this work, the general trend in King County, as in many areas in western Washington, has been towards less forest harvest over the past few decades. More discussion of timber harvest within the Snoqualmie River basin and potential impacts on flooding is included in Section 3.5.3.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County ES-v Land development? Increased impervious surface associated with urban development is known to affect flooding by increasing the magnitude, and accelerating the timing, of stormwater runoff. However, given the relatively small portion of the Snoqualmie River basin that has been urbanized (less than 4.5 percent of the basin land area), it is unlikely that urbanization has had a significant effect on river flows. Furthermore, the basin has progressively urbanized over decades, making it difficult to detect development-induced changes in flows at the Snoqualmie River gages. Increasingly stringent stormwater management standards have likely helped mitigate hydrologic impacts of recent large urban developments. Discussion of potential impacts of development, including “tightlines” that convey peak flows directly from the Snoqualmie Ridge and other development sites, is included in Section 3.5.2. Analysis of peak flood characteristics at the Snoqualmie River gages near Snoqualmie and Carnation did not identify statistically significant trends towards faster flood arrival time or higher flood peaks, despite the historical development.

River capital improvement projects? Flood effects from King County’s Farm Pad program, the Upper Carlson project, the Chinook Bend project, the McElhoe Pearson project, and the timber piling removal at river mile 31 (Neal Road Southeast closure) have been either minimal (based on project-specific “zero-rise” analyses) or minor (based on the characteristics and scale of these projects). In addition, many of these projects include post-project monitoring to look for unanticipated impacts to the river and its floodplain, and associated changes in flooding. For example, post-project monitoring of localized flood impacts near the Upper Carlson project was in progress (King County 2016a) at the time this report was written. Additional discussion of these projects is included in Section 3.5.5.

Localized flood impacts change from event to event based on the hydrologic characteristics of each flood, including storm progression, rainfall intensity, duration, and areal extent. Local changes in sediment deposition and bank erosion may also cause different patterns of flooding in different events. Because no two floods are the same, it can be difficult to attribute differences in observed flooding to a specific project as opposed to simply differing flood characteristics or different channel conditions unrelated to the project. Some of these considerations are illustrated in Section 4 – Anatomy of a Flood. Full understanding of localized effects typically requires a site-specific investigation that is beyond the scope of this study.

Climate change, and changes in precipitation patterns? Climate change projections for the Puget Sound area suggest that seasonal flows will shift, causing lower flows in summer, higher flows during annual peak floods (typically in winter), and higher flows in fall and spring (Mauger et al. 2015). This shift appears to be consistent with observed trends in the Snoqualmie River basin towards increased flooding in the spring, fall, and winter, which are evident in daily flow records at several gages in the basin (see Section 3.3) and in trends in daily precipitation at gages proximate to the basin (see Section 3.5.1).

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Regardless of climate change impacts, variation in precipitation patterns between different storms likely accounts for much of the variability in flooding seen in the historical record and especially during recent floods. This phenomenon is described in Section 4.

Projects at Snoqualmie Falls? Two major projects were recently completed at Snoqualmie Falls: modifications to Puget Sound Energy’s weir at Snoqualmie Falls (the PSE Project) completed in 2012, and removal of upstream flow obstructions by the US Army Corps of Engineers (the 205 Project) completed in 2004. These projects have increased the magnitude of peak flows downstream of the Falls and slightly decreased flood travel time between the Falls and Carnation. Hydraulic simulations indicate a maximum increase in discharge of 1,200 cubic feet per second in the 100-year5 peak flow downstream from the PSE and 205 projects, and peak stages at Carnation occurring 20 minutes earlier (WSE and Herrera 2016). Peak 100-year water surface elevations also increased by a maximum of 0.1 foot downstream of the Falls and decreased by 1.5 feet upstream in the city of Snoqualmie. The decrease in upstream water levels results in a significant reduction in flood storage volume—by as much as 2,500 acre-feet during the 100-year flood event. This upstream flood storage reduction is the cause of the higher downstream flows and water levels. The effects of the PSE and 205 projects were investigated in detail in an earlier phase of this study, the report for which can be downloaded from the King County website: .

Lack of sediment management? King County recently compared available Snoqualmie River channel cross-section surveys to assess bed level changes in sediment monitoring reaches downstream of Carnation and Fall City (below the Tolt River and Raging River confluences, respectively). That investigation found no clear trend in sediment deposition between 1997 and 2004, average increases in the elevation of the channel bed of 0.6 to 0.7 foot between 2004 and 2011, and an average decrease in the elevation of the channel bed of 0.3 foot in the Fall City reach between 2011 and 2015 (2015 data are not available for the Carnation reach). Comparison of recent survey data to sparse data available from a 1965 US Army Corps of Engineers channel survey also suggest little evidence of widespread or consistent change in sediment levels over the last 50 years.

Hydraulic modeling detailed in Section 3.5.4 suggests that if the channel bed level were increased by 2 feet within the monitoring reaches downstream of Fall City and Carnation to represent hypothetical sediment deposition, the water surface would rise by a maximum of 0.4 foot at any modeled cross section during the simulated 100-year event, and a maximum of 0.8 foot during the simulated 10-year event.6 The reach-wide average increases in water surface elevation would be less than 0.2 foot (2.4 inches) in either event. Based on new channel monitoring survey data (King County 2017), comparison of those data to the sparse 1965 survey data, and deposition calculated from sediment-transport modeling (Booth et al. 1991), it would

5 A 100-year event is a flood that has a 1 percent chance of occurring, on average, in any given year. 6 A 10-year flood event has a 10 percent chance of occurring, on average, in any given year.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County ES-vii likely take several decades for 2 feet of sediment to deposit in the main stem Snoqualmie River in the vicinity of Carnation and Fall City.

While reach-wide sediment aggradation (i.e., deposition that raises the river bed) was not found, peak stages at the Carnation gage have increased by about 1 foot since the 1960s. This includes recent increases due to sediment deposition near the gage after significant flood events in November 2006 and January 2009 (King County 2017). Sedimentation has likely had an effect on the relationship between stage and flooding over time and may explain residents’ observations of changes in the relationship between stage at the Carnation gage and flooding elsewhere in the valley. Additional discussion of sediment effects on the Carnation gage are included in Section 2.5.7 and 3.3.1.

Zero-rise reporting shows that a project won’t impact flooding, but I’ve seen changes at my property. Why? Zero-rise analysis is a process used to demonstrate that a proposed project will not create a measurable rise in 100-year flood levels. For projects in the floodplain of the Snoqualmie River, this is most often completed using the existing Federal Emergency Management Agency hydraulic model, although some projects may also use more sophisticated (two dimensional [2D]) modeling, or look at a broader range of flow conditions (e.g., 50-year, 10-year, or 2-year floods) to evaluate impacts at a finer scale and over a range of flows. Zero-rise analysis is only required to address project impacts during the 100-year (regulatory) flood and does not typically include assessment of future changes in flood hydrology (due to climate change or other factors), capture ongoing changes in channel configuration caused by natural river processes, or evaluate flooding caused by high flows originating on tributary streams.

Why has the flooding pattern on my property changed? The evaluation of changes in flooding at a parcel level was beyond the scope of this study. Flow records are used as a reasonable proxy for local flooding impacts, but flow data may not perfectly correlate to impacts such as road closures, how long fields stay wet, flow velocity, or flood depth at a given location.

Variability in flood impacts at a basin scale was examined by comparing three recent flood events, two of which had similar peak flows at the Snoqualmie gage, but very different impacts in terms of road closures and peak water levels throughout the lower valley. This discussion is included in Section 4. A comparison of relative HWM elevations between recent flood events is included in Section 5.

Discussion of a number of FAQs relating to local flooding patterns is included below:

Are flow velocities in the river higher? Flow velocities were not specifically analyzed as part of this investigation. While reach-wide average velocities in the river channel are likely to remain fairly consistent because of the relatively constant channel slope of the Snoqualmie River, local flow velocities can be affected by changes in bed configuration, bank erosion, obstructions in the river, and backwater

July 2018 ES-viii Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

conditions (which are impacted by downstream channel constrictions or tributary influences). All these factors can change from flood to flood.

Has sediment accumulated in the river? See earlier discussion, “Lack of sediment management?”

Why is more sediment transported onto my property? Localized sediment impacts were not evaluated as part of this study. Recent sediment deposition may be tied to large flood events such as those that occurred in November 2006 and January 2009, which have been linked to sediment level increases in the channel (Section 3.5.4 provides additional discussion on this topic). These events would have also carried high suspended sediment loads, including sands and silts.

Why are flood marks on my property higher for the same given peak stage at the USGS gage? Site-specific evaluations were not conducted as part of this study. However, depending on proximity of a given property to the gage, the relationship between the stage recorded at a gage and the corresponding flood stage on specific properties can be dramatically different from flood to flood. Variation in historical peak stage values between the Carnation and Snoqualmie gages is discussed in Section 2.5.5. Water surface elevations during floods can also vary over a short distance, due to floodplain dynamics such as flow obstructions, surface roughness, and tributary inflow. This effect is illustrated by a comparison of high water marks (HWMs) from the January 2015 flood in Section 4.5. A comparison of relative HWM elevations between recent flood events is included in Section 5. A changing relationship between gage height and flood peak marks at a particular location may also result from changes to the stage-discharge relationship (rating) at USGS gages. Gage ratings are regularly recalibrated based on new field measurements to account for changes to channel morphology that alter the relationship between river stage and flow magnitude at the gage (see Section 2.2). For this reason, local HWMs likely correlate better with the USGS estimate of peak flow rather than with the recorded stage.

As discussed in Section 2.5.6, and as noted above, the Carnation gage is known to be problematic at high flows, and the flow reported at a particular stage may vary widely depending on the magnitude and timing of the event.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County ES-ix

1. INTRODUCTION

Watershed Science & Engineering (WSE) conducted analyses to evaluate possible historical changes in hydrology and the resultant effects on flooding in the Snoqualmie River basin. The intent of this investigation was twofold: to determine the extent to which flood hydrology has changed over time and to investigate contributing factors at a level of detail sufficient for King County, in consultation with stakeholders, to prioritize further analysis and possible mitigation.

The investigation consisted primarily of three separate tasks: 1) a review of the US Geological Survey (USGS) gaging program in the Snoqualmie River basin, which is the primary source of data used to assess hydrology trends in this study; 2) an evaluation of hydrologic trends and potential factors influencing flood hydrology; and 3) an “anatomy of a flood” discussion to examine the development, progression, and impacts of three recent flood events within the lower Snoqualmie River valley (Snoqualmie Valley). The intent of the third task is to explore the variability and uniqueness of recent flood events to illustrate some of the challenges and limitations inherent in making a generalized assessment of flooding patterns and trends. This report summarizes the work conducted and findings related to the three tasks. A fourth task involving analysis of historical highwater mark data was subsequently completed in response to public input on the draft version of this report.

The remainder of this report presents the following information:

 Section 2 reviews the USGS gaging system on the Snoqualmie River and identifies potential problems.

 Section 3 discusses hydrological trends and potential future conditions in the river basin.

 Section 4 compares three recent floods to demonstrate the effects of event-specific factors on the resultant flooding.

 Section 5 compares high water marks (HWMs) from recent large floods to the stage at the Carnation gage and to water surface elevations predicted by the FEMA Flood Insurance Study (FIS) flood model.

 References are listed in Section 6.

Supplemental technical information is provided in appendices at the end of this report.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 1

2. REVIEW OF SNOQUALMIE RIVER GAGES AND DATA

Historical flood estimates and real-time gaging data from the USGS constitute a key resource for flood hazard evaluation by King County Department of Natural Resources and Parks. However, interpretation of the data for policy purposes (e.g., flood warning, land management) requires a thorough understanding of the USGS gaging program and its limitations. The following section provides an overview of the current USGS gaging program in the Snoqualmie River basin in King County, Washington, including a critical review of gages on the main stem Snoqualmie River and key tributaries; the main tributaries are the North Fork, South Fork, and Middle Fork Snoqualmie River; the Raging River; and the Tolt River. This section also provides recommendations for improving the program and the reliability of the gaging record.

USGS gages considered in this analysis are listed in Table 1; their locations are shown in Figure 1.

Table 1. USGS Gages in the Snoqualmie River Basin. Gage No. Gage Description Period of Record 12141300 Middle Fork Snoqualmie River near Tanner, WA 1962–2016 12141500a Middle Fork Snoqualmie near North Bend, WA 1907–1932 12142000 North Fork Snoqualmie River near Snoqualmie Falls, WA 1929–2016 12143000a North Fork Snoqualmie River near North Bend, WA 1907–1937,1960–1977 12143400 South Fork Snoqualmie River above Alice Creek Near Garcia, WA 1960–2016 12143600 South Fork Snoqualmie River at Edgewick, WA 1962–1965, 1983–2016 12144000 South Fork Snoqualmie River at North Bend, WA 1907–1974b, 1984–2016 12144500 Snoqualmie River near Snoqualmie, WA 1958–2016 12145500 Raging River near Fall City, WA 1945–2016 12147500 North Fork Tolt River near Carnation, WA 1952,1959–2016 12147600 South Fork Tolt River near Index, WA 1959–2016 12148000 South Fork Tolt River near Carnation, WA 1952,1969–2016 12148300 South Fork Tolt River below regulating basin near Carnation, WA 1982–2016 12148500 Tolt River near Carnation 1928–2016 12149000 Snoqualmie River near Carnation, WA 1928–2016 12150400 Snoqualmie River at Duvall, WA 2000–2016 a Discontinued gage location. b Temporal gaps exist in this period of record.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 3

Source: USGS 2016 Discontinued gages and stage-only gages are not included on Figure 1. Figure 1. USGS Gaging Stations Located Within the Snohomish River Basin, Including the Snoqualmie River and Tributaries.

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2.1. AVAILABLE INFORMATION

Representatives from King County, USGS, and WSE met on October 27, 2015, to discuss the existing USGS Snoqualmie River basin gaging program. In coordination with the USGS, WSE subsequently obtained additional historical background information, including indirect discharge measurement summaries, special reports, rating tables, rating curves, and other details available for key gages. WSE reviewed this information along with the published discharge records to identify potential discrepancies and anomalies in the data.

2.2. OVERVIEW OF USGS GAGING PROGRAM

Adhering to strict gaging and reporting standards, the USGS produces consistent records of river stage (water surface elevation) and discharge (flow rate). The Snoqualmie River basin gaging program is managed out of the USGS Northwest Washington Field Office in Ferndale. USGS historical flow records are the most comprehensive records available for the Snoqualmie River basin. Gage records at many locations date back to the 1920s, with a few sites dating to 1909 or earlier.

The current USGS gaging program includes 14 active (published) stream gages on the Snoqualmie River; the South, Middle, and North forks of the Snoqualmie River; and major Snoqualmie River tributaries including the Tolt and Raging rivers (Table 1). A typical stream gage consists of a device that measures the elevation of the water surface (stage), a data logger, and a transmitter that broadcasts the data to the USGS at a defined interval. The USGS converts stage data to discharge using a site-specific “rating curve,” which relates stage at that location to flow based on a set of actual discharge measurements. Both stage and discharge are then publicly reported via the USGS website. At many locations, stage data are downloaded remotely and discharge is computed and reported in near real-time. These data are considered provisional (subject to change) until they have been reviewed and edited by the USGS and subsequently published. Data are typically published within 6 months of the end of the water year7 (September 30).

The USGS collects field measurements of discharge at gaging sites every 6 to 8 weeks on average to support rating curve development. If field measurements suggest a change in conditions affecting the rating curve, a “shift” may be applied. This shift allows new measurements to be related to the existing rating curve. Long-term changes at the gage site may warrant development of a new rating curve. Over time, several ratings and shifts may be applied at a single site.

An example rating curve, “Rating Curve 8” for the Snoqualmie River near Snoqualmie, is shown in Figure 2 (USGS 2015). The shape of the curve was developed by the USGS based on physical

7 The water year is the 12-month period from October 1 of the previous year through September 30. For example, water year 2016 is October 1, 2015, through September 30, 2016.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 5 discharge measurements.8 The dashed portion of the curve represents an extrapolation to estimate rating at discharges that exceed the highest measured discharge.

Figure 2. Example Rating Curve (Rating Curve 8) for the Snoqualmie River Gage Near Snoqualmie.

2.3. GAGE ISSUES AND UNCERTAINTY

The largest area of uncertainty with the existing gage record is the accuracy with which reported peak flows match actual peak flows. Within the range over which physical discharge measurements have been made, the flow estimates are generally assumed to be within 5 to 10 percent of actual flow. When a recorded stage is higher than the stage of the highest physically measured discharge, the USGS estimates the corresponding flow based on an extrapolation of the rating curve (see Figure 2). However, the extrapolated part of the curve is an approximation, and may not account for hydraulic characteristics such as overbank flow, water bypassing the gage, or superelevation at the gage site (i.e., bank-to-bank variations in the water surface due to flow around a bend). The USGS tries to limit uncertainty by restricting rating

8 The curve reflects the current site rating, and the shape of the curve gives more weight to the most recent discharge measurements, which represent current conditions at the site. Figure 2 includes discharge measurements after the gage was moved in September 1999.

July 2018 6 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

curve extrapolation to 2.5 times the highest measured discharge;9 however, even this level of extrapolation can result in a high degree of uncertainty in extrapolated flows.

Uncertainty is also associated with reported peak flows that were determined by methods other than a standard gage reading, for example peak flows reconstructed from high water marks (HWMs) following a gage malfunction, or peaks flows determined by indirect methods before a gage was in place. Variability in the method used to determine peak flow adds uncertainty and makes it difficult to discern whether differences within the gage record are due to actual flow differences or to the methods used to make the estimate. Uncertainty is also introduced when a gage is moved to a new location, such as to the opposite riverbank, which may be subject to slightly different flow characteristics.

Comparing peak discharges at different gages in the Snoqualmie River basin is complicated, and direct comparison of individual flood events based solely on peak flow magnitude is not necessarily meaningful. The basin encompasses a large drainage area (603 square miles above gage number 12149000 near Carnation) containing numerous tributaries with various floodplain characteristics, and two events with a similar recurrence interval10 can cause different flooding effects in the river valley depending on a number of factors including:

 Total flow volume and flood duration (flashy versus prolonged)

 Contribution and timing of flow from tributaries

 Storage distribution and flow attenuation within the floodplain

 Temporal changes to basin characteristics (e.g., logging, land development)

 Localized changes in channel cross section

 Downstream hydraulic conditions including backwater influences of the Skykomish River

These and other factors make it a challenge to interpret gage records and predict corresponding flood impacts.

9 A stage exceeding the “2.5 times” criterion requires additional review. The USGS may decide to extend the rating curve to estimate the peak or, alternatively, to calculate the peak via indirect methods. If the USGS feels the resulting flow estimate is of low quality, it may downgrade the quality of the record, code the peak as an estimate, or decide not to publish a value for the peak discharge (M. Mastin, USGS, personal communication, December 22, 2015). 10 Recurrence interval describes the likelihood of a flood occurring in any given year (e.g., a 100-year flood has a 1 percent chance of occurring in any given year).

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 7 2.4. SUMMARY OF SNOQUALMIE RIVER BASIN GAGE RECORDS

The following section describes current and historical USGS gaging stations in the Snoqualmie River basin and peak flood flows in the record. Summaries for each of the sites listed in Table 1 are based on data from the USGS website, Annual Data Reports (USGS 2014),11 and discussion with USGS staff. “Key” gages for this assessment include the Middle Fork Snoqualmie near Tanner (Middle Fork Tanner gage), the South Fork Snoqualmie above Alice Spring near Garcia (South Fork Garcia gage), the North Fork Snoqualmie near Snoqualmie Falls (North Fork gage), the Snoqualmie River near Snoqualmie (Snoqualmie gage), and the Snoqualmie River near Carnation (Carnation gage). Stage and flow at these stations are reported on King County’s Flood Warning website (King County 2015) and used to inform early flood warnings and road closures. Descriptions for other tributary gages are included in Appendix A.

2.4.1. Middle Fork Snoqualmie Near Tanner, WA (12141300)

The Middle Fork Snoqualmie near Tanner (Middle Fork Tanner gage) is one of three gages used to compute the “Sum of Snoqualmie River Forks” (also called the Sum of the Forks) flow values used by the King County Flood Warning Center to determine the flood phase and support early flood warning for the lower Snoqualmie Valley. The gage was installed in 1961 and measures flow from a drainage area of 154 square miles. There are no known issues with this gage, although failure of the sensor during floods in January 2015 and February 2016 caused it to miss the flood peak (D. Miller, USGS, personal communication, November 20, 2015). The USGS has addressed these failures by replacing much of the gaging equipment and is now closely monitoring the gage during flood events (J. Greene, USGS, personal communication with Chris Ewing, King County, August 2, 2016). The maximum discharge reported at the site is 31,700 cubic feet per second (cfs); it was recorded on November 6, 2006, and corresponds to a gage height of 15.32 feet. Following the flood of November 23, 1959 (before the Middle Fork Tanner gage was in place), the USGS used HWMs to estimate a peak discharge of 49,000 cfs at a location 6 miles downstream of the gage location.

2.4.2. Middle Fork Snoqualmie Near North Bend, WA (12141500) – Discontinued

The Middle Fork Snoqualmie near North Bend gage began operation in 1907, and measured flow from a drainage area of 169 square miles. The gage was discontinued in 1932. Peak, daily, monthly, and yearly flow statistics are available for the period of record. The maximum discharge recorded at this site was 26,700 cfs, recorded on November 23, 1909.

11 Reference provided for Snoqualmie River Near Carnation, WA Gage 12149000. 2013 Annual Data Reports were also used for other operational gages.

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2.4.3. North Fork Snoqualmie River Near Snoqualmie Falls, WA (12142000)

The North Fork Snoqualmie River near Snoqualmie Falls is the second gage included in King County’s Sum of Snoqualmie River Forks reporting and flood warning. The gage was installed in 1929 and measures flow from a drainage area of 64.0 square miles. The gage was moved 1,500 feet upstream in 1961 to the current site. No data quality issues are known or reported at this location. The maximum discharge recorded at the gage is 17,100 cfs; it was recorded on January 7, 2009, and corresponds to a gage height of 13.42 feet. The highest physical discharge measurement of 13,700 cfs was collected by USGS staff on November 22, 1959.

2.4.4. North Fork Snoqualmie River Near North Bend, WA (12143000) – Discontinued

The North Fork Snoqualmie near North Bend gage began operation in 1907, and captured flow from a drainage area of 95.7 square miles. The gage was taken offline in 1937, but gaging at this site started again 1960 and continued until 1977. Daily records end in 1971, while peak flows are reported through 1977. The maximum discharge recorded at the gage was 15,800 cfs; it was recorded on November 23, 1909.

2.4.5. South Fork Snoqualmie River Above Alice Creek Near Garcia, WA (12143400)

The South Fork Snoqualmie River near Garcia gage (South Fork Garcia gage) is the third and final gage used to support King County’s Sum of the Forks reporting and early flood warning system. The gage was installed in October 1960, and measures flow from a drainage area of 41.6 square miles. There are no known data quality issues at this site. The highest recorded peak is 8,910 cfs; it was recorded on November 6, 2007, and corresponds to a gage height of 18.68 feet. The highest physical discharge measurement of 7,660 cfs was collected on November 24, 1990.

2.4.6. Sum of Snoqualmie River Forks (Sum of Gages 12141300, 12142000, 12143400)

King County reports the Sum of the Forks by combining real-time flood estimates from three gages: the Middle Fork Snoqualmie near Tanner, the North Fork Snoqualmie near Snoqualmie Falls, and the South Fork Snoqualmie River above Alice Creek near Garcia. As mentioned above, there are no known data quality issues with any of these gages. This flow does not represent the true flow at the confluence of the Snoqualmie forks however, since it does not take into account travel time from the gage locations nor inputs from minor tributaries between the gages and the confluence.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 9 2.4.7. South Fork Snoqualmie River at Edgewick, WA (12143600)

The South Fork Snoqualmie River at Edgewick gage (Edgewick gage) was installed downstream of the South Fork Garcia gage in 1962 and measures flow from a drainage area of 65.9 square miles. The gage operated from July to September 1962 and from March 1963 to September 1965; it has also been in continuous operation since October 1983. The gage was moved from the left bank to the right bank in 2004, on the same datum.12 The USGS does not report any data quality issues at this gage; however, inconsistencies have been noted in the relationship between peak flows at this gage and downstream gage number 12144000, South Fork Snoqualmie River at North Bend (King County 2014). A discussion of these inconsistencies is included later in this report. The highest recorded peak flow is 16,500 cfs; it was recorded on November 6, 2006, and corresponds to a gage height of 15.32 feet. The highest physical discharge measurement of 8,000 cfs was collected on November 29, 1995.

2.4.8. South Fork Snoqualmie River at North Bend, WA (12144000)

The South Fork Snoqualmie River at North Bend (North Bend gage) has been in operation since 1907, but its location has moved a number of times and the record contains many temporal gaps. It has been in continuous operation at its current location since 1984, and the current site measures flow from a drainage area of 81.7 square miles. Inconsistencies have been noted in the relationship between peak flows at this gage and upstream gage number 12143600, South Fork Snoqualmie River at Edgewick (King County 2014). A discussion of these inconsistencies is included elsewhere in this document. The highest recorded peak flow is 13,600 cfs; it was recorded on November 6, 2006. The highest physical discharge measurement of 7,930 cfs was collected on November 7, 2006.

2.4.9. Snoqualmie River Near Snoqualmie, WA (12144500)

The Snoqualmie River gage near Snoqualmie (Snoqualmie gage) is located downstream from Snoqualmie Falls. Daily and peak flow reporting at the site began in August 1958, although intermittent records of monthly flow are available as early as 1898. This is the first gage on the main stem Snoqualmie River below the confluence of the North, South, and Middle forks, and measures flow from a total drainage area of 375 square miles. Until 1999, the gage was on the west (left, looking downstream) bank of the river downstream from Puget Sound Energy (PSE) Powerhouse Number 2. In 1999, the gage was moved to the right bank, where it remains today.

12 The gage maintained the same “zero” elevation as the previous gage, so that gage heights can be related to the existing record and rating curve without conversion. Occasionally, USGS gages are surveyed into an existing datum (e.g., National Geodetic Vertical Datum of 1929 [NGVD 29]), while others are on a local or approximate datum (e.g., 780 feet above NGVD 29).

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There are no known data quality issues with the current gage; however, there is significant uncertainty regarding the reported 1990 flood peak of 78,800 cfs13, which appears inconsistent when compared to peak flows measured at other Snoqualmie River basin gages during the same event. More discussion of that event is included below. The highest physical discharge measurement of 38,100 cfs was collected on December 9, 2015.

2.4.10. Snoqualmie River Near Carnation, WA, (12149000)

The Snoqualmie River gage near Carnation (Carnation gage) has been in operation since October 1928, and measures flow from a drainage area of 603 square miles. The maximum discharge recorded at this site is 82,900 cfs; it was recorded on January 8, 2009, and corresponds to a gage height of 62.21 feet. The highest physical discharge measurement is 50,500 cfs; it was recorded on February 10, 1951, and corresponds to a gage height of 58.68 feet.

2.4.11. Snoqualmie River at Duvall, WA (12150400)

The Snoqualmie River at Duvall is a stage-only gage site (flows are not reported). The drainage area above the gage is 645 square miles. The gage has been in operation since October 2000. The maximum gage height of 45.18 feet was recorded on January 8, 2009. This station is used for flood warning purposes only and does not start operating until the stage reaches about 29 feet (NGVD 29).

The stage measured at the Duvall gage is susceptible to backwater influence of the Skykomish River; the gage is located only 5 miles upstream of the confluence of the Snoqualmie and Skykomish rivers. It is also known that a large portion of flood flow leaves the main channel upstream of the Duvall gage and therefore does not pass the gage site in the main river channel. This condition does not directly impact reporting since the USGS does not provide an estimate of flow at this location.

2.5. NOTED ANOMALIES IN GAGE RECORDS

Significant anomalies noted in the existing USGS gaging record are discussed below, including a description of potential sources of error or uncertainty.

13 The USGS recently revised the November 1990 event peak downward from 78,800 cfs to 74,300 cfs. This change occurred after finalization of this analysis; however, the current estimate remains high compared to historical flow relationships and observations during the flood, and the revision does not significantly affect the analysis or conclusions herein.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 11 2.5.1. Middle Fork Snoqualmie Near Tanner

The flood of record at the Middle Fork Tanner gage location was estimated by the USGS using indirect discharge techniques for a 1959 flood that predates gage installation. As shown in Figure 3, the 1959 event appears to be an outlier when compared to annual flood peaks recorded since installation of the Middle Fork Tanner gage in 1961. Overall, annual peak flow reported for the Middle Fork Tanner site has ranged from 44 to 70 percent of the corresponding peak at the Snoqualmie gage with the exceptions of 1959 and 1990,14 where the peak at the Middle Fork Tanner gage represented 80 percent and 38 percent of the peak at the Snoqualmie gage, respectively.

November 1959

November 1990

Figure 3. Relationship of Peak Annual Flows at the Middle Fork Tanner Gage and Snoqualmie Gage.

14 The 1990 event is a noted anomaly at the Snoqualmie gage, as discussed later in this report.

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The 1959 flood peak estimate of 49,000 cfs for the Middle Fork Tanner gage site was developed by the USGS using the slope-area method based on high water marks and channel cross sections surveyed in early 1960 (USGS 1960). Potential sources of error or uncertainty with the estimate include reliability of the HWMs and selection of an appropriate channel roughness value. The location where HWMs and channel characteristics were determined was downstream of a river bend, and HWMs indicate 3 feet of bank-to-bank variation in water surface elevation; this is likely the result of both superelevation and high turbulence associated with the event. The final estimate of 49,000 cfs would require average channel velocities of 16 feet/second, and a unit discharge of over 300 cfs per square mile of drainage area, both of which appear very high but not necessarily unreasonable for a large flood event. While the 1959 peak flow cannot likely be validated 50 years after the estimate was made, it appears high and should be viewed with caution and with due consideration of the methods and assumptions used to make the estimate.

2.5.2. South Fork Snoqualmie Near North Bend

Another anomaly is the relationship between peak flows at the South Fork Snoqualmie gages near North Bend and at Edgewick. As shown in Figure 4, annual peak flows recorded at the downstream North Bend gage before 2004 tend to be higher than at the upstream Edgewick gage; however, since 2004 peak flows have been consistently higher at the Edgewick gage over the entire range of flows. It is unlikely that peak flows would actually decrease between Edgewick and North Bend because the North Bend gage is located downstream, it captures a larger contributing drainage area, and the river lacks significant floodplain storage that would attenuate the flood peak between these two gages.

The Edgewick gage was moved from the left bank to the right bank on March 17, 2004, corresponding closely to the change in relationship to the North Bend gage. However, the USGS has not found a problem that would cause them to suspect flow reporting at the new Edgewick gage location: the gage was leveled to the same datum as the old gage, and a high flow measurement made on November 7, 2006, matched closely to the existing stage-discharge curve (King County 2014).

The USGS suspects that water is bypassing the North Bend gage during high flows, causing the gage to under-report discharge (M. Mastin, USGS, personal communication, October 27, 2015). Levee overtopping was documented in the reach between the gages during peak annual events in both 2006 and 2009, but does not account for all the flow reduction during these events (King County 2014). A drawdown in the water surface has also been observed at the North Bend gage, which means that localized water surface impacts are causing the stage reported by the pressure transducer to be lower than the actual river stage; this would tend to make the North Bend gage under-report, and as a result the USGS has been applying a correction factor at the site since 2009 (King County 2014).

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 13

2006 Flood Event

2009 Flood Event

Figure 4. Difference in Peak Annual Flow Between the North Bend Gage and the Edgewick Gage.

2.5.3. Sum of the Forks

As shown in Figure 5, the peak coincident flow for the Sum of the Forks tends to be similar in magnitude to the corresponding peak at the Snoqualmie gage. Positive values in Figure 5 represent an increase in peak flow between the Sum of the Forks and the downstream Snoqualmie gage during 27 peak annual flood events recorded at the Snoqualmie gage since 1987. In general, flood peaks at the Snoqualmie gage were slightly larger than those for the Sum of the Forks, although the magnitude of increase varies widely. As shown in Figure 5, the November 1990 event is a significant outlier. Although King County and others have previously attempted to reconcile the confounding data from the November 1990 flood, the cause of its divergence from the other values shown in Figure 5 has not been determined.

July 2018 14 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

1990 Flood Event

Figure 5. Difference in Peak Annual Flow Between the Sum of the Forks and the Snoqualmie Gage Near Snoqualmie Since 1987.

2.5.4. Snoqualmie River Near Snoqualmie 1990 Peak

The peak flow in the November 1990 flood was, by far, the largest flood ever reported at the Snoqualmie gage. The stage recorder was destroyed during the event, causing it to miss the flood peak; the USGS reported the peak based on high water marks left at the gage location. The USGS reported peak of 78,800 cfs greatly exceeds peak flows reported at upstream and downstream gages during the same event. Table 2 shows reported peak flows at the Snoqualmie gage, Carnation gage, and Sum of the Forks during a number of historical high-flow events, including November 24, 1990.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 15 Table 2. Peak Flow Comparison at Key Gages. USGS Reported Peak Flow (cfs) Date Sum of Forksa Snoqualmie Near Snoqualmie Snoqualmie Near Carnation 12/3/1975 50,800b 51,800 52,100 11/24/1986 49,950b 58,100 57,100 11/24/1990 48,250 78,800 65,200 11/1995 47,960 50,200 62,400 2/9/1996 43,740 51,700 61,600 11/7/2006 53,500 55,000 71,800 1/7/2009 54,100 60,700 82,900 1/17/2011 34,740 37,900 51,600 11/17/2015 44,210 48,000 51,700 a Sum of peak flows at gages 12141300, 12142000, and 12143400; see Table 1 and Figure 1. b Sum of instantaneous peak flow for each gage during event (not peak sum of coincident flow) because continuous flow records are not available before 1987. Other Sum of Forks values are from King County (2015).

King County and WSE have noted that the November 1990 peak at the Snoqualmie gage seems out of line with other gage records for the same event. In a 2012 memorandum, the USGS “… agree[d] that there is ancillary information that suggests the published peak discharge for the November 1990 flood appears to be too high”; however, they could not produce justification to adjust the peak (USGS 2012).

Anecdotal information provided by Mr. Bob Barnes, who retired recently from a long tenure at PSE, supports the theory that the actual 1990 flood peak was lower than the currently reported flow of 78,800 cfs (Barnes, personal communication, November 16, 2015). The following information is based on Mr. Barnes’ observations:

 Water surface elevations near the gage site in November 1990 were lower than those in January 2009 (the January 2009 flood event peak is currently reported at 60,700 cfs).

 There was at least 2 feet of flow superelevation in the water surface from the north side (lower) to the south side (higher) of the Snoqualmie River channel near the PSE Powerhouse during the 1990 event.

Mr. Barnes supported his observation of water surface elevations by noting that the January 2009 flood inundated a lawn area located north of PSE Powerhouse 2, and that the inundation was sufficient to prompt a PSE shutdown of the powerhouse; the powerhouse is located just upstream from the gage location. According to Mr. Barnes, the powerhouse lawn was not inundated during the November 1990 flood event, and the powerhouse remained in operation throughout that event.15

15 Mr. Jory Oppenheimer of PSE confirmed that PSE logbooks show the powerhouse was taken offline during flooding in 2006 and 2009. PSE could not verify whether the powerhouse remained in operation during the 1990 flood event (J. Oppenheimer, PSE, personal communication, December 22, 2015).

July 2018 16 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

The superelevation observed at the powerhouse could be significant if associated water surface impacts continued downstream and affected the USGS gage location. Until 1999, the gage was located on the left bank of the river downstream from the powerhouse, at a site where superelevation would tend to increase the stage at the gage relative to the average river stage and thus result in artificially high flow estimates. To evaluate the potential impact of superelevation at the gage site, WSE constructed a simple 2D Hydrologic Engineering Center River Analysis System (HEC-RAS) hydraulic model for the river reach near the Snoqualmie gage, and ran the model to simulate the 1990 flood event. Model results showed pronounced water surface superelevation along the west bank of the river channel near the PSE powerhouse, confirming Mr. Barnes’ observations. However, simulated superelevation was minor to nonexistent downstream of the powerhouse near the gage location, suggesting that superelevation did not likely impact the 1990 peak flow estimate. Results also show no water surface impacts from superelevation on the right bank, where the gage was moved in 1999, and remains today.

The USGS recently discovered a 0.45-foot error in the measurement of a high water mark from the November 24, 1990, flood at the gage site. This change would adjust the peak stage down from 21.55 feet to 21.1 feet, and adjust the peak flow estimate down from 78,800 cfs to 74,300 cfs (M. Mastin, USGS, personal communication, October 27, 2015).

2.5.5. Snoqualmie River Near Snoqualmie Rating 7

Since 1958, the USGS has applied eight different rating curves at the Snoqualmie River gage near Snoqualmie. “Rating 7”, used from November 1998 to September 2011, was dramatically different from earlier rating curves and is not consistent with high flow measurements collected since 1958. Use of Rating 7 results in notably lower estimates of the flood peaks during the 1998 to 2011 period, including for extreme floods in November 2006 and January 2009.

The three most recent rating curves (Ratings 6, 7, and 8) are shown in Figure 6, along with peak flow measurements collected since 1958. The upper portion of Rating 7 is shifted to the left of the other ratings, indicating a lower flow at a given stage. Rating 7 was developed from a small number of flow measurements and did not account for past ratings (M. Mastin, USGS, personal communication, November 16, 2017). Rating 8 provides a much better fit to measured flow data. Application of Rating 8 to peak stages measured in November 2006 and 2009 would increase peak flow estimates for those events by 5 to 10 percent.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 17

Figure 6. Recent Snoqualmie Gage Ratings Versus Measured Discharge.

The data shown in Figure 7 illustrate how stage rating at the Snoqualmie gage has changed over time across a range of flows. There was little change in the stage-discharge relationship at the gage for the first 40 years of operation; however, the application of Rating 7 in 1998 is marked by a significant jump in stage for flows above 20,000 cfs. Rating 8 was applied in November 2011 and returned the rating closer to the levels seen in earlier ratings. Based on a recent discussion with USGS staff (M. Mastin, USGS, personal communication, November 15, 2017) there is not enough information to determine why Rating 7 was drawn the way it was, and the USGS does not believe that a retroactive correction to Rating 7 is warranted. Therefore, historic data for November 1998 to September 2011 are still based on Rating 7.

July 2018 18 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Application of Rating 7

Figure 7. Gage Height (Stage) Over Time at the Snoqualmie Gage.

2.5.6. Snoqualmie Near Carnation

A known issue at the Carnation gage is that a large portion of flood flow leaves the main channel upstream of Carnation Farm Road (see Photo 1) and therefore does not pass the gage site. The existing field measurement program does not account for the overbank flow (D. Miller, USGS, personal communication, October 27, 2015). This condition is a source of inaccuracy in flow reporting for large flood events when the river overtops its banks upstream of the gage site.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 19

Photo 1. Viewing Upstream to the Northeast Carnation Farm Road Bridge During the November 1995 Flood. Significant Flow is Bypassing the Carnation Gage, Which was Located just Downstream of the Bridge on the Left Bank.

Flow reporting at the Carnation gage is further confounded by unsteady flow conditions, which produce a hysteresis effect16 at this gage site. Specifically, flows during the rising limb of a flood hydrograph are under-reported and those during the falling limb are over-reported. At the time of this analysis, the USGS web page for the Carnation gage included the following note regarding this issue:

At this site, near-real-time provisional discharges greater than 9,000 ft3/s (gage heights greater than about 50 ft) are not reliable due to unsteady flow conditions during high flows. Under these conditions, a loop stage-discharge rating exists such that the true discharge is generally greater than indicated on a rising hydrograph and less than

16 When a flood wave moves down a river channel and through a given cross section of the channel, the effect of the wave when moving from upstream of the cross section is to increase the flow velocity at the cross section. When the flood peak passes, the rear of the wave increases the backwater conditions and so reduces the velocity at a given discharge at the cross section. The result is that, for the same stage, the discharge is higher during rising stage than during falling stage. This phenomenon is called hysteresis. For some rivers, especially those with low channel gradient and complex floodplain geomorphology such as at the Carnation gage, these effects will be manifest as distinctive loops in the stage–discharge relationship.

July 2018 20 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

indicated on a falling hydrograph. Corrections are applied following a high-flow event but data remain provisional until approved. (Message posted February 20, 2015.)

The USGS continues to develop a new looped rating curve to apply at specific gage water levels. However, this correction, which was first implemented in 2015, must be applied manually during the rising limb of a flood and is based on the forecasted peak flow at the gage, and thus is influenced by the accuracy of the forecast issued by the National Weather Service Northwest River Forecast Center (NWRFC). Despite these issues, the King County Flood Warning Center continues to use real-time flow reporting at this site as a source of best available data during a flood event.

2.5.7. Shifts in Carnation Gage Stage-Discharge Relationship

The rating at the Carnation gage has shifted significantly over time, indicating a changing relationship between stage and flow. Figure 8 shows stage versus time at the Carnation gage for several representative flows. Stage has increased for all flows since the 1960s, with a total increase of more than 1 foot at flows below 20,000 cfs between 1960 and 2015. Stage shows less variability since approximately 2000, and has reached a relative equilibrium that is consistently at or above the pre-1960 dredging stage.

Figure 8. Gage Height (Stage) Over Time at the Carnation Gage.17

17 Gage heights are based on historical USGS rating curves and shifts.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 21 Stage at low flows changes in response to river channel adjustments including sediment deposition and bed scour. Increased stage for flows below approximately 20,000 cfs likely represents physical changes within the channel over time. Decreased stages in the 1960s were observed after 200,000 cubic yards of material was dredged from the “Tolt River Delta” (King County, 2017). Additional discussion of sediment impacts at the Carnation gage is included in Section 3.5.4.

Stage at higher, out-of-bank flows is less sensitive to physical changes within the river channel. Interpretation of stage changes at higher flows is also complicated by unsteady flow conditions at the Carnation gage. The current rating is considered “poor” for discharges above 20,000 cfs (see Section 2.5.6); high flow ratings are based on a relatively small number of measurements, making high flow stages and shifts subject to much more uncertainty than those for lower flows.

2.5.8. Relationship Between Snoqualmie and Carnation Gages

Historical gaging records do not show a consistent relationship between concurrent annual peak flood flows at the Snoqualmie and Carnation gages, as indicated in Figure 9. Of the 50 peak annual events observed at both gages, 33 were higher at the Carnation gage than at the Snoqualmie gage (positive value) and 17 were higher at the Snoqualmie gage than at the Carnation gage (negative value). Of the 17 flow values that were higher at the Snoqualmie gage, nine were recorded in the 14-year span between 1977 and 1990, which may indicate an issue with the gage data. The reason for the variation in flow differences between Snoqualmie and Carnation is unclear, but could be differences in flood characteristics (such as flashy versus prolonged high flows), differences in tributary inflows, differences in the accuracy of the reported gage data at different periods in the record, variations in floodplain storage, and other as yet unknown factors.

July 2018 22 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Figure 9. Difference Between Peak Annual Flows at the Snoqualmie River Gages Near Snoqualmie and Carnation.

2.6. RECOMMENDATIONS FOR IMPROVING DISCHARGE RECORDS AND STREAMFLOW GAGING

Improving the discharge gaging records and streamflow gaging in the Snoqualmie River basin could be accomplished as follows:

 Implement measures to increase confidence in existing discharge estimates and stage-discharge rating curves.

o Update existing rating curves to account for flow known to bypass the Snoqualmie gage near Carnation and the South Fork Snoqualmie gage at North Bend. This effort should include physical measurements of bypassing flow (if present) during field measurements at the gage site, as well as indirect estimation of peak flow bypassing the gage following a flood event. The stage-discharge relationship should then be updated to reflect the total flow. This effort would best be completed under the direction of USGS.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 23 o Develop a strategy to obtain more physical discharge measurements during large floods to improve discharge rating curves for high-flow estimation. This effort could be completed and implemented cooperatively among USGS, King County Rivers Section, and King County Flood Control District.

 Use hydraulic modeling of the Snoqualmie River to supplement recorded information. Unsteady simulation of historical flood events based on reported USGS flows would allow comparison of modeled results and observed flood conditions to perform the following:

o Check the reasonableness of USGS estimated peak flows. These modeling results could especially be used to help assess the accuracy of flow estimates during larger less frequent floods.

o Confirm the interaction of USGS flow estimates at different gages.

o Support estimation of peak flows at gages where records are missing or incomplete.

o Provide insight into the interaction of water surface elevation and flow during flood events that exceed the highest physically measured discharge at a gage location. Doing so would allow the USGS to check existing rating curve extrapolations, or to extend rating curves based on modeled results.

o Estimate the frequency and flood magnitude where flow bypasses are initiated. This action would be helpful at gage locations including Snoqualmie gage near Carnation and the South Fork Snoqualmie gage at North Bend.

 Maintain existing gages to extend the gage record at the current locations. The current gaging program provides good coverage of the Snoqualmie River and major tributaries, and the length and consistency of the record is an important factor for flood frequency analysis, identification of trends, and interpretation and investigation of gage anomalies.

Existing gages should be flood-proofed to the extent possible to prevent malfunction and improve their reliability during extreme flood events. This enhancement will help avoid missing data in the long-term historical stage record, and reduce uncertainty associated with indirect discharge estimates (e.g., November 1990 at the Snoqualmie gage, 1959 at the Middle Fork Tanner gage). The USGS has already taken steps to address recent gage malfunction at the Middle Fork Tanner gage.

 Add one or more gages within the lower Snoqualmie Valley. In providing more data for flow comparisons, additional gages would improve understanding of the integrity of data from individual gages, and add redundancy. Denser gage coverage would provide a means to check the consistency of preliminary data and support more thorough comparison of gage records and reported peak flows. Multiple gages on the South Fork

July 2018 24 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Snoqualmie River, for example, allowed the USGS to identify flow that was bypassing the North Bend gage, and highlighted a potential issue with the observed flow relationship between the North Bend and Edgewick gages. It would be difficult, however, to find a location in the lower Snoqualmie River valley where flow during a large flood would not be able to bypass the gage location. Thus, limited gaging resources and effort should be directed towards maintaining and improving gaging at the existing locations; adding gage locations should be a lower priority.

 Add one or more stage-only gages with telemetry to provide additional real-time resolution to river stage data during flood events. Additional gage locations reporting river stage would be useful to residents in tracking flood events and preparing for flood impacts. Two potential locations to consider are the Preston-Fall City Road Southeast bridge over the Snoqualmie River at Fall City (located on the Snoqualmie River upstream of the confluence with the Tolt River), and the Northeast Tolt Hill Road bridge over the Snoqualmie River near Carnation (located on the Snoqualmie River downstream of the confluence of the Raging River and Tokul Creek). These locations would provide resolution between the existing Snoqualmie and Carnation gages, and would account for inflow from the Raging River. Stage-only gages provide real-time flood stage information, but are less costly to install and maintain than flow gages since flow measurement and rating curve development are not required.

2.7. RECOMMENDATIONS FOR KING COUNTY USE OF GAGE DATA FOR FLOOD WARNING

It is recommended that King County continue to rely on USGS stage and flow reporting to inform early flood warning in the Snoqualmie River valley. Despite the uncertainties and anomalies discussed above, the existing USGS gaging program is the best source of real-time flood information on the Snoqualmie River. Stage reporting is generally very reliable; peak flow estimation for large flood events is often based on rating curve extrapolation well beyond the largest measured flow and is thus subject to considerable uncertainty.

Based on the foregoing assessment of gages in the basin, the following recommendations are offered:

 Continue to use the Sum of the Forks as the basis for flood phase determination and early flood warning. No known data quality issues are associated with the three gages used in the Sum of the Forks, and peak flows reflected in the Sum of the Forks appear to correlate well to peak flows at the Snoqualmie gage (with the exception of the 1990 flood event).

 Continue to use the current peak flow estimate for flood frequency analysis for the Snoqualmie River gage near Snoqualmie. The magnitude of the 1990 flood event at this location remains uncertain. The peak flow estimate for the November 1990 flood

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 25 was recently revised downward to 74,300 cfs, but may still be too high based on comparison to historical gage records and 1990 flood observations. The current peak flow estimate should be used for flood frequency analysis (where conservatism is prudent).

 Use caution when interpreting peak flow information for large floods (greater than 9,000 cfs) at the Carnation gage. The current gaging program does not directly account for the portion of the flow known to bypass the gage at discharges above approximately 50,000 cfs. Additionally, real-time flow reporting by the USGS can only approximate the updated looped rating curve under development for this gage; therefore, final flow reporting may vary significantly from real-time data currently used in King County’s flood warning system. Stage information at this site is expected to remain reliable.

 Expand the amount of web-based real-time detailed information available to the public for flood levels and flow rates. Increasing access to detailed information would help ensure that farms, residents, and businesses can be safer during floods.

o Develop a distributed data collection program that expands and institutionalizes the web-based social network that exists. Landowners could be an important resource in implementing the program. This effort would best be developed and implemented cooperatively by Snoqualmie Valley Preservation Alliance (SVPA), the King County River and Floodplain Management Section, and the King County Flood Control District.

o Collect, store, and share flood-related data for analysis over time. Data should include high water marks, photos, and flood observations.

 Continue to include forecasted flow and stage hydrographs from the National Weather Service Northwest River Forecast Center (NWRFC) on King County’s Flood Warning website. These forecasts define potential flood magnitude and duration, which help residents anticipate how flood conditions may develop. The NWRFC flood forecast is based on a runoff and river routing model that incorporates simulated basin conditions, observed streamflow, precipitation, temperature, and forecasted meteorological parameters to estimate flow at each of the key USGS gage locations. Because the model is only run once daily,18 forecasted stage and flow will become misaligned with observed stage and flow when flooding develops differently than predicted. Despite these limitations, the NWRFC forecasts are a useful tool for tracking potential flooding.

18 The NWRFC flood forecast may be updated (rerun by replacing forecasted parameters with observed parameters) if the forecast diverges significantly from the observed conditions during large flood events, but the timing and frequency of this update is at the discretion of the NWRFC. The river forecasts are not archived, and the routing model is not made available to the public.

July 2018 26 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

3. HYDROLOGIC TRENDS AND POTENTIAL FUTURE CONDITIONS IN THE SNOQUALMIE RIVER

WSE completed an investigation to characterize the nature and strength of hydrologic trends in the Snoqualmie River basin and their effect on flooding. This effort included a review of existing literature on hydrologic trends in the Puget Sound region, an analysis of streamflow and precipitation data for the Snoqualmie River basin, an evaluation of long-term hydrologic trends for the lower Snoqualmie River, and an investigation of key factors that may be influencing those trends.

3.1. SUMMARY OF FINDINGS

Evaluation of precipitation and streamflow data for the Snoqualmie River basin since 1962 suggests that there is generally less flow in summer, more flow in spring and late fall, and an increase in the magnitude of extreme floods. However, few of the trends are statistically significant at the 90 percent significance level.19 The trends are generally consistent with changes in precipitation and flow projected by current climate change research (Mauger et al. 2015), which suggest a continued shift in flood timing that may result in even lower flow volumes in summer, higher flow volumes in winter, and more extreme annual peak flooding (typically in winter). Analysis of peak flood characteristics at the Snoqualmie River streamflow gages near Snoqualmie and Carnation did not yield statistically significant evidence of trends towards faster flood travel time or faster flood response.

Human activities including development and forest harvest are known to affect flooding by increasing the magnitude and timing of peak flows. Land use changes in the basin between 1996 and 2010 have increased impervious surface in the watershed by 27 percent (National Oceanic and Atmospheric Administration [NOAA] 2016); however, only roughly 9.2 square miles of the basin is covered by impervious surface (1.3 percent), and stormwater management regulations implemented after 2005, such as those applied to the Snoqualmie Ridge development, have helped mitigate effects from the recent large-scale urban developments. In addition, the impact from logging has likely declined with the decades-long decrease in timber harvest in the basin.

Major projects at Snoqualmie Falls (i.e., modifications to PSE’s weir at Snoqualmie Falls [the PSE Project] and removal of upstream flow obstructions by the US Army Corps of Engineers [the 205 Project]) increased the magnitude of peak flows downstream of Snoqualmie Falls, and

19 A 90 percent significance level means there is at least a 9 in 10 chance the trend is real.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 27 slightly decreased flood travel time between the Falls and Carnation. Hydraulic simulations indicate a maximum increase of 1,200 cfs in the 100-year peak flow downstream from the projects, and the earlier occurrence of peak flood stage (20 minutes earlier) at Carnation (WSE and Herrera 2016). Simulated peak 100-year water surface elevations were increased by a maximum of 0.1 foot downstream of the Falls, and decreased by 1.5 feet upstream in the city of Snoqualmie. A significant loss of flood storage volume, totaling as much as 2,500 acre-feet during the 100-year flood event, was estimated to have occurred upstream of the Snoqualmie Falls as a result of the projects at the Falls. This upstream flood reduction is the cause of the higher downstream flows and water levels. Impacts from the Farm Pad program, the Upper Carlson project, the Chinook Bend project, the McElhoe Pearson project, and channel maintenance at river mile (RM) 31 have been either minimal (based on no-rise analysis) or minor (based on the nature and scale of the project. (Descriptions of these recent capital projects are provided in Section 3.5.5.) Specific localized impacts (or lack thereof) of projects in the river and floodplain, and associated changes in flooding, may become clearer as monitoring continues. Post-flood monitoring, for example of localized flooding impacts near the Upper Carlson project, is still under way (King County 2016a).

While human activities may account for some variability in flood characteristics over time, this investigation did not find sufficient statistically significant evidence of overall flooding trends in the Snoqualmie Valley. Much of the flood-to-flood variation experienced by residents and recorded by gages can likely be explained by the combined variations in antecedent soil saturation and ground temperature, and differing patterns of precipitation including snowfall, storm duration, and areal distribution of rainfall within the watershed. In general, observed trends in flow and precipitation data are not statistically significant to the 90 percent significance level, but could still be indicative of long-term trends that are not currently evident in the available data record. Any emerging trends in hydrologic conditions and flooding will become more evident with time.

3.2. BACKGROUND

Flooding is a regular occurrence within the Snoqualmie River basin. Annual floods inundate farmland and overtop minor roads, while larger and less frequent floods can cover the floodplain from valley wall to valley wall. The increased frequency and magnitude of recent flood events and local observations of changes in flood characteristics have caused Snoqualmie Valley residents to question whether the nature of floods is changing within the Snoqualmie River basin.

A recent analysis of King County streamflow and precipitation data from 1962 to 2008 (King County 2010) found an upward trend in annual maximum daily flows at six USGS gaging stations in the Snoqualmie River basin, as well as upward trends in annual maximum daily precipitation at long-term precipitation stations proximate to the basin. However, the study concluded that there was not sufficient statistically significant evidence to state that these trends were reliable (King County 2010). In general, there is considerable uncertainty about whether local changes in

July 2018 28 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

extreme flooding and precipitation result from changing climate or from natural decade-to-decade variability (Salathé et al. 2014).

Residents of the Snoqualmie Valley have reported changing flood patterns along the Snoqualmie River, including more frequent and extreme flooding, faster flood progression, and more damaging flooding in the spring and fall. Reported changes are often attributed to increased upstream development, forestry practices, sediment deposition (or lack of sediment management), and upstream projects such as those constructed by the US Army Corps of Engineers and PSE just upstream of Snoqualmie Falls and by King County at the Chinook Bend Natural Area.

To assess hydrologic trends in the Snoqualmie River basin, and their potential effect on hydraulic changes, WSE did the following:

 Consulted representatives of the USGS, the NWRFC, and the University of Washington Climate Impacts Group (CIG) to gather the current state of knowledge on precipitation, flooding, and conditions affecting the Snoqualmie River basin (a summary of these interviews is included in Appendix B).

 Reviewed existing literature regarding hydrologic trends in the Puget Sound region.

 Assessed trends in peak flood frequency and magnitude using USGS peak and mean daily flood records, and compared the rate of flood level rise and flood travel time for peak annual flood events at the Snoqualmie River gages near Carnation and near Snoqualmie.

 Assessed trends in peak flood stage at the Snoqualmie River gages near Carnation and near Snoqualmie.

 Completed a trend analysis of daily precipitation data and mean daily streamflow data in the Snoqualmie River basin between 1962 and 2015, which is the common period of record for the gages analyzed.

 Completed a trend test to evaluate the frequency of smaller floods, and possible trends towards more frequent or severe flooding in the early fall and late spring.

 Examined the potential role of changes within the basin on flood magnitude and timing, including forestry, land development, instream capital projects, precipitation patterns, and climate change.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 29 3.2.1. Historical Peak Flows at Key Streamflow Gages

Analysis of USGS flow data focused on five “key” USGS streamflow gages King County uses to provide flood information and early flood warning for the Snoqualmie River:

 North Fork Snoqualmie River near Snoqualmie Falls (North Fork gage)

 Middle Fork Snoqualmie River near Tanner (Middle Fork Tanner gage)

 South Fork Snoqualmie River above Alice Springs near Garcia (South Fork Garcia gage)

 Snoqualmie River near Snoqualmie (Snoqualmie gage)

 Snoqualmie River near Carnation (Carnation gage)

The 10 largest peak annual flows reported for each of these five Snoqualmie River gages are listed in Table 3.

Table 3. Ten Highest Observed Annual Peak Flowsa at Key Snoqualmie River Gages. Middle Fork South Fork Carnation Gage Snoqualmie Gage Tanner Gage Garcia Gage North Fork Gage Water Flow Water Flow Water Flow Water Flow Water Flow Rank Year (cfs) Year (cfs) Year (cfs) Year (cfs) Year (cfs) 1 2009 82,900 1991 74,300b 2007 31,700 2007 8,910 2009 17,100 2 2007 71,800 2009 60,700 2009 31,200 1987 8,450 1932 15,800 3 1991 65,200 1987 58,100 1978 30,200 1991 8,000 1996 14,500 4 1996 61,600 2007 55,000 1991 30,100 1996 7,450 2007 14,200 5 1932 59,500 1978 53,800 1987 28,900 2009 7,440 1960 13,700 6 1933 59,000 1976 51,800 1996 27,400 2016c 7,400 1935 13,400 7 1987 57,100 1996 51,700 2016c 27,300 1963 7,090 1945 13,400 8 2016c 56,200 2015 50,100 1975 24,900 1989 6,380 1951 13,200 9 2015 53,900 2016c 49,500 1990 24,400 1978 6,370 1987 12,600 10 1951 52,200 1975 48,100 1976 23,700 1976 6,190 1955 12,200 a Annual peak flow represents the maximum instantaneous flow occurring in a single water year. Therefore, significant floods (i.e., Nov 2008) are not included in this table when a larger flood (i.e., Jan 2009) occurred in the same water year. b Value recently revised downward from 78,800 cfs by the USGS. c Preliminary value. Approved peak flow records for water year 2016 (October 1, 2015, to September 30, 2016) have not been published officially by the USGS.

Many residents have reported that flooding in the Snoqualmie Valley is getting worse. This view has been bolstered by a number of recent large flood events. Four of the ten largest peak annual floods recorded at gages near Carnation and Snoqualmie occurred within the last 10 years, including events in water years 2007, 2009, 2015, and 2016. While recent flooding stands out as some of the most extreme, a number of the largest peak annual floods (Table 3) occurred much earlier in the record and have been well distributed over the entire record of the

July 2018 30 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

gages analyzed. Peak flow magnitude (1962 through 2015) appears to have increased over time, but statistical evidence to verify the trend is not sufficient.20

The relatively short record at most of these five gages (Table 4) makes it difficult to determine whether recent large flood events reflect a true upward trend in flooding or simply natural decadal variation in flood magnitude and frequency. This ambiguity is demonstrated graphically in Figure 10 and Figure 11, which show peak annual flows recorded at the Carnation and Snoqualmie gages, respectively. Figure 10 shows annual peak flows recorded at the Carnation gage since recordkeeping began in 1929, with horizontal lines to represent the 2-year and 10-year recurrence flood magnitudes. The high flows in water years 2007 and 2009 are the largest events in the record at this gage, and all flood events exceeding 60,000 cfs have occurred since 1990. Significant flood events, with magnitudes greater than 30,000 cfs, have also been recorded in almost every decade since recordkeeping began, including top 10 events in the 1930s, 1950s, 1980s, 1990s, 2000s, and 2010s. The period from 1931 through 1934 included some of the most consistently high annual peaks, including two of the top ten events in the record. Despite the visual appearance of an increase in peak flows as seen in Figure 10, there is no statistically significant evidence that peak annual flood magnitude has increased at the Carnation gage.

Annual peak flows recorded at the gage near Snoqualmie are shown in Figure 11; recordkeeping began in 1958. No obvious visual or statistical trend in peak flood magnitude exists for the dataset at this location.

Table 4. Available Data Record at Key Snoqualmie River Gages. Available Data Record Gage Location Mean Daily Flow Annual Peak Flow Short-interval Flow Snoqualmie 1907–2015 1958–2015 1988–2015 Carnation 1929–2015 1929–2015 1988–2015 North Fork 1929–2015 1930–2015 1988–2015 Middle Fork Tanner 1961–2015 1961–2015 1988–2015 South Fork Garcia 1961–2015 1961–2015 1988–2015 Raging River 1945–2015 1945–2015 1988–2015

20 Sufficient evidence of a trend was defined as statistically significant to the 90 percent significance level.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 31

Figure 10. Observed Peak Annual Floods at Snoqualmie River Near Carnation Gage (1929–2015).

July 2018 32 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Figure 11. Observed Peak Annual Floods at Snoqualmie River Near Snoqualmie Gage (1958–2015).

3.3. TRENDS IN MAGNITUDE AND TIMING OF FLOOD FLOWS AT KEY STREAMFLOW GAGES

Although extreme flood events are often the focus of flood hazard studies and floodplain regulations, smaller more frequent flood events also adversely impact the Snoqualmie Valley. These impacts include damage to property (e.g., structures, cropland, crops, soil, livestock), disruption of agricultural activities, changes to the river morphology (e.g., bank erosion, sediment deposition), and temporary road closures. Many valley residents have reported that the magnitude and frequency of smaller flood events is not only increasing but also shifting to earlier in the fall and later in the spring. Valley residents have expressed interest in a trend assessment of spring and fall flooding, as smaller floods outside the large-flood season are often reported as having significant impacts to cropland and property.

Analysis of trends in flood magnitude (or frequency analysis of flow data) in the Snoqualmie River basin is confounded by the relatively short flow record, limitations in the accuracy of reported flows, and noted anomalies in the USGS gaging record (see Section 2). Flows estimates exceeding 20,000 cfs at the Carnation gage are rated “poor” by the USGS because of complicated floodplain storage effects at the site. The Snoqualmie gage was destroyed during

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 33 the November 1990 flood event, and the reported peak is suspected to be too high21. The Middle Fork Tanner gage has malfunctioned during several recent flood events, missing the flood peak. Although the accuracy of reported flows at USGS gages is generally within 5 to 10 percent (USGS 1992), it is not clear that this level of accuracy is always attained at all locations in the Snoqualmie basin or particularly for large floods. However, despite these limitations and anomalies, the USGS dataset represents the best and most comprehensive record of flooding in the valley, and provides a reasonable basis for comparison of flow trends over time.

A 2010 evaluation of the trend in mean daily flow at selected USGS gaging stations throughout King County (King County 2010) suggests shifts to earlier snowmelt and runoff (i.e., from spring to winter) and increases in the magnitude of extreme flow events. The analysis discerned an increase in maximum annual flow at each of the key gaging stations in the Snoqualmie River basin, but that increase was not found to be a statistically significant trend.

WSE extended the 2010 trends analysis for the Snoqualmie River basin gages using approved mean daily flow records through 2015. Although short-interval (typically 15-minute) flow data are available at the gages beginning in 1988, the mean daily records provide a much longer period of record, are not marked by the significant data gaps typical of the 15-minute data, and are therefore more conducive to analysis of changes over time. Trends tests conducted by WSE focused on annual and monthly flow magnitude, including annual (water year) average flow, annual (water year) maximum daily flow, monthly average flow, monthly maximum flow, and annual (water year) peak flow (based on annual instantaneous, 15-minute, peak flow rather than mean daily flow). Available flow records at the key gages are listed in Table 4. The Raging River near Fall City gage (USGS 12145500; Raging River gage) was included in this analysis to be consistent with the previous work by King County (2010), and peak annual stage records were evaluated at the Carnation and Snoqualmie gages to identify trends in the highest stage recorded during any given year.

Trend analysis was also completed to test for patterns in the frequency of occurrence of smaller flood flows, using threshold values of 80 percent of the 2-year22 mean daily flow, the 2-year mean daily flow, and the 5-year mean daily flow. Mean daily flow data from the key gages were reviewed to determine the number of days each year that the threshold flow values were exceeded, and a trend test was completed to assess changes in frequency of occurrence over time. Flow threshold values are shown in Table 5 along with peak flow frequency values at the key gages. At the Snoqualmie gage, 80 percent of the 2-year threshold represents a mean daily flow value of 17,600 cfs, which captures five of the six significant floods on the Snoqualmie River in water year 2016 (October 31, 2015; November 13, 2015; November 17, 2015; December 9, 2015; February 15, 2016).

21 The reconstructed flow of 78,800 cfs was recently reduced to 74,300 cfs, but remains anomalously high compared to event peaks recorded elsewhere in the basin. See Section 2.5.4. 22 This threshold was selected to approximate an annual flood event.

July 2018 34 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Table 5. Flow Frequency for Key USGS Gages in the Snoqualmie River Basin. Middle South North Raging Carnation Snoqualmie Fork Fork Fork River Return Flow Frequency Flow (cfs) Flow (cfs) Flow (cfs) Flow (cfs) Flow (cfs) Flow (cfs) 10-Year Peak Flow 54,400 52,900 26,700 7,000 12,600 3,500 50-Year Peak Flow 76,500 72,800 34,200 9,900 16,900 5,200 100-Year Peak Flow 86,100 81,100 37,100 11,000 18,700 6,000 500-Year Peak Flow 109,300 100,300 43,100 13,700 22,700 7,900 80% 2-Year Mean Daily Flow 20,100 17,600 9,200 2,000 3,600 1,100 2-Year Mean Daily Flow 25,200 22,000 11,500 2,500 4,500 1,400 5-Year Mean Daily Flow 36,500 32,300 15,950 3,700 6,400 1,975 Source: WSE (2016) Based on approved USGS peak flow records through 2013.

All trends were evaluated over the common period of record for the gages (1962 through 2015) using the nonparametric Mann-Kendall trend test. This test was developed by the USGS and is conventionally used for trend evaluation in the field of hydrology (USGS 2006). Computations were performed using a FORTRAN program developed by the USGS (2006) and modified by King County (2010). The strength of identified trends is based on the statistical significance level (p). Selection of a low p value to define significance helps to avoid erroneous identification of a trend when none exists.23 This analysis used the following definitions to qualify the evidence for the trend, consistent with the 2010 analysis by King County (2010):

 p ≥ 0.10 – No reliable statistical evidence against the null hypothesis that a trend exists

 0.05 ≤ p < 0.10 – Weak evidence against the null hypothesis that a trend exists

 0.01 ≤ p < 0.05 – Strong evidence against the null hypothesis that a trend exists

 p ≤ 0.01 – Very strong evidence against the null hypothesis that a trend exists.

In other words, a minimum statistical significance level of 90 percent (p <0.10) was used to qualify the presence of a trend. This definition is broader than the typical 95 percent significance threshold (p <0.05) commonly used in scientific literature.

3.3.1. Results

Results of the gage data analysis for flow trends are presented in Figure 12. The direction of a change in the data is shown by an arrow (up = positive, down = negative). The statistical

23 Known in statistics as a “Type I” error, or a “false positive” result, which can occur due to weakness of the trend, the methodology, or the shortness of the record and the level of statistical significance chosen. “Type II” errors—when the test fails to detect an existing trend—are also possible but more difficult to detect. Both Type I and Type II errors become less likely with a longer record.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 35 significance of the change is shown by cell color; absence of color indicates that a change does not represent a statistically significant trend (i.e., p ≥ 0.10).

Figure 12. Flow Trends Analysis Results for Six Snoqualmie River Gages.

All gage records show an increase in annual maximum daily flow and annual peak discharge; however, none of these trends are statistically significant. Peak annual stage shows an increasing trend at both the Snoqualmie and Carnation gages for the period 1962 to 2015 and for the full period of record. However, this trend is statistically significant for only the full period of record at the Carnation gage.

Trend tests on monthly flow data reveal increasing monthly average flow at all gages for the months of March, April, May, October, and November, with weak to strong statistical

July 2018 36 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

significance in only March, April, and November. Decreasing monthly average flows are indicated at all gages for the months of December, January, February, July, August, and September, with weak to very strong statistical significance for most of the gages in only August and September. At most locations, the trend direction is the same for monthly maximum and monthly average flow. Decreases in monthly maximum flow in August are statistically significant at the Middle Fork Tanner, South Fork Garcia, Snoqualmie, and Carnation gages. Increases in March maximums are supported by weak to strong evidence of trends at the North Fork Snoqualmie Falls, Middle Fork Tanner, and Snoqualmie gages.

All gages show an increasing trend in the number of days exceeding threshold flow values (80 percent of the 2-year mean daily flow, the 2-year mean daily flow, and the 5-year mean daily flow). The Carnation gage shows a statistically significant trend at the 80 percent of the 2-year and at the 5-year flow thresholds. The North Fork Snoqualmie Falls gage shows statistically significant increasing trends in the number of days with flow over the 2-year threshold, and the Middle Fork Snoqualmie near Tanner gage shows a statistically significant increasing trend in the number of days with flow over the 5-year threshold.

3.3.2. Discussion

Trends identified in the extended record are generally consistent with King County’s (2010) analysis, which suggests a shift to generally lower flows in summer, higher flows in winter, increasing annual peak flows, and a relatively consistent annual average flow. With the exception of decreasing summer flows, however, most of these trends are not statistically significant. The statistically significant increase in peak annual stage at the Carnation gage likely reflects a combination of a trend towards increasing annual peak flows (which is not statistically significant) combined with increasing stage at the Carnation gage for a given flow due to sedimentation (see Section 2.5.7 for discussion of sedimentation issues).

Observations by Snoqualmie Valley residents that flooding in the spring and fall appears to be increasing in magnitude are generally corroborated by increases in monthly maximum flow in March, April, May, October, and November, including statistically significant trends in maximum flow in March at three locations and in April at one location. Changes in monthly maximum flow are not statistically significant at any gage except the Raging River in October and May—the months for which resident farmers expressed specific concern.

Trend analysis suggests that flood events above 80 percent of the 2-year mean daily flow threshold are occurring more frequently; however, this change is not a statistically significant trend except at the Carnation gage, where the statistical evidence is weak. The number of days when flow exceeded the threshold at the Carnation gage is plotted in Figure 13, and the corresponding number of days at the Snoqualmie gage (where no significant trend was detected) is shown in Figure 14. During years without a vertical line in Figures 13 and 14, a flood that exceeded 80 percent of the 2-year threshold did not occur. The number of days when flow exceeded the 2-year and 5-year mean daily flow thresholds are indicated with green and red line segments, respectively.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 37

Figure 13. Number of Days Exceeding 80 Percent of the 2-Year (20,100 cfs), the 2-Year (25,200 cfs), and the 5-Year (36,500 cfs) Mean Daily Flow at the Carnation Gage (1962–2015).

July 2018 38 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Figure 14. Number of Days Exceeding 80 Percent of the 2-Year (17,600 cfs), the 2-Year (22,100 cfs), and the 5-Year (32,300 cfs) Mean Daily Flow at the Snoqualmie Gage (1962–2015).

Increasing trends in the number of days exceeding the 2-year and 5-year mean daily flow thresholds indicate that these flood events are occurring more frequently, are lasting longer, or both, consistent with observations of some residents in the valley.

A trend in the timing of early fall and late spring flows was more difficult to analyze, as high-flow events during these periods occur so infrequently. In the history of the Snoqualmie gage, mean daily flow exceeded 80 percent of the 2-year recurrence flow on only 4 days in October, and never in May.

The best indicators of changes in fall and spring flooding are trends in monthly maximum and monthly average daily flow. For these parameters, flow over time trends upward in March, April, and May (spring) as well as in October and November (fall). Statistically significant trends (weak to strong) are found in monthly maximums in March at the North Fork Snoqualmie Falls, Middle Fork Tanner, and Snoqualmie gages (Figure 12). Overall, this finding suggests an upward trend in maximum flow, which would likely correspond to an increase in the frequency and magnitude

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 39 of fall and spring flooding. This finding is consistent with the observations of farmers in the valley.

In comparing the relative timing of the maximum annual flow event, King County (2010) found no significant trend at any of the gages analyzed. Visual examination of the peak flow timing presented in Figure 15, however, suggests higher variability in peak flow timing after about 1980. Higher variability in the timing of the peak annual flow is illustrated by greater distances between plotted points.

July 2018 40 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Year

Source: King County (2010) Figure 15. Timing of Annual (Water Year) Maximum Flow for the Period 1962–2008 at the Key Snoqualmie River Gages.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 41 3.4. FLOOD ARRIVAL TIME

Flood characteristics such as the rate of rise of flow rate on the flood hydrograph and travel time of the peak rate down the valley can be important for early flood warning and flood preparation. For this study, the rate of rise was defined as the increase in flow divided by time from the initial rise of the flood hydrograph to the flood peak. Initial rise was visually determined as the time when the flood hydrograph first began a rapid ascent towards the flood peak, as shown in Figure 16.

90,000 Time at Peak 80,000

70,000

60,000

50,000

40,000

Discharge, cfs Discharge, Initial Rise 30,000

20,000

10,000

0 1/6/2009 1/7/2009 1/8/2009 1/9/2009 1/10/2009 Date

Figure 16. Example Start and Peak of a Flood Hydrograph Used for the Rate-of-Rise Analysis.

The rate of rise for annual peak floods was assessed using short-interval flow data (15-minute time increments), which are available starting in 1988. The calculated rates of rise at the Snoqualmie and Carnation gages for all peak annual flood events between 1988 and 2015 are depicted in Figure 17. Showing considerable scatter, the rate of rise generally ranges from 500 cfs/hour to more than 2,000 cfs/hour, excluding a significant outlier in January 2015 at the Snoqualmie gage (more than 3,000 cfs/hour). Visual comparison reveals neither a consistent upward nor downward trend, further indicating that the January 2015 event is an outlier.

July 2018 42 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

3500 Carnation Snoqualmie 3000 Linear Trend (Carnation) Linear Trend (Snoqualmie) 2500

2000

1500 Rate of Rise cfs/hr of Rise Rate

1000

500

0 1985 1990 1995 2000 2005 2010 2015 Water Year

Figure 17. Rate of Rise of Annual Peak Flood Events at Snoqualmie and Carnation Gages Since 1988.

The travel time between flood peaks as measured at the Snoqualmie and Carnation gages was also compared for each peak annual flood since 1988 (Figure 18). Travel time varied between 4 hours and 19.5 hours, with an average near 12 hours. Many of the most recent annual flood peaks exhibited a shorter travel time but not to the degree of a consistent downward trend; of all the floods evaluated, peak flood travel times in 2006 and 2011 were the longest. Travel time could not be evaluated for events in water years 1991, 2004, or 2007; peak timing was not reported for the Snoqualmie gage in water year 1991, or for the Carnation gage in 2004 or 2007.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 43

Figure 18. Time Between Peaks at Snoqualmie and Carnation Gages for Annual Peak Flood Events at the Snoqualmie Gage Since 1988.

3.5. POTENTIAL CAUSES OF HYDROLOGIC CHANGE

The presence of hydrologic trends in the Snoqualmie River basin is difficult to detect for two primary reasons:

 Extreme events, by definition, are infrequent in nature.

 Streamflow records in the Snoqualmie River basin are relatively short.

Analysis presented in Section 3.3.1 provides evidence of trends towards decreasing flow in the summer as well as consistent, yet not generally statistically significant, trends towards increasing flows in the spring and fall, and increasing annual maximum flows (typically in winter). Attributing these trends to specific drivers is complicated by the influence of changing basin characteristics, including land development and forestry practices. Although studies assessing the effects of land use change show a clear connection between changing land use patterns, runoff, and flood frequency (Wissmar et al. 2004; Matheussen et al. 2000), it has proven more difficult to separate the role of climate variations from changes in land cover (e.g., Bowling et al. 2000).

July 2018 44 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Changes within the Snoqualmie River basin that affect hydrology include new development; land use conversion; forest harvest and regrowth; annual and decadal variation in precipitation and climate patterns; and localized changes to the river morphology including bank erosion, sediment deposition, and sediment management. Each of these factors affects basin hydrology, but it was beyond the scope of this study to quantify those impacts. Potential drivers influencing hydrology and flooding in the Snoqualmie Valley based on current knowledge are described in the following sections. The likely magnitude of impacts is also noted.

3.5.1. Precipitation

Climate projections for the Puget Sound region suggest that warming temperatures will result in decreasing summer precipitation, and a shift from rain-snow–dominant24 to rain-dominant winter precipitation (Mauger et al. 2015). Changing precipitation patterns combined with earlier snowmelt is projected to move streamflow timing earlier in the year. This results in less snowpack, ultimately causing small increases in total annual streamflow but large increases in winter streamflow (Hamlet et al. 2013). Figure 19 (from Hamlet et al. 2013) shows projected changes in the seasonal timing of streamflow in the Snohomish River basin based on global climate modeling. The direction and timing of forecasted changes are consistent with apparent trends in King County precipitation and streamflow data found by King County (2010), although most of the trends identified by King County are not statistically significant.

WSE completed an analysis of precipitation data in King County using daily gage records from 1962 through 2014 for eight long-term precipitation gages in King and Snohomish counties. Trends in monthly total, monthly maximum daily, annual total, and annual maximum daily precipitation were evaluated using the Mann-Kendall test methodology.

24 A rain-snow–dominant basin has a ratio of peak snow water equivalent (SWE) to October to March precipitation of 0.1 to 0.4. (i.e., 10 to 40 percent of the winter precipitation falls as snow). Rain-dominant basins have a ratio of <0.1 (from Hamlet et al. 2013).

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 45

Source: Hamlet et al. (2013) Notes: Projected flow timing in 2040 (orange) and 2080 (red) based on an assumed future of continued economic growth and reliance on a balance of both fossil and non-fossil energy sources (A1B scenario), based on variable infiltration capacity modeling. Figure 19. Projected Changes in Seasonal Timing of Streamflow in the Snohomish River Basin.

Results of trend tests on the extended data record are presented in Figure 20. These results generally match trends described in King County (2010): evidence of downward trends in monthly total and monthly maximum precipitation in the months of July and September, and evidence of upward trends in total and maximum precipitation in the months of March, April, May, October, and November.

Changes in annual total precipitation are mixed (records from five gages show an increase and records from three gages show a decrease), with statistically significant trends at only the Everett and South Fork Tolt precipitation gages. With the exception of the North Fork Snoqualmie Falls gage, annual maximum daily precipitation has increased at the gages analyzed, with statistically significant trends at the SeaTac, Everett, and South Fork Tolt gages.

July 2018 46 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Station Annual Total Precipitation Annual Maximum Precipitation Legend Everett <0.01 Very Strong Trend Monroe 0.01-0.05 Strong Trend Startup 0.05-0.10 Weak Trend Sea-Tac >0.10 No Significant Trend Snoq Falls 0.05-0.10 Weak Trend Buckley 0.01-0.05 Strong Trend Cedar Lake <0.01 Very Strong Trend SF Tolt Monthly Total Station Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Everett Monroe Startup Sea-Tac Snoq Falls Buckley Cedar Lake SF Tolt Monthly Maximum Station Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Everett Monroe Startup Sea-Tac Snoq Falls Buckley Cedar Lake SF Tolt Figure 20. Precipitation Trends Analysis Results for Eight Regional Gages with Long-Term Records.

While changes in precipitation are generally not statistically significant for most months and precipitation gage locations (i.e., p ≥ 0.10), comparison of Figures 9 and 17 reveals that decreasing monthly total and monthly maximum precipitation at all gages in July, August, and September coincide with downward trends in monthly flow found at several key Snoqualmie Valley streamflow gage stations. Precipitation increases in October, November, March, April, and May are evident in the record at most gages, likewise coinciding with upward trends in monthly average and maximum flow during the same months. King County (2010) suggests that patterns of precipitation and flow may be indicative of a shift to earlier seasonal runoff and greater magnitude of extreme precipitation and flow events, which would be consistent with forecast modeling and projections for continued climate change.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 47 Increases in precipitation variability have been shown to increase flood risk in rain- and rain-snow–dominant basins in western Washington (Hamlet and Lettenmaier 2007), and regional climate models have suggested increases in future flood risk resulting from more extreme storms as well as a shift from snow-dominant to rain-dominant precipitation. The latter shift results from increasing temperatures, which cause freezing levels to rise, more precipitation to fall as rain, and more of the basin to contribute runoff during a storm event (Salathé et al. 2014). In rain-dominant and mixed rain- and snow-dominant watersheds such as the Snoqualmie River basin, the heavier rainfall linked to this shift is expected to increase flooding (Mauger et al. 2015).

Most of the largest precipitation events in western Washington over the past 60 years have been associated with narrow bands of concentrated moisture originating in the Pacific Ocean known as atmospheric rivers or “pineapple express” storms (Warner et al. 2012). The January 8 and 9, 2009, storm that brought 3 to 8 inches of rainfall to the lower Snoqualmie River valley was associated with this phenomenon and resulted in record flooding. A study of four large rivers in western Washington found that atmospheric rivers were associated with most large floods in the last 30 years (Neiman et al. 2011). Major rainfall events over western Washington are occurring more frequently (Mass et al. 2011), and some projections suggest that atmospheric river events will increase in intensity and frequency throughout the next century, potentially occurring 4 times as frequently by the 2080s as in the historical time period (Mauger et al. 2015). This increase would have a significant impact on extreme flooding in the Snoqualmie River basin.

While changing precipitation patterns may be linked to changes in flooding, many other factors determine whether an extreme rainfall event will result in extreme flooding. These factors include soil saturation, ground temperature, antecedent snowfall, watershed size, amount of flood storage present in a watershed or reach of river, and the areal extent of rainfall in the watershed. The connection between changing precipitation patterns and changing flooding should become clearer over time, as the precipitation and streamflow periods of record lengthen.

3.5.2. Land Development

Conversion of large areas of land for urban development is correlated to increases in runoff and streamflow. Land development increases impervious surface area and direct runoff of precipitation, while reducing evapotranspiration, infiltration, and flood storage. This combination of changes results in both faster peak flow timing and larger volumes of runoff entering the river. The effect of development on peak flows is generally proportionately greater for low-recurrence interval floods (linked to moderate amounts of precipitation) than for less frequent high-recurrence interval floods (linked to higher than normal amounts of precipitation). These relative effects occur because heavy rainfall in an undeveloped watershed tends to produce saturated soil conditions that reduce or eliminate the capacity of the soil to absorb additional rainfall, causing more of the rain to run off and become streamflow, in the same way that impervious surfaces cause more runoff.

July 2018 48 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Development represents a relatively small portion of the total Snoqualmie River basin area, where primary land uses include forestland and rural residential (Snohomish County Surface Water Management et al. 2015). Snoqualmie River basin land coverage is shown in Table 6, based on the Coastal Change Analysis Program (C-CAP) analysis of Landsat satellite imagery in 1996, 2001, 2006, and 2010 (NOAA 2016). The developed area in the basin increased by 5.6 square miles between 1996 and 2010. While this represents a 23 percent increase in developed land, the increase in developed area in the basin represents only an additional 0.8 percent of the total basin area. Total impervious surface is estimated to have increased by 1.9 square miles (0.3 percent of the basin) over the same period (NOAA 2016).

Table 6. Land Cover in the Snoqualmie River Basin. Area (mi2) Change in Area 1996–2010 Percent of Basin Land Cover 1996 2001 2006 2010 mi2 Area High/Medium Intensity Developed 3.6 3.8 4.8 5.2 1.6 0.2 Low Intensity Developed 15.4 16.9 17.9 18.3 2.9 0.4 Open Space Developed 5.6 5.8 6.4 6.7 1.1 0.2 Total Developed 24.5 26.5 29 30.1 5.6 0.8 Grassland 28.3 28.5 24.2 28.5 0.2 0.0 Agriculture 17.6 18.0 17.8 17.9 0.3 0.0 Forested 490.1 485.4 488.1 473.4 -16.7 -2.4 Scrub/Shrub 90.4 92.8 92.5 100.2 9.8 1.4 Woody Wetland 13.5 13.6 13.6 13.5 0.0 0.0 Emergent Wetland 2.9 2.8 2.8 3.0 0.1 0.0 Barren Land 16.9 16.6 16.2 17.6 0.7 0.1 Open Water 9.8 9.8 9.8 9.7 -0.1 0.0 Source: NOAA (2016)

The largest urban development that occurred in the basin during this period is the Snoqualmie Ridge development, which covers approximately 3.3 square miles. The City of Snoqualmie has applied strict stormwater management regulations to developments to mitigate adverse impacts. Runoff within Phase II of the Snoqualmie Ridge development, for example, is collected in stormwater detention ponds whose outflow to receiving streams is not to exceed predevelopment levels for flow durations up to the 50-year peak flow (Derek Stuart, Northwest Hydraulic Consultants [NHC], personal communication, June 7, 2016). This standard makes it highly unlikely that runoff from newly developed urban areas will substantially affect flood flows in the Snoqualmie River, as long as the detention ponds are maintained to achieve their intended flow control performance.

Another common comment of valley residents is that high-flow bypasses, also known as “direct discharge” or “tightlines,” constructed to carry stormwater runoff directly from developed areas to the Snoqualmie River without significant detention have been the source of increased downstream flooding. According to the environmental impact study for two such tightlines within the Snoqualmie Ridge Phase I development, “no increase in peak flood levels in the

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 49 Snoqualmie River would occur from the high flow bypass…” as the direct hydraulic connection provided by these tightlines allows peak runoff to reach the Snoqualmie River 2.25 to 4.5 hours earlier than it would have in the predevelopment condition, and well before the flood peak on the Snoqualmie River reaches that location (Snoqualmie 1995). A total combined peak flow capacity of 350 cfs carried by the Snoqualmie Ridge Phase I tightlines represents approximately 0.4 percent of the 100-year flood peak at Snoqualmie (approximately 81,100 cfs), or 0.7 percent of the 10-year flood peak (approximately 53,000 cfs).

This type of stormwater conveyance system can be allowed under a Washington State Department of Ecology (Ecology) “exemption.” The exemption restricts the use of tightlines to areas in close proximity to (within 0.5 miles of) the receiving river, and to rivers with a drainage area over 100 square miles, where the impact of the direct runoff will be relatively minor (Ecology 2010). After some investigation, a January 2008 Snoqualmie Flood-Farm Task Force report concluded that the “tightlines from the Urban Planned Developments appear not to be the issue they were perceived to be based on available data” (King County 2008); however, they continue to be a point of intense scrutiny.

3.5.3. Timber Harvest

Forestry represents the primary land use designation within the Snoqualmie River basin (Snohomish County Surface Water Management et al. 2015). Historical timber harvest data specific to the Snoqualmie River basin were not found; however, the Washington State Department of Natural Resources (WDNR) has reported yearly timber harvest for King County since 1965. As shown in Figure 21, reported timber harvest in King County peaked in the early 1970s, and has generally declined since 1987. Despite fairly consistent total forest cover (accounting for both harvest and regrowth) in the basin between 1996 and 2006 (Table 6), total forest cover decreased by approximately 16.7 square miles (2.4 percent of the basin area) between 1996 and 2010. Most of loss is due to conversion of forested area to grassland, development, scrubland, and barren land (NOAA 2016).

July 2018 50 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

600,000

500,000

400,000

300,000

200,000

100,000

0

Timber Harvest, Thousand Board Feet Scribner rule) Scribner Feet Board Thousand Harvest, Timber

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year Source: WDNR (2015) Figure 21. Reported Timber Harvest in King County 1965–2014.

Direct impacts of timber harvest activities on streamflow are difficult to measure because of the interplay among many variables (Bowling et al. 2000). A recent literature review (Perry et. al. 2016) found no consensus on the nature of the relationship between forest harvesting and significant increases in peak flows in large drainage basins or during extreme flood events. Although it is generally agreed that peak runoff increases following timber harvest, there is less certainty about impacts during large floods given the rarity of those events and the nature of the flood-producing rainfall. In general, conclusive evidence that extreme floods are significantly affected by forest practices is not found in the literature (Perry et. al. 2016).

Streamflow may be altered following timber harvest by decreased evapotranspiration and increased soil moisture; increased snowmelt during rain-on-snow events; and decreased travel time for subsurface flow intercepted by logging roads, ditches, and soil compacted during logging. Studies on post-harvest recovery suggest that changes in peak flows may persist for 10 years and, in some cases, remain detectable for up to 30 years (Perry et. al. 2016). Given the King County timber harvest totals shown in Figure 21, the heaviest impacts of timber harvest on streamflow would have occurred in the 1970s and 1980s, and potentially into the 1990s, with harvest consistently lower and recovery rates thus fairly stabilized since that time. An uptick in timber harvest totals in 2014 reflects the largest harvest in more than 20 years, but still below pre-1990 levels. It is not clear how much of this harvest occurred within the Snoqualmie basin. C-CAP land cover data for the Snoqualmie River basin suggest a spike in timber harvest between 2006 and 2010 (the last year for which C-CAP data are available). Although a direct correlation

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 51 to flood impact cannot be drawn, the 2006 through 2010 loss of 14.7 square miles of forest cover corresponds to a period covering two of the largest flood events in the basin (water years 2007 and 2009; see Table 3). These flood events were not unique to the Snoqualmie River basin, however, and resulted from large winter storms that brought extreme flooding to much of western Washington (e.g., Mastin et. al. 2010).

3.5.4. Sediment

King County monitors sediment levels in the Snoqualmie River to evaluate the effects of sediment on flooding and to inform sediment management decisions. This monitoring program targets two reaches along the Snoqualmie River, one below Carnation and one below Fall City. These reaches are immediately downstream of the Tolt and Raging rivers, which, along with Tokul Creek, represent the major coarse sediment inputs to the Snoqualmie River below Snoqualmie Falls (Booth et al. 1991).

Comparison of available survey data suggests that minor to moderate channel changes occurred between 1997 and 2004. Both increases and decreases in average channel bed elevation were noted within each reach, with no identified trend in sedimentation (King County 2017). Between 2004 and 2011, deposition caused elevation of the channel bed to increase by an average of 0.7 foot in the Fall City reach and an average of 0.6 foot in the Carnation reach. Between 2011 and 2015, however, the Fall City reach experienced an average decrease in channel bed elevation of 0.3 foot (2015 data are not available for the Carnation reach). The sedimentation within these reaches between 2004 and 2011 was likely tied to extreme flood events (one each in November 2006 and January 2009), which would have caused increased sediment movement from the tributaries into the main stem. The reduction in bed level within the Fall City reach between 2011 and 2015 may indicate that this sediment is continuing to work its way downstream.

King County evaluated longer term sediment trends by comparing recent data to US Army Corps of Engineers channel survey data from 1965. Although the 1965 survey sections are too sparse for a comprehensive comparison, bed levels in 1965 are sufficiently similar to those measured recently to suggest little evidence of pervasive changes over the last 50 years (King County, 2017). This finding indicates that the 2004 through 2011 increases in bed levels are due to recent large floods rather than a reach-wide trend in sediment aggradation.

The potential magnitude of water surface impacts resulting from reach-wide sediment deposition was quantified by updating the existing Snoqualmie River hydraulic model to represent a hypothetical 2-foot increase in bed levels25 in the Carnation and Fall City reaches. Bed elevations were increased at each cross section from RM 18.7 to RM 23.88 and from RM 30.84 to RM 34.85, as shown in Figure 22. The model was then run to simulate the 10-year

25 Booth et al. (1991), estimated bed aggradation rates of between 1 and 2 feet per century within the subject reaches; however, recent comparison of current and historical bed survey has shown little evidence of pervasive changes in bed level during the last half century (King County 2017).

July 2018 52 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

and 100-year flood events. Results show a maximum increase in water surface of 0.8 foot during the 10-year simulation and of 0.4 foot during the 100-year flood simulation, with reach-wide average increases of less than 0.2 foot (2.4 inches) in both events.

Elevation (ft) Elevation

Legend Existing model cross section With 2 feet of sediment

Figure 22 Example Cross-Section Adjustment to Account for 2 Feet of Sediment Deposition.

Although King County’s sediment monitoring program has not found evidence of systematic reach-wide sediment aggradation in the Carnation or Fall City monitoring reaches, short-term and localized changes can still affect water surface elevation and flood impacts. Specific gage analysis26 at the Carnation gage shows that the stage-flow relationship at the gage has changed over time, including recent increases and decreases consistent with observed bed level increases and decreases in the Carnation reach (King County 2017). Overall, stage at flows above 10,000 cfs has increased by approximately 1 foot at the Carnation gage since the 1960s, as shown in Figure 8, which corresponds to the time when dredging was last completed in the surrounding reach (King County 2017). Figure 23, from the King County (2017) report, shows average bed elevations measured near the gage (Stossel bridge) between 1965 and 2011,

26 Specific gage analysis was based on calculated gage shifts through time at the Carnation gage (12149000). A gage shift is the difference between the gage height measured at the time of each historical discharge measurement and the gage height that would be predicted based on the most recent rating curve.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 53 including recent increases in bed elevations within a mile upstream and downstream of the gage.

Source: King County (2017) Figure 23 Average Bed Elevations of Snoqualmie River Near Snoqualmie Gage Based on River Surveys in 1965, 1997, 2004, and 2011.

Continued monitoring is recommended to better understand patterns of sediment deposition and to inform potential future flood risk reduction actions, which could include dredging, levee setbacks, home elevations, acquisition of affected properties, or some combination thereof.

3.5.5. Large Capital Projects

Large capital projects constructed in the river and its floodplain represent evident and purposeful changes to river conditions. These projects typically undergo substantial scrutiny and hydraulic design analysis to predict potential effects on flooding. The following discussion highlights recent projects in the river and its floodplain, and the associated hydraulic analyses to assess effects on flooding.

July 2018 54 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Flood impacts were recently simulated for two projects at Snoqualmie Falls and the upstream river reach (WSE and Herrera 2016). The combined effect of the PSE and 205 projects was an estimated cumulative reduction in flood storage upstream of the Falls of 2,500 acre-feet during the 100-year flood, sending approximately 1,200 cfs of additional flow downstream at the peak of the 100-year flood event. Pre- and post-project hydraulic simulations indicated that the two projects would result in a maximum increase in peak stage of approximately 0.1 foot downstream of the projects, a maximum increase in peak stage of approximately 0.08 foot at Carnation, and a maximum decrease of 1.5 feet at the city of Snoqualmie. The simulation also predicted that peak flood stage at Carnation would occur 20 minutes earlier relative to pre-project conditions.

Under the King County Farm Pad program, fill has been placed within the floodplain of the Snoqualmie River for construction of approximately 30 elevated farm pads for relief to livestock and farm equipment during large flood events. Requests to construct farm pads are evaluated on a case-by-case basis. Each evaluation includes a review of potential alternatives and means of mitigating flood risk without placing additional fill in the floodplain. Pad designs are subjected to a zero-rise evaluation, including hydraulic modeling to ensure that the proposed pad size and placement are compliant with zero-rise requirements in the King County Code. Zero-rise modeling is performed using unsteady-state methods so that flood impact assessments can account for floodplain storage loss due to farm pad construction.

In 2014, King County completed its Upper Carlson project, removing 1,600 feet of levee and revetment along the bank of the Snoqualmie River near the Fall City Natural Area to reconnect the river with its historical floodplain. The zero-rise analysis completed for this project demonstrated that the project would not impact 100-year flood levels, and additional hydraulic modeling and sediment transport analysis of lower flood flows indicated that project impacts were not likely to extend beyond isolated areas in and near the channel (King County 2016a). At present, King County is finishing its review of flood impacts following the January 2015 flood event, and has recommended continued monitoring to more fully understand potential flood impacts and guide future actions in the Upper Carlson project reach.

The Chinook Bend restoration project removed 1,100 linear feet of levee and included both excavation and fill within the floodplain. Zero-rise and compensatory storage analyses were completed to demonstrate that the project would not increase flood levels; levee removal studies showed that slight reductions in water surface elevation would result upstream of the project, near the Stossel bridge and Carnation gage (Mansfield 2009). Modeled impacts of the levee removal ranged from a water surface elevation decrease of up to 0.03 foot during the 100-year event to more than 0.1 foot during the 2-year event. While this change would impact water levels at the Carnation gage, noted issues with the Carnation gage (discussed in Section 2.5) likely have had much greater impacts on the accuracy of the gage.

Additional minor projects completed in and near the river channel include the McElhoe Pearson Restoration project near Carnation and the Snoqualmie channel maintenance project at RM 31. The McElhoe Pearson project, which removed a portion of an existing levee and excavated an outlet channel to the Snoqualmie River, provides juvenile salmon access to an existing wetland

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 55 and thus increases habitat for rearing and flood refuge. A roadside berm was also constructed to maintain the pre-project level of flood protection for the adjacent road and properties. The Snoqualmie channel maintenance project removed wood pilings from the river channel and repositioned natural wood that had racked on the upstream pilings. Available project documentation does not indicate that any analysis of flood impacts was undertaken.

Except for the PSE and 205 projects, there is little evidence that projects constructed in the river and its floodplain have had a measurable effect on valley flooding outside of the local project vicinity. The PSE and 205 projects are estimated to have increased peak flows, caused slightly elevated peak flood levels, and caused slightly decreased peak flood travel time for areas downstream from the Falls.

3.6. SUMMARY AND CONCLUSIONS

Trends in precipitation and flow data for the Snoqualmie River basin suggest that there is less runoff in summer, more runoff in spring and fall, and increased extreme flood magnitude. While few of the apparent trends identified in this investigation are considered to be statistically significant, they are generally consistent with changes in precipitation and flow projected by current research on climate change (Mauger et al. 2015). They also suggest a continued shift in flood timing, potentially resulting in yet lower flows in summer, higher flows during annual peak floods, and higher flows in fall and spring.

Human activities including land development and forest harvest are known to affect flooding by increasing the magnitude and peak flow timing of runoff. Analysis of peak flood characteristics at the Snoqualmie River streamflow gages near Snoqualmie and Carnation did not yield statistically significant evidence of trends towards faster flood travel time or faster flood peaks. While development has increased impervious area in the basin, it makes up a minor component of the basin’s total drainage area. Furthermore, increasingly stringent stormwater management standards, such as those applied to the Snoqualmie Ridge development, have helped mitigate the hydrologic impact of large urban developments. In addition, the impact of forestry practices has likely declined with the decades-long decrease in King County timber harvest.

Long-term trends in sediment deposition are not evident based on available surveys; however, King County is continuing to monitor sediment levels in reaches near Carnation and Fall City, where sediment inputs from the Tolt and Raging rivers are known to occur. Recent increases in channel bed elevations were noted at some cross sections within these reaches between 2004 and 2011; however, a 2015 survey for the Fall City reach suggests that levels have been decreasing since 2011. The hypothetical impacts of 2.0 feet of persistent sediment deposition throughout these reaches was estimated through hydraulic modeling to cause a 0.0 to 0.4 foot of rise in water surface during the 100-year event, and 0.0 to 0.8 foot of rise during the 10-year event, with reach-wide average increases of less than 0.2 foot (2.4 inches) in both events. The greatest rise was seen upstream of the Stossel bridge, Tolt bridge, and the SR 203 bridge at Fall City. Sedimentation has likely impacted stages near the Carnation gage, and reduced the flow rate at which overbank flooding begins to occur at this location.

July 2018 56 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Major projects at Snoqualmie Falls (the PSE and 205 projects) increased peak flow magnitudes downstream of Snoqualmie Falls, and slightly decreased flood travel time between the Falls and Carnation. Impacts from the Farm Pad program, the Upper Carlson project, the McElhoe Pearson project, Chinook Bend project, and channel maintenance at RM 31 have been minimal (based on no-rise analysis) or minor (based on project nature and scale). Project-specific impacts (or lack thereof) in the river and its floodplain, and associated changes in river flooding, may become clearer as monitoring continues.

Although human activities may explain some of the variability in flood characteristics observed by valley residents, the hydrologic trends investigation described in this section did not find overall statistical evidence to support flooding trends. Flood-to-flood variation is most likely explained by the interplay of multiple factors in addition to human activities, including changing patterns of precipitation due to both natural variability and climate change, and variations in ambient soil saturation, ground temperature, snowfall, storm duration, and rainfall extent in the basin. As more data accumulate, emerging trends in hydrologic conditions and flooding may become clearer.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 57

4. ANATOMY OF A FLOOD

WSE examined data from three recent flood events in the Snoqualmie River basin to prepare a detailed description of flooding characteristics in the lower Snoqualmie River valley. Flood events on January 7 and 8, 2009; January 5 and 6, 2015; and December 8 and 9, 2015, were selected for this comparison in coordination with King County and the Snoqualmie Valley Preservation Alliance. Flood hydrographs and high water marks (HWMs) from the events were also compared to simulated 10-year and 100-year flow data from the Snoqualmie-Skykomish unsteady hydraulic model developed with the US Army Corps of Engineers HEC-RAS program, which is used as best available data for the lower Snoqualmie River valley.

The goal of this work was to provide examples demonstrating the variability among floods in their antecedent conditions, storm development, and flood progression, and the effects of these variations on flood impacts. The analysis included a comparison of impacts based on USGS gaging records, road closure data, and HWMs collected following the events; it did not include a detailed comparison of localized effects (e.g., whether a particular property was under water) or flooding impacts on tributaries to the main stem Snoqualmie River.

4.1. SUMMARY OF FINDINGS

Variations in peak flood magnitude and volume reported at the USGS Snoqualmie River gages near Snoqualmie and Carnation during the January 2009, January 2015, and December 2015 flood events correlate well with variations in precipitation patterns and flood development in the tributary sub-basins. Of the three floods evaluated, the most extreme flooding in the lower Snoqualmie River valley occurred during the January 2009 flood event following intense and widespread rainfall, including significant rain-on-snow. The January 2009 flood peak at Carnation (82,900 cfs) was exacerbated by the timing of flooding on the Tolt River near Carnation, which peaked at about the same time as the peak on the Snoqualmie River near Carnation and caused backwater in the Snoqualmie River upstream of the confluence of the two rivers.

The January 2015 flood event resulted from a relatively short and intense period of rainfall that led to a quick rise in flood stage. Flow increases were even more dramatic because of the dry conditions and low base flow that preceded the event. Because the January 2015 event had a relatively small flow volume, its peak was attenuated substantially by the time it reached Carnation, where the reported flood peak flow was 53,900 cfs.

The December 2015 flood event followed many days of steady rainfall that caused elevated base flows in the Snoqualmie River at the start of the flood. Although the flood peak at the Snoqualmie gage was smaller than that of the January 2015 flood, the duration and total flood volume was much larger. The flood peak persisted downstream through the valley and increased

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 59 in magnitude by the time it reached the gage near Carnation, where the peak flow reached 56,200 cfs. The December 2015 flood was also unique in that upstream tributary gages recorded two distinct flood peaks on December 8 and 9, which transformed into a single flood peak as the flood wave moved downstream to the Snoqualmie and Carnation gages.

The extent and duration of impacts during these three flood events correlate to both peak magnitude and total flood duration. Road closures, measured as total road miles closed and duration of closures, were greatest during the January 2009 flood and least during the January 2015 flood. In general, HWMs from the three floods are similar to water surface elevations predicted by hydraulic modeling. HWMs from the January 2009 event are close to simulated water surface elevations in the 100-year Snoqualmie-Skykomish HEC-RAS model; the January 2015 HWMs are close to or slightly below the simulated water surface elevations in the 10-year flood event; and HWMs from the December 2015 event are close to or slightly above the simulated water surface elevations in the 10-year flood event.

4.2. BACKGROUND

The extents of the Snoqualmie River basin and major sub-basins are delineated in Figure 24. Figure 24 also shows the location of the streamflow gages used in this analysis.

The descriptions of flood anatomy are based on review of the following data for the flood events of January 7 and 8, 2009; January 5 and 6, 2015; and December 8 and 9, 2015:

 Streamflow records from USGS gaging stations at North Fork Snoqualmie River near Snoqualmie Falls, Middle Fork Snoqualmie River near Tanner, South Fork Snoqualmie River above Alice Springs near Garcia, Snoqualmie River near Snoqualmie, Snoqualmie River near Carnation, and Tolt River near Carnation; locations are shown in Figure 24.

 Flooding observations provided by a number of valley residents.

 HWM surveys.

 Precipitation records from National Weather Service (NWS) gages; the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS); and the Natural Resources Conservation Service Snow Telemetry (Sno-Tel) network.

 Quantitative precipitation estimates (QPEs) from NOAA’s Advanced Hydraulic Prediction Service.

 King County flood records including road closure information, HWMs, and flood warning center activation.

 Storm summaries and monthly weather newsletters from the Office of the Washington State Climatologist.

July 2018 60 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Tolt River near Carnation 12148500 Snoqualmie River near Carnation 12149000

North Fork Snoqualmie River near Snoqualmie Falls 12142000

Snoqualmie River near Snoqualmie 12144500

Middle Fork Snoqualmie River near Tanner 12141300

South Fork Snoqualmie River above Alice Springs near Garcia 12143400

Figure 24. Snoqualmie River Basin, Sub-basins, and Key USGS Gaging Locations.

4.3. FLOOD EVENT DESCRIPTIONS

Floods in the Snoqualmie River basin typically occur between November and February and are attributable to heavy rain or rain-on-snow events. Extreme floods in the basin are always driven by atmospheric river (also known as “pineapple express”) storm systems, which funnel tropical moisture to western Washington and result in warm temperatures and heavy rainfall. Flood events in January 2009, January 2015, and December 2015 were among the largest in the historical record in the Snoqualmie River basin. Each ranks in the top 10 peak annual flows recorded at the USGS Snoqualmie River streamflow gages near Snoqualmie and near Carnation (see Table 7), but vary in flood development and flood impacts. Flood peaks that occurred in January 2009, January 2015, and December 2015 are highlighted in Table 7. The characteristics of these floods are described below.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 61 Table 7. Ten Highest Observed Annual Peak Flows at Key Snoqualmie River Gages. Snoqualmie Near Snoqualmie Near Middle Fork South Fork North Fork Carnation Snoqualmie Tanner Garcia Snoqualmie Falls Water Water Flow Water Flow Water Flow Water Flow Rank Year Flow (cfs) Year (cfs) Year (cfs) Year (cfs) Year (cfs) 1 2009 82,900 1991 74,300a 2007 31,700 2007 8,910 2009 17,100 2 2007 71,800 2009 60,700 2009 31,200 1987 8,450 1932 15,800 3 1991 65,200 1987 58,100 1978 30,200 1991 8,000 1996 14,500 4 1996 61,600 2007 55,000 1991 30,100 1996 7,450 2007 14,200 5 1932 59,500 1978 53,800 1987 28,900 2009 7,440 1960 13,700 6 1933 59,000 1976 51,800 1996 27,400 2016 7,400b 1935 13,400 7 1987 57,100 1996 51,700 2016 27,300b 1963 7,090 1945 13,400 8 2016 56,200b 2015 50,100 1975 24,900 1989 6,380 1951 13,200 9 2015 53,900 2016 49,500b 1990 24,400 1978 6,370 1987 12,600 10 1951 52,200 1975 48,100 1976 23,700 1976 6,190 1955 12,200 Note: Middle Fork gage malfunctioned in January 2015 event. Green indicates January 2009 event, red indicates January 2015 event, and blue indicates December 2015 event. Note that the December 2015 event occurred in water year 2016. a Value recently revised downward from 78,800 cfs by the USGS. b Preliminary value. Approved peak flow records for water year 2016 (October 1, 2015, to September 30, 2016) have not been published officially by the USGS.

4.3.1. January 2009

This event was driven by an atmospheric river with a narrow band of warm, moist air that brought heavy rainfall to much of western Washington between January 5 and January 7. Heavy rainfall and warming temperatures in the Snoqualmie River basin melted lowland snow that had accumulated during December storms, adding to the total runoff. The resulting flood was the largest in the historical record at the Carnation gage (86-year record) and the North Fork Snoqualmie gage near Snoqualmie Falls (85-year record), the second largest at the Snoqualmie gage (58-year record) and at the Middle Fork Tanner gage (54-year record), and the fifth largest at the South Fork Garcia gage (54-year record) (see Table 7). Flood impacts in King County were severe, with millions of dollars in reported damage to homes, businesses, and schools (Catchpole 2009). Statewide flood impacts from the January 2009 storm included evacuations along the Carbon and Puyallup rivers, closure of Interstate 5 in Chehalis, and a presidential disaster declaration for nine counties in Washington, including King County (Mastin, et. al. 2010).

The January 2009 flood hydrograph for each gage used in this analysis is shown in Figure 25. The Sum of the Forks hydrograph represents the sum of the Middle Fork Tanner, North Fork Snoqualmie Falls, and South Fork Garcia gage hydrographs. Flood peak progression can be tracked downstream with the individual fork gages peaking midday on January 7, the Snoqualmie gage peaking late on the 7th, and the Carnation gage peaking in the early morning on January 8. The USGS gage on the Tolt River near Carnation remained near 10,000 cfs for over 24 hours before peaking at 13,800 cfs almost coincident with the timing of the peak of the

July 2018 62 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

Snoqualmie at Carnation gage. This phenomenon contributed to the large increase in flood volume between Snoqualmie and Carnation, and resulted in increased backwater flooding observed by residents upstream of the Tolt River and Snoqualmie River confluence (e.g., Geary Eppley, personal communication, July 6, 2016). The Tolt River generally peaks much earlier during major flood events than the Snoqualmie River near Carnation, but a late spike in flow from the Tolt River (see Figure 25) coincided closely with the peak of the event at Carnation.

Figure 25. Observed Hydrographs at Key Gages for January 2009 Flood Event.

There has been much conjecture as to the cause of the flow spike on the Tolt River and one reason often cited is operations at the South Fork Tolt reservoir. However, operational changes could not possibly have resulted in the observed downstream flow spike on the Tolt River because the South Fork Tolt dam is not equipped with a controlled spillway; flood flows are passed via an uncontrolled overflow spillway (WSE 2014). Furthermore, the spike in flow seen on the Tolt River near Carnation can be traced back upstream to the North Fork Tolt River gage, and similar spikes in flow are also seen in the records at the Middle Fork Tanner and North Fork Snoqualmie gages (see blue vertical line in Figure 25), suggesting that the spike was due to a late increase in precipitation intensity, as was recorded at the NOAA Snoqualmie Falls rain gage (NOAA 2009).

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 63 4.3.2. January 2015

This atmospheric river event brought significant rain and warming temperatures to the Snoqualmie River basin. The heaviest rainfall occurred over a relatively short period, beginning on the morning of January 4 and continuing into the early hours of January 5. As shown in red shading in Table 7, the resulting flood was the ninth largest peak annual flood in the historical record at the USGS streamflow gage near Carnation and the eighth largest at the gage near Snoqualmie. The event was not in the top 10 annual peak floods at the South Fork Garcia or North Fork Snoqualmie Falls gages, and the Middle Fork Tanner gage failed very early in the event, missing most of the flood hydrograph, including the peak.

Flood hydrographs for this event at various gages are shown in Figure 26. The storm followed a relatively dry period with lower than average streamflows, making the quick rise of the flood event appear even more dramatic. Residents along the Fall City reach of the Snoqualmie River downstream of Fall City commented on the quick rise and fast flow velocities associated with the event (King County 2016a). Figure 17 in Section 3.4 illustrates the unusually rapid rate of rise of this event compared to other historical floods. Unlike the January 2009 event, the timing of peak flows on the Tolt River was more or less in line with that measured at the South Fork and North Fork gages, and peak Tolt River flows reached the Snoqualmie River well before the flood peak at the Snoqualmie near Carnation gage. The Middle Fork gage malfunctioned during the event, and no data are shown past January 4.

Note: The Middle Fork gage (MF Snoq) malfunctioned during the event, and no data is reported for that location after January 4. Figure 26. Observed Hydrographs at Key USGS Gages for January 2015 Flood Event.

July 2018 64 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County

4.3.3. December 2015

This atmospheric river event brought warm temperatures and consistent heavy rain to the Snoqualmie River basin between December 6 and 9, 2015. The storm followed a period of wet weather, including three significant flood events in October and November that had saturated soils and increased base flows in the Snoqualmie River. The flood included two distinct peaks at the North Fork Snoqualmie Falls, Middle Fork Tanner, and South Fork Garcia gages on December 8 and 9 (Figure 27). As the flood progressed downstream, flow attenuation and timing differences in the tributary inflows transformed the two upstream peaks into a single peak, which arrived at the Snoqualmie gage late on the morning of December 9 and at the Carnation gage by mid-afternoon of the same day. Preliminary flood discharge values indicate that the December 2015 event was the eighth largest peak annual flood on record at the Carnation gage, the ninth largest at the Snoqualmie gage, the seventh largest at the Middle Fork Tanner gage, and the sixth largest at the South Fork Garcia gage. The event was not in the top 10 peak annual flood events at the North Fork Snoqualmie Falls gage, and was not a significant flood event on the Tolt River. Flood hydrographs are depicted in Figure 27.

Figure 27. Observed Hydrographs at Key USGS Gages for December 2015 Flood Event.

4.4. STORM COMPARISON

Precipitation totals for the three flood events are shown in Figure 28 and Table 8. Figure 28 depicts 48-hour precipitation totals based on NOAA QPE data. Table 8 shows 48-hour precipitation totals and the change in snow water equivalent (SWE) at six gage locations within or proximate to the Snoqualmie River basin. This table includes data for three Sno-Tel sites; a

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 65 North Bend CoCoRaHS station; and NWS stations at Snoqualmie Falls, SeaTac Airport, and the South Fork Tolt Reservoir. The location of the stations listed on Table 8 are shown in Figure 28.

Figure 28. 48-Hour Quantitative Precipitation Estimates in the Snoqualmie River Basin for January 2009, January 2015, and December 2015 Flood Events.

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Table 8. 48-Hour Precipitation and Snow Accumulation Related to the January 2009, January 2015, and December 2015 Flood Events. January 7–8, 2009 January 5–6, 2015 December 8–9, 2015 Elevation Precip. Δ SWE Precip. Δ SWE Precip. Δ SWE Location (data source) (feet) (inches) (inches) (inches) (inches) (inches) (inches) Ollalie Meadows (Sno-Tel) 4,030 17.0 4.6 6.2 -0.1 8.9 0.8 Alpine Meadows (Sno-Tel) 3,500 16.7 2.3 9.7 0.1 7.3 -1.0 Skookum Creek (Sno-Tel) 3,300 19.0 0.0 10.3 -1.1 8.4 -0.7 South Fork Tolt Res (NWS) 2,000 6.9 -5.0a 4.8 0.0 5.1 0.0 North Bend (CoCoRaHS) 525 10.6 0.0 3.5 0.0 7.0 0.0 SeaTac (NWS) 370 2.3 0.0 0.7 0.0 2.7 0.0 Snoqualmie Falls (NWS) 134 2.8 0.0 1.9 0.0 4.1 0.0 Note: Station totals are generally from 7 am to 7 am. a Value represents snow depth not SWE. Precip. = precipitation Δ SWE = change in snow water equivalent

The January 2009 event included large quantities of widespread and persistent precipitation throughout the basin. The 48-hour precipitation totals ranged from 2.3 inches in the lower watershed to as much as 19 inches at higher elevations. SWE totals at two of the three Sno-Tel sites indicate that a portion of the total precipitation was held in the snowpack at higher elevations. The South Fork Tolt Reservoir precipitation gage is the only location showing 48-hour snowmelt between January 7 and 8 (Table 8); although the data are not shown in Table 8, snowmelt also occurred elsewhere, but earlier (January 5 and 6), at the gages at North Bend (9 inches), Snoqualmie Falls (1 inch), and SeaTac (3.5 inches). It is likely that a significant amount of snow also melted at intermediate elevations during the event, although specific records are not available.

QPE totals during the January 2015 event (Figure 28) show that 48-hour precipitation varied from about 2 inches in the lower watershed to 8 inches at higher elevations. Rainfall was most intense over the upper basin areas including the Middle, South, and North Fork Snoqualmie River, and the Tolt River basin, and was more moderate over the lower basin below the forks. CoCoRaHS and NWS gage observations show that there was no snowpack to melt during the event at lower elevations (site elevations between 134 and 2,000 feet), but Sno-Tel sites indicate some higher elevation snowmelt during the event. Although 48-hour rainfall is used for storm comparisons in Table 8 and Figure 28, most of the rainfall during the January 2015 event actually fell during a 24-hour period.

QPE 48-hour precipitation totals during the December 2015 event ranged from 3.5 inches in the lower watershed to more than 10 inches at higher elevations. Some snowmelt likely added to the runoff totals, as indicated by reduced snowpack (0.7 to 1.0 inches) at two of the three Sno-Tel sites. Precipitation was most intense over the Middle, South, and North Fork basins, and over the upper Tolt River basin, with less intense but still significant rainfall over the lower Snoqualmie River valley. Flooding due to rain during the 48-hour period between December 8

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 67 and 9 was exacerbated by significant rainfall during the preceding 48 hours (i.e., December 6 and 7), which increased base flow levels (as shown in Figure 27).

4.5. COMPARISON OF FLOOD CHARACTERISTICS

Four-day hydrographs for each of the flood events are shown in Figure 29 for the Snoqualmie gage and Figure 30 for the Carnation gage. The simulated 10-year and 100-year flood hydrographs from the Snoqualmie-Skykomish HEC-RAS hydraulic model are included for comparison. All of the hydrographs have been adjusted to pass through 20,000 cfs at time 24 hours, allowing an at-gage comparison of conditions preceding the flood, flood rise, time to flood peak, and cumulative flood volume (represented by the total area under the hydrograph curve). Peak flow, antecedent flow, and the time between peaks at the Snoqualmie and Carnation gages are also summarized in Table 9. Total flood volume is calculated as the cumulative flow above 20,000 cfs, which corresponds to a moderate flooding threshold (flood phase 3) at the Sum of the Forks (flood phases are described at the end of this section).

The Snoqualmie-Skykomish model is a one-dimensional (1D) unsteady flow HEC-RAS model developed for floodplain mapping as part of the 2006 FEMA flood insurance study (FIS), then altered to include farm pads that have been permitted and constructed along the Snoqualmie River over the past few years. The flow hydrographs in the FIS model were developed from historical flow data and adjusted such that simulated 10- and 100-year flood stages throughout the valley matched stage frequency analyses based on simulations of historical events (NHC 2006). The simulated 10- and 100-year flood provide a useful benchmark for flood comparison; however, the simulated events were developed for floodplain mapping purposes and do not represent actual flood events. The shape of the 100-year hydrographs are based on the 1990 flood event, which was determined to most closely match 100-year flood stages within the study area, and the hydrographs for the 10-year event are based on the December 1977 flood (NHC 2006); however, each of the event hydrographs were further modified to reproduce corresponding stage frequencies. In the discussion below, the January 2009 flood is compared to the simulated 100-year event, and the January 2015 and December 2015 floods are compared to the simulated 10-year event.

The 2009 event hydrograph was more drawn out and less flashy than the simulated 100-year event. Total flood volume in the January 2009 event was greater than the simulated 100-year flood hydrograph volume at the Snoqualmie gage, but very similar to the simulated 100-year flood hydrograph volume at the Carnation gage. Peak flow at both locations during the January 2009 flood was lower than that for the corresponding simulated 100-year flood. The observed lag time between the flood peaks at the Snoqualmie and Carnation gages in the January 2009 event was 6 hours and 15 minutes compared to 4 hours in the modeled 100-year event.

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Figure 29. Comparison of Flood Hydrographs at Snoqualmie River Near Snoqualmie Gage Location.

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Figure 30. Comparison of Flood Hydrographs at Snoqualmie River Near Carnation Gage Location.

Table 9. Flood Characteristics at Snoqualmie and Carnation Gage Locations. Event January January December Category Location 2009 100-Year 2015 2015 10-Year Flow at Time “0” (cfs) Snoqualmie 3,260 21,780 1,700 5,240 2,930 Carnation 5,330 25,850 2,890 6,950 6,200 Flood Volumea (acre-ft) Snoqualmie 179,000 162,000 74,000 96,000 99,000 Carnation 304,000 303,500 97,000 131,000 147,000 Peak Flow (cfs) Snoqualmie 60,700 79,100 50,100 49,500 51,700 Carnation 82,900 92,000 53,900 56,200 57,900 Lag Time (hr:min) Snoqualmie to 6:15 4:0 9:0 4:15 5:0 Carnation a Total flood volume that occurred while the flow rate was above 20,000 cfs.

The January 2015 event rose rapidly to its peak and then quickly began to recede. Although its peak flood magnitude was similar to that of the simulated 10-year event at the Snoqualmie gage, its total flood volume was about 75 percent of the simulated 10-year flood at that location. At the Carnation gage, the January 2015 flood peak was smaller than the 10-year event, and the total flood volume was 66 percent of the simulated 10-year event. The observed lag

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time between the peak flow at the Snoqualmie and Carnation gages in the January 2015 event was 9 hours, compared to 5 hours in the simulated 10-year event.

Relative to the January 2015 event, the December 2015 flood peak was slightly lower at the Snoqualmie gage, but higher at the Carnation gage. The December 2015 event was generally more similar to the simulated 10-year event. Flood volumes at the Snoqualmie and Carnation gages were 97 percent and 89 percent of the modeled 10-year event, respectively, and peak lag time between the gages was 4 hours and 15 minutes compared to 5 hours in the 10-year modeled event.

Available HWMs for the three events are compared to modeled 100-year (for January 2009) and 10-year (for January 2015 and December 2015) water surface elevations in Figures 31 through 33. HWMs were marked by valley residents and County staff, and were surveyed by the County in April 2016. The graphs report the difference, in feet, between the surveyed HWM and the modeled stage, specifically the surveyed HWM elevation minus the modeled maximum water surface elevation at the same location. HWMs that are higher than the modeled water surface are thus positive, and HWMs that are lower than the modeled water surface are negative. The x-axis in Figures 31 through 33 is in river miles upstream from the confluence with the Skykomish River (Snoqualmie Falls is at approximately RM 40).

Figure 31. Difference Between HWM Elevations from January 2009 Flood and the Simulated 100-Year Flood Event.

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Figure 32. Difference Between HWM Elevations from January 2015 Flood and the Simulated 10-Year Flood Event.

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Figure 33. Difference Between HWM Elevations from December 2015 Flood and the Simulated 10-Year Flood Event.

HWMs from the January 2009 event are compared to the 100-year modeled event in Figure 31. Most are within 2 feet of the simulated 100-year water surface elevation, with some scatter but no clear trend. Although the January 2009 peak flow at the Carnation gage was much lower than the simulated 100-year event, HWMs near RM 27, just upstream of the gage location, are very close to the modeled 100-year event water surface elevation.

HWMs from the January 2015 event are compared to the simulated 10-year water surface elevation in Figure 32. In general, the January 2015 HWMs show significant scatter, with marks ranging from 3 feet below to 3 feet above the modeled 10-year water surface within a very short length of the river in the vicinity of Fall City. Somewhat more of the observed HWMs are below the modeled 10-year water surface than above it. However, given the significant scatter in the observed HWMs, the data cannot be used reliably except to note the obvious presence of very localized issues not reflected in the modeling. This observation both corroborates the local variability reported by valley residents, and highlights the likelihood that errors and uncertainty are associated with the HWM observations. It also underscores the limitations of using a 1D hydraulic model to approximate flow conditions accross a complex floodplain cross section.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 73 HWMs from the December 2015 event (Figure 33) are generally close to the modeled 10-year water surface elevation. The observed HWMs range from 1.5 feet below to 2 feet above the modeled 10-year water surface elevation with the exception of one value, which is more than 4 feet above the simulated 10-year water surface elevation. Otherwise, the HWMs generally corroborate the hydraulic modeling, as the December 2015 event was close to a 10-year flood in most areas. Variability in the y-axis values (i.e., difference between observed HWM and simulated water surface elevation) over a short distance (x-axis) may indicate localized effects not captured by the hydraulic model or errors in the HWM data.

King County has established flow thresholds to define flood severity and likely flood impacts. Flood phases are based on flow at the Sum of the Forks, as noted in Table 10. The time that elapses between one phase and the next—flood phase progression—varies from flood to flood.

Table 10. Snoqualmie River Flood Phases at the Sum of the Forks. Flood Phase Flow (cfs) Status 1 6,000 Internal alert 2 12,000 Minor flooding 3 20,000 Moderate flooding 4 38,000 Major flooding

Figure 34 compares flood phase progression of the three events and the modeled 10-year and 100-year events at the Snoqualmie gage. The most noticeable difference between the three events is the time to progress from flood phase 1 to flood phase 2, which took 1.25 hours in January 2015 but over 18 hours in December 2015. This slow progression in December 2015 was largely due to antecedent wet conditions that resulted in base flows in excess of the 6,000-cfs threshold for flood phase 1; that is, the river was in flood phase 1 before the December 2015 flood began. The three events were fairly similar to one another for the progression from flood phase 2 to phase 3 (2 to 3.25 hours) and the progression from flood phase 3 to phase 4 (6 to 6.5 hours).

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Figure 34. Duration Between Flood Phases at the Snoqualmie River Gage at Snoqualmie.

4.6. ROAD CLOSURES

The extent of road closures within the lower Snoqualmie River valley during the three events based on King County records and aerial photography taken during the floods is shown in Figure 35. Road closures totaled 37.5 miles of roads in the lower valley in January 2009, 21 miles in January 2015, and 24.6 miles in December 2015. The average road closure lasted approximately 4.6 days in January 2009, 1.8 days in January 2015, and 4.4 days in December 2015. Road closure impacts were thus most severe for the January 2009 event. Although the January 2015 and December 2015 events had similar flood peaks at Snoqualmie, the latter event reached a higher peak at Carnation, had a much greater total flood volume, and thus resulted in more extensive and longer road closures than occurred in January 2015.

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Figure 35. Road Closures During January 2009, January 2015, and December 2015 Flood Events.

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4.7. SUMMARY

The preceding descriptions of three recent large flood events on the Snoqualmie River shows how each flood was unique with corresponding unique impacts on water surface elevations and road closures, and therefore residents, in the Snoqualmie valley. The storms that triggered these flood events had warm air temperatures and heavy rainfall in common but differed in magnitude, spatial distribution, timing, and antecedent conditions, and thus resulted in different patterns of flooding and flood impacts.

The most extreme flooding in the lower valley occurred during the January 2009 flood event following intense and widespread rainfall, including significant rain-on-snow. In contrast, the January 2015 flood event was both smaller and of shorter duration, resulting in a quick rise and fall of the flood hydrograph and significant attenuation of the flood peak as flows moved downstream. The December 2015 flood event was preceded by several days of steady rainfall, resulting in a longer duration of high flows and greater total flood volume than the January 2015 event. Although the peak of this event was lower at Snoqualmie than that in January 2015, the flood peak persisted downstream and increased in magnitude by the time it reached the gage near Carnation.

The extent and duration of impacts during the three events correlate to both peak magnitude and total flood duration. Road impacts, as measured by total road miles closed and duration of closures, were the greatest during the January 2009 event and least during the January 2015 event. Observed high water marks from the January 2009 event are generally similar to simulated water surface elevations in the 100-year Snoqualmie-Skykomish HEC-RAS model flood event, January 2015 HWMs are close to or slightly below the simulated 10-year event, and HWMs from the December 2015 event are close to or slightly above the simulated 10-year event. The December 2015 and January 2015 floods illustrate how two flood events of similar magnitude, based on gage data, can result in widely variable local impacts due to variations in flood development, progression, timing, and duration.

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5. HIGH WATER MARK EVALUATION

This section discusses the use and interpretation of HWMs when describing flooding trends. Section 5.1 presents the available data. Sections 5.2 and 5.3 examine the relationship of observed HWMs to those at the Carnation gage (Section 5.2) and to those predicted by simulated 10- and 50-year flood events (Section 5.3). Findings are discussed in Section 5.4.

5.1. HIGH WATER MARK SURVEY

HWMs are valuable for characterizing the impact of floods on the landscape, structures, and people. Consistently collecting HWMs at specific locations can also facilitate analysis of changes in flooding over time, comparison of flood levels at different locations, and calibration and verification of hydraulic models and floodplain mapping. King County conducted ground surveys in early 2016 to collect elevations of HWMs identified by Snoqualmie Valley residents and County staff. A total of 70 HWMs were surveyed at 17 sites owned or maintained by valley residents corresponding to 12 different flood events. Another 16 HWMs were surveyed from marks made by County staff. Surveyed elevations are included in Table 11 for recent and historical flood events dating back to 1990—flood events are sorted according to reported peak flow magnitude at the Carnation gage. HWM locations are shown in Figure 36.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 79 Table 11. High Water Mark Survey Data. Elevation (feet, NAVD 88) River 100-Year 50-Year 10-Year Jan. Nov. Nov. Nov. Feb. Nov. Dec. Jan. Nov. Dec. Oct. Mile Location FISa FISa FISa 2009 2006 1990 2008 1996 1995 2015 2015 2015 2010 2015 5.82 Kosters #1 49.6 48.2 44.7 47.48 47.01 48.52 46.68 5.86 Kosters #2 49.6 48.2 44.7 47.49 47.03 48.48 46.67 5.94 Roetcisoender 49.6 48.2 44.7 47.45 48.85 9.18 Duvall Gage 50.6 49.4 46.1 48.76 47.86 45.49 46.26 47.94 45.61 45.13 45.39 45.01 43.92 11.30 Haack 52.0 51.0 49.0 47.80 47.38 45.34 14.40 Erickson-Brown 53.8 52.9 50.3 52.20 15.00 Erickson-Brown 54.4 53.6 51.0 52.53 16.50 Oxbow Farm 55.4 54.5 52.0 55.95 55.34 54.70 54.07 53.04 17.60 Robertson 55.8 54.9 52.6 57.31 19.90 Camp Corey 58.5 57.8 55.0 56.67 56.04 56.18 21.87 Carnation Gageb 64.2 63.8 62.4 64.71 63.78 63.2 62.99 62.84 62.8 62.28 61.74 61.36 61.26 59.2 21.88 KC RFMS 64.20 63.80 62.40 61.35 21.90 KC RFMS 67.90 67.20 65.10 61.60 22.00 Mason #1 68.0 67.3 65.3 66.09 65.12 22.04 Mason #2 68.0 67.4 65.4 66.30 23.41 Tolt MacDonald 75.5 74.8 72.9 74.80 73.30 72.83 72.19 71.52 70.76 70.16 23.90 RFMS 79.2 78.5 76.6 74.98 26.70 Eppley 79.6 78.9 76.7 79.64 78.28 76.83 76.76 76.30 75.73 26.80 Strom 80.0 79.2 77.2 79.73 78.79 76.86 27.30 Casey 80.2 79.4 77.4 80.59 78.23 78.23 27.72 Tregoning 80.6 79.8 77.8 80.40 78.64 78.26 27.77 Haakenson 80.8 80.0 78.1 80.98 79.96 79.19 79.19 31.60 Keller 84.9 84.2 82.5 83.20 82.57 81.33 81.07 35.27 Groshell 103.8 103.4 101.6 103.53 103.01 102.74 101.94 102.43 102.27 101.99 101.92 101.67 37.75 KC RFMS 119.6 118.9 116.6 119.61 37.78 Yankacy 119.6 118.9 116.6 123.11 121.19 38.08 Snoqualmie Gage 125.4 124.7 122.3 125.97 125.31 126.1 123.76 123.52 123.32 123.53 123.62 123.31 121.19 121.3 a FIS flood profiles elevations are approximate, Source: FEMA (2017). b Elevations at USGS Carnation gage are based off an assumed datum conversion and were not surveyed.

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Figure 36. Location of Lower Snoqualmie Highwater Marks.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 81 The accuracy of each HWM elevation depends on the accuracy of the survey as well as the quality and interpretation of the mark. Seed or mud lines left on a building typically provide a clearer elevation than debris lines or cut lines on a bank, and care must be taken to ensure the mark is not impacted by localized effects such as waves, floodborne debris accumulation nearby, or water “run-up” and/or drawdown on opposing sides of an obstruction to flow that would cause it to not be representative of the surrounding water surface (see Figure 37). The USGS notes the uncertainty of collected HWM elevations using a rating of excellent, good, fair, poor, or very poor (Koenig, 2016). The USGS’ relatively common rating of “fair” corresponds to a vertical uncertainty of ±0.2 feet (It is unclear whether the USGS considers only survey error when assigning ratings, or if survey errors as well as errors in the identification and interpretation of high water indicators are considered. In WSE’s experience, the precision associated with a “fair” rating (i.e., ±0.2 feet) is unlikely to be consistently achieved when HWMs are field-identified after a flood.

Source: Koenig et al. (2016) Figure 37. Seed Line on a Building (Left), and Water Surface Run-up and Drawdown, Which Impact HWM Interpretation (Right).

Most of the HWMs collected by valley residents were clearly marked on stable vertical surfaces such as building walls or doors. Many HWMs were marked on interior walls so are less likely to have error associated with run-up or drawdown effects. Some HWMs that had ambiguous or erroneous dates noted were excluded from the analysis. Other HWMs were included in the analysis but carry more uncertainty because they were provided based on the resident’s memory rather than a visible mark (e.g., that the January 2009 flood rose to the top of a stair step).

A dense collection of HWMs around the Upper Carlson floodplain restoration site near RM 32.5 was also considered but excluded from the following analysis. The elevations of 28 HWMs collected by King County staff after the January 2015 flood in this area varied by several feet within a relatively short distance along the river (see Section 4.5 and Figure 32). Many of these HWMs were interpreted from debris lines left on natural features (such as a trees or grass) and carry greater uncertainty in the high water level that occurred than most residents’ HWMs due to the considerations described above. The elevation range amid the numerous HWMs surveyed in the Upper Carlson site area illustrate the variable nature of water surface dynamics in a fast-

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moving river. Similar variation was seen in HWM data collected by the USGS after the January 2009 flood event (USGS 2018), including marks that vary by more than 3 feet from one side of the river to the other. Elevation accuracy is inherent in HWM data collection and analysis.

5.2. COMPARISON OF HWMS TO CARNATION STAGE

HWMs were used to evaluate changes in flood elevations over time by focusing on locations where HWMs have been recorded in multiple flood events. This approach reduces uncertainty associated with spatial variation in HWMs. Figure 38 shows how peak water surface elevations at eight locations compared to peak stage at the USGS Carnation gage (RM 21.87) for eight large flood events. Locations include the USGS Snoqualmie gage (RM 38.08), the Groshell property (Snoqualmie Falls Golf Course) (RM 35.27), the Eppley property (RM 26.70), Tolt MacDonald Park (RM 23.4), Oxbow Farm (RM 16.50), the USGS Duvall gage (RM 9.18), and the Kosters property (RM 5.82).

To facilitate comparisons among the data, at each location the HWM elevation for each flood event was subtracted from the corresponding HWM elevation at that location for the January 2009 flood (y-axis on the graph). Resulting values were then compared to the change in HWM elevation at the Carnation gage (x-axis on the graph). For example, the HWM elevation at Tolt MacDonald Park recorded after the November 1990 flood was approximately 2 feet lower than the HWM recorded at the same location after the January 2009 flood, while the HWM at the Carnation gage for the November 1990 flood was approximately 1.5 feet lower than it was for the January 2009 flood. Data for each location were then fit with a linear trendline. No trendline was fit to data from the Kosters property because only three HWMs were available at that location, and that is not enough data points to draw a trend line for the comparison.

HWM relationships between the Carnation gage and nearby Tolt MacDonald Park (RM 23.41) are relatively consistent. HWMs recorded at the Eppley property (RM 26.70) and Groshell property (RM 35.27) are also fairly consistent with their best-fit trend lines, with the exception of the 2008 flood event. Hydraulic conditions during floods at Duvall (RM 9.18) and the Kosters property (RM 5.82) are affected by the coincidence (or lack thereof) of flooding on the Skykomish River, which joins the Snoqualmie to form the Snohomish downstream, and are therefore less correlated to stage at Carnation. This phenomenon can be seen for the 1990 and 1995 flood events, when coincident flooding on the Skykomish River caused substantial backwater in the lower Snoqualmie River and elevated HWMs at the Kosters property. HWMs at the Snoqualmie gage (38.08) also show less correlation to the Carnation gage data. The lower correlation is likely due to the significant (15-mile) distance between these two gage sites, the numerous tributaries that discharge to the Snoqualmie River between the two gages, and the variable nature of flood attenuation resulting from different flood characteristics as described in Section 4.

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Figure 38. Relative Change in Stage at HWM Locations Compared to Change in Stage at the Carnation Gage During the January 2009 Flood.

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HWMs at the Snoqualmie gage and Groshell property (located approximately 3 miles downstream) show a fairly consistent relationship to each other, with the exception of the 1990 flood. The 1990 flood was an anomaly in the Snoqualmie gage record both because the gage failed during the event and because the estimated USGS peak flow at Snoqualmie was much larger than the corresponding peak flow at the Carnation gage. The 1990 HWM at the Groshell property appears consistent with the trendline for the Groshell property, while the HWM at the Snoqualmie gage is 1.5 feet higher than the best-fit trend line for that gage location. This absence of fit to the trend line further suggests that the HWM and flow for the 1990 flood event at the Snoqualmie gage may have been overestimated as discussed in Section 2.5.4.

HWMs for the 2008 flood event are consistently low at all locations compared to the best-fit trend lines, possibly indicating an error in the reported stage at Carnation or an anomaly at the Carnation gage during that event. For example, channel changes during the November 2006, December 2007, and/or November 2008 flood event(s) could have altered the stage relationship at the Carnation gage.

Overall, the comparison of HWM elevations at several locations in the valley to high water levels observed at the USGS Carnation gage does not show a clear pattern of recent flooding being consistently higher or lower than the best-fit trend lines drawn from the available HWM data.

5.3. COMPARISON OF HWMS TO FEMA FIS FLOOD MODEL PREDICTIONS

Table 12 presents 10- and 50-year flood elevations predicted via hydraulic modeling documented in the Snoqualmie River Flood Insurance Study (FIS) (FEMA 2017) and HWM elevations from recent flood events, allowing a direct comparison of simulated versus observed events, and an indirect comparison of flood-to-flood variability in water levels throughout the valley. The following discussion is fairly dense because there are many variables affecting HWMs and comparison of recorded HWMs, and simulated flooding is inherently nuanced. Peak flows for each actual flood event (HWM data) and the simulated 10- and 50-year flood events (FIS model results) are listed at the top of the table—actual flood events are sorted according to reported peak flow magnitude at the Carnation gage. Data from Table 12 are shown graphically in Figures 39 and 40. Note that HWMs in areas downstream of Duvall are often affected by backwater flooding from the Skykomish River.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 85 Table 12. Comparison of HWMs to FEMA FIS Flood Model Predictions. 50-Year 10-Year (FIS) (FIS) Jan. 2009 Nov. 2006 Nov. 1990 Nov 2008 Feb 1996 Nov 1995 Dec 2015 Jan 2015 Nov 2015 Peak Flow (cfs) 71,000 51,700 60,700 55,000 74,300 47,000 51,700 50,200 49,500 50,100 48,000 Snoqualmie Gage Peak Flow (cfs) 82,400 58,200 82,900 71,800 65,200 63,100 61,600 61,200 56,200 53,900 46,600 Carnation Gage HWM Elevation Minus 50-Year River Mile Location FIS Elevation (feet) HWM Elevation Minus 10-Year FIS Elevation (feet) 5.82 Kosters #1 -0.72 -1.19 0.32 1.98 5.86 Kosters #2 -0.71 -1.17 0.28 1.97 5.94 Roetcisoender -0.75 0.65 9.18 Duvall Gage -0.64 -1.54 -0.61 0.16 1.84 -0.49 -0.97 -0.71 11.30 Haack -1.20 -1.62 14.40 Erickson-Brown -0.70 15.00 Erickson-Brown 1.53 16.50 Oxbow 1.45 0.84 2.70 2.07 1.04 17.60 Robertson 2.41 19.90 Camp Corey -1.13 1.04 1.18 21.87 Carnation Gage 0.91 -0.02 -0.60 0.59 0.44 0.40 -0.12 -0.66 -1.04 22.00 Mason #1 -1.21 -2.18 22.04 Mason #2 -1.10 23.41 Tolt MacDonald 0.00 -1.50 -1.97 -0.71 -1.38 -2.14 -2.74 26.70 Eppley 0.74 -0.62 0.13 0.06 -0.40 -0.97 26.80 Strom 0.53 -0.41 -0.34 27.30 Casey 1.19 0.83 0.83 27.72 Tregoning 0.60 0.84 0.46 27.77 Haakenson 0.98 -0.04 -0.81 1.09 31.60 Keller -1.00 -1.63 -1.18 35.27 Groshell 0.13 -0.39 -0.66 0.34 0.83 0.67 0.39 0.32 0.07 37.78 Yankacy 4.21 4.59 38.08 Snoqualmie Gage 1.27 0.61 1.40 1.46 1.22 1.02 1.23 1.32 1.01 FIS = Flood insurance study (FEMA 2017)

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Figure 39. Difference Between HWM Elevations from the Three Largest Snoqualmie River Floods and the 50-Year Flood Event Simulated with the FIS Model.

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Figure 40. Difference Between HWM Elevations from the November 1995, February 1996, and November 2008 Floods and the Simulated 10-Year Flood Event.

Figure 39 compares HWMs for the January 2009, November 2006, and November 1990 events with elevations from the 50-year FIS flood model simulation. The January 2009 event typically produced the highest HWMs, and the November 1990 event the lowest. This pattern is consistent with the relative magnitude of peak flows recorded at the Carnation gage (see Table 12). One exception is that the HWMs for the 1990 flood event are highest near Duvall, which is due to the large and coincident flooding that occurred on the Skykomish River in 1990 resulting in a backwater effect that extended upstream of Duvall. The only other exception is that the HWM for the 1990 flood event is highest at the Snoqualmie gage, possibly the result of a gaging anomaly—the Snoqualmie gage failed during the 1990 event, and peak stage and flow estimated by the USGS following the event appear to be too high. Observations by PSE personnel indicate that the peak water surface near the Snoqualmie gage location was actually higher in 2006 and 2009 than in 1990 (see Section 2.5.4), which would be consistent with the ordering of HWMs collected at the downstream Groshell golf course property (RM 35.27). The USGS high flow rating curve used at the Snoqualmie gage from 1998 to 2011 also likely caused flows to be underestimated for the 2006 and 2009 floods (see Section 2.5.5). Given this uncertainty, it is possible that a corrected flow record would cause the ranking of peak flows at

July 2018 88 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

the Snoqualmie gage to match the order reported at other locations, and it seems likely that the reported HWM for the 1990 flood event at the Snoqualmie gage is too high.

The simulated 50-year peak flow at the Carnation gage (82,400 cfs) is similar to the reported January 2009 peak flow at the Carnation gage (82,900 cfs); however, the January 2009 HWMs are 0.9 foot above the simulated 50-year flood elevation at the gage location, and approximately 0.5 to 1.2 feet above the simulated 50-year elevation at properties between RM 26.7 and RM 27.7. The 2009 HWMs are also approximately 1.1 to 1.2 feet lower than the simulated 50-year flood elevation at the Mason property (RM 22) located just upstream from the Carnation gage, and equal to the simulated 50-year flood elevation at Tolt MacDonald Park (RM 23.4). HWMs from all available flood events are consistently higher than the simulated 50-year flood water surface profiles near the Oxbow Farm (RM 16.5) and Robertson properties (RM 17.6), and low at the Keller property (RM 31.6).

Figure 40 compares HWMs for the November 2008, February 1996, and November 1996 flood events to elevations simulated for the 10-year flood event with the FIS model. HWMs from the observed flood events are similar to the simulated 10-year peak flood elevation at both the Snoqualmie and Carnation gages (see Table 12). The HWMs are closely grouped at most locations and are within approximately 1 foot of the simulated 10-year flood water surface elevation. HWMs from all three flood events included in Figure 40 are above the simulated 10-year flood peak water surface elevation at the Carnation gage, which is consistent with the relative magnitude of peak flows at that location.

Two anomalies can be noted in Figure 40: 1) the 2008 HWM at Oxbow Farm (RM 16.5) is more than 2.5 feet high relative to the FIS model prediction, and 2) the Groshell property (RM 35.27) is the only location upstream of Duvall where reported HWMs for the November 2008 flood event were lower than those from the 1995 and 1996 flood events. This anomaly may indicate an error with the recorded HWMs at that location; however, there are too few nearby HWMs to allow a more detailed comparison, and there are few sites with HWMs from all three events.

Although the 2008 flood event produced the highest HWM at the Snoqualmie gage, the corresponding USGS peak flow estimate in that event is less than peak flow estimates for the 1995 and 1996 floods at that location due to the change in gage rating. However, the USGS- reported peak flow estimate for the 2008 flood is likely too low due to a possible error with that gage rating (see Section 2.5.5).

5.4. DISCUSSION

HWMs for various recent flood events were compared at locations throughout the valley, and to simulated flood elevations from the FIS hydraulic model. HWMs near the Carnation gage tend to correlate well to stage at the Carnation gage, while HWMs closer to Snoqualmie Falls and Duvall show much more scatter and are likely more influenced by other factors such as overbank flood flow attenuation, tributary inflow between Snoqualmie and Carnation, and backwater effects from the Skykomish River.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 89 Comparison of HWMs to water surface elevations predicted by the FIS model show that the model is likely underpredicting water surface in some areas while overpredicting in others. Areas where the model may be underpredicting peak water surface elevations include Oxbow Farm (RM 16.5), the Robertson property (RM 17.6), and the Yankacy property (RM 37.78), where HWMs for all events are consistently higher than any modeled water surface elevation. This may indicate that the FIS model is not well calibrated in these areas, that complex flow patterns causing the HWMs are not well represented by the FIS model, or that potential errors are present in the HWM data. It should be noted that the FIS hydraulic model was developed and calibrated in 2006, preceding the 2006 and 2009 flood events, and therefore its calibration did not address HWMs from those ensuing flood events. While overall relationships between HWMs from different flood events appear reasonable with respect to relative flood event magnitude, comparisons of HWMs at the valley scale is limited by the relatively small number of locations where a consistent and long-term record of HWM elevations is available. We recommend continued HWM data collection following all significant flood events.

5.5. HIGH WATER MARK PROFILES

Figures 41 to 44 show HWMs from recent flood events alongside the water surface elevation profiles generated by the FEMA FIS model for the 10-, 50-, and 100-year flood events. Figures 41 and 42 include HWMs from the largest three recorded flood events: January 2009, November 2006, and November 1990. Figures 43 and 44 compare HWMs from the November 2008, February 1996, and November 1995 flood events. These figures were developed by King County as part of the HWM data collection program and provide useful information on how HWMs from large flood events compare to each other and to modeled water surfaces.

In general, the January 2009 HWMs correlate well with the FIS model 50-year flood water surface elevation for most river reaches while HWMs from the November 1990 and November 2006 flood events fall between the simulated 10-year and 50-year water surface profiles. The November 1995, February 1996, and November 2008 floods all correlate well to the simulated 10-year flood event in most reaches. As stated previously, downstream of Duvall, HWMs are influenced by the occurrence (or lack) of backwater flooding emanating from the Skykomish River, and therefore show less correlation to the FIS model water surface elevation profiles.

July 2018 90 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

Figure 41. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 2006, January 2009, and November 1990 Flood Events, Downstream of Carnation Gage.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 91

Figure 42. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 2006, January 2009, and November 1990 Flood Events, Upstream of Carnation Gage.

July 2018 92 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

Figure 43. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 1995, February 1996, and November 2008 Flood Events, Downstream of Carnation Gage.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 93

Figure 44. Comparison of FIS Model Water Surface Elevation Profiles and HWMs from November 1995, February 1996, and November 2008 Flood Events, Upstream of Carnation Gage.

July 2018 94 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

6. REFERENCES

Booth, D.B., K. Bell, and K.X. Whipple. 1991. Sediment transport along the South Fork and main stem of the Snoqualmie River. King County Surface Water Management Division. Seattle, Washington.

Bowling, L.C., P. Storck, and D.P. Lettenmaier. 2000. Hydrological effects of logging in western Washington, United States. Water Resources Research 38(11):3223–3240. DOI: 10.1029/2000WR900138. November 2000.

Catchpole, D. and L. Geggel. 2009. Living with flooding part 1: a valley endures, Sno Valley Star, Reprinted by King County Flood Control District, December 16, 2009.

Ecology. 2010. Implementing the flow control standard in Ecology’s western Washington municipal stormwater permits, frequently asked questions. Washington State Department of Ecology, Water Quality Program. May 2010.

FEMA. 2017. Flood Insurance Study. King County Washington and Unincorporated Areas. Federal Emergency Management Agency, FIS No. 53033CV001B, Preliminary. September 15, 2017.

Hamlet, A.F. and D.P. Lettenmaier. 2007. Effects of 20th century warming and climate variability on flood risk in the western US Water Resources Research 43(6). W06427, DOI: 10.1029/2006WR005099.

Hamlet, A.F, M.M. Elsner, G.S. Mauger, S.Y. Lee, I. Tohver, and R.A. Norheim. 2013. An overview of the Columbia basin climate change scenarios project: approach, methods, and summary of key results. Atmos.-Ocean 51:392-415. DOI: 10.1080/07055900.2013.819555.

King County. 2008. Snoqualmie Flood-Farm Task Force Report. King County Department of Natural Resources and Parks, Water and Land Resources Division, Seattle, Washington. January 2008.

King County. 2010. Climate change impacts on river flooding: state-of-science and evidence of local impacts. Prepared by Curtis DeGasperi, King County Department of Natural Resources and Parks, Water and Land Resources Division, Seattle, Washington.

King County. 2014. South Fork Snoqualmie River levee characterization report river mile 2.0 – 5.9. South Fork Snoqualmie River levee repair and reconstruction plan appendices. King County Department of Natural Resources and Parks Water and Land Resources Division, Seattle, Washington. June 2014.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 95 King County. 2015. Snoqualmie River flooding information, King County Water and Land Services Flood Services website, King County, Washington. Accessed December 23, 2015. .

King County. 2016a. Draft – Post flood analysis of flood effects in the Snoqualmie at Fall City (SAFC) project reach for the January 5–6, 2015 flood event. King County Water and Land Resources Division, Seattle, Washington. February 21, 2016.

King County 2016b. Flood frequency analysis of King County rivers with an emphasis on the January 2009 floods. Prepared by Curtis DeGasperi, King County Water and Land Resources Division, Seattle, Washington. August 2016

King County. 2017. Snoqualmie River 1997 to 2015 in-channel sediment monitoring. King County Department of Natural Resources and Parks, Water and Land Resources Division, River and Floodplain Management Section, Seattle, Washington. March 2017.

Koenig, T.A., J.L. Bruce, J.E. O’Connor, B.D. McGee, R.R. Holmes, Jr., R. Hollins, B.T. Forbes, M.S. Kohn, M.F. Schellekens, Z.W. Martin, and M.C. Peppler. 2016,. Identifying and preserving high-water mark data: US Geological Survey Techniques and Methods, book 3, chapter A24. .

Mansfield. 2009. RE: Chinook Bend restoration project compliance with King County Code. Letter to Gary Downing. King County Department of Natural Resources and Parks, Water and Land Resources Division, Seattle, Washington. June 1, 2009.

Mass C., A. Skalenakis, and M. Warner. 2011. Extreme precipitation on the West Coast of North America: is there a trend? Journal of Hydrometeorology 12:310–318.

Mastin, M.C., A.S. Gendaszek, and C.R. Barnas. 2010. Magnitude and extent of flooding at selected river reaches in western Washington, January 2009: US Geological Survey Scientific Investigations Report 2010-5177.

Matheussen B., R.L. Kirschbaum, I.A. Goodman, G.M. O'Donnell, and D.P. Lettenmaier. 2000. Effects of land cover change on streamflow in the interior Columbia River Basin (USA and Canada). Hydrological Processes 14(5):867–885. DOI: 10.1002/(SICI)1099-1085(20000415).

Mauger, G.S., J.H. Casola, H.A. Morgan, R.L. Strauch, B. Jones, B. Curry, T.M. Busch Isaksen, L. Whitely Binder, M.B. Krosby, and A.K. Snover. 2015. State of knowledge: climate change in Puget Sound. Prepared for the Puget Sound Partnership and the National Oceanic and Atmospheric Administration. University of Washington, Climate Impacts Group, Seattle, Washington. DOI: 10.7915/CIG93777D.

NHC. 2006. Flood insurance mapping study for the Snoqualmie River and Skykomish River, King and Snohomish Counties, Washington. Prepared for Federal Emergency Management Agency. Northwest Hydraulic Consultants, Seattle, Washington. April 2006.

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Nieman P.J., L.J. Schick, F.M. Ralph, M. Huges, and G. Wick. 2011. Flooding in western Washington: the connection to atmospheric rivers. Journal of Hydrometeorology 12:1337–1358. DOI: 10.1175/2011JHM1358.1.

NOAA. 2009. Hourly Precipitation Data, Washington, January 2009. National Oceanic and Atmospheric Administration National Climate Data Center, Volume 59, Number 1, ISSN 0364-6912.

NOAA. 2016. Coastal Change Analysis Program (C-CAP) land cover atlas. 2016. Accessed June 29, 2016. .

Perry, G., J. Lundquist, and D. Moore. 2016. Review of the potential effects of forest practices on stream flow in the Chehalis River basin – preliminary draft. University of Washington, Department of Civil and Environmental Engineering, Seattle, Washington, and University of British Columbia, Departments of Geography and Forest Resources Management, Vancouver, British Columbia.

Salathé, E., A. Hamlet, C. Mass, S. Lee, M. Sumbaugh, and R. Steed. 2014. Estimates of twenty-first-century flood risk in the Pacific Northwest based on regional climate model simulations. Journal of Hydrometeorology 15(5):1881–1899. DOI: 10.1175/JHM-D-13-0137.1.

Snohomish County Surface Water Management, King County Snoqualmie Watershed Forum Staff, and Tulalip Tribes Natural Resources Department. 2015. Snoqualmie Basin protection plan 2015. December 2015.

Snoqualmie, City of. 1995. Draft supplemental environmental impact statement, City of Snoqualmie, Washington. April 1995.

US Geological Survey. 1960. 12-1415. Middle Fork Snoqualmie River near North Bend, WA, Snohomish River Basin Flood of November 23, 1959. Field notes and slope-area calculations.

US Geological Survey. 1992. Policy statement on stage accuracy. Technical Memorandum No. 93.07. US Geological Survey, Office of Surface Water. December 4, 1992.

US Geological Survey. 2006. Computer program for the Kendall family of trend tests. Scientific Investigations Report 2005-5275.

US Geological Survey. 2012. Memo to the Record: Snoqualmie River near Snoqualmie, WA, Station Number 12144500, Mark Mastin, USGS WAWSC, Tacoma, Washington. July 31, 2012.

US Geological Survey. 2014. Water-resources data for the United States, Water Year 2013: US Geological Survey Water-Data Report WDR-US-2013, site 12149000. .

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions – King County 97 US Geological Survey. 2015. National Water Information System rating curve, Rating ID 8, Snoqualmie River near Snoqualmie, WA, Station Number 12144500. Accessed December 4, 2015. .

US Geological Survey. 2016. Map of the basin, online interactive map of USGS gaging in the Snohomish and Snoqualmie River Basin. .

US Geological Survey. 2018. FEMA high water marks – western Washington flood, January 2009. Accessed January 8, 2018. .

Warner, M.D., C.F. Mass, and E.P. Salathé. 2012. Wintertime extreme precipitation events along the Pacific Northwest coast: climatology and synoptic evolution. Monthly Weather Review 140:2021–2043. DOI: 10.1175/MWR-D-11-00197.1.

WDNR. 2015. Washington State timber harvest reports. Washington State Department of Natural Resources. Updated December 9, 2015. .

Wissmar, R.C., R.K. Timm, and M.G. Logsdon. 2004. Effects of changing forest and impervious land covers on discharge characteristics of watersheds. Environmental Management 34(1):91–98.

WSE. 2014. Existing condition hydrologic and hydraulic analysis, Lower Tolt River. Prepared for King County Department of Natural Resources and Parks, Water and Land Resources Division by Watershed Science & Engineering, Seattle, Washington. February, 21, 2014.

WSE and Herrera. 2016. Evaluation of effects of Snoqualmie Falls projects on downstream flooding. Prepared for King County Department of Natural Resources and Parks, Water and Land Resources Division by Watershed Science & Engineering and Herrera Environmental Consultants, Seattle, Washington. April 29, 2016.

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APPENDIX A

USGS Gage Summaries, Tributary Gages

SUMMARY OF TRIBUTARY GAGE RECORDS

Major tributaries to the Snoqualmie River below the Sum of the Forks include the Tolt River and the Raging River. The Tolt River is the larger of the two, and flows into the Snoqualmie River at the city of Carnation, approximately 2 miles upstream of the main stem Snoqualmie River gage near Carnation. Although the Tolt is capable of producing significant flood flows (the flood of record on the Tolt gage near Carnation is 17,400 cfs), it does not generally contribute significantly to peak floods on the main stem Snoqualmie River. This is because peak flows from the Tolt tend to reach the Snoqualmie River before the main stem flood wave peak has reached the confluence location. The same is true for the Raging River, which has a much smaller flood of record of 6,220 cfs. A summary of current and historical USGS gaging stations on the Tolt and Raging Rivers is included below.

Raging River Near Fall City, WA (12145500)

The Raging River gage has been in intermittent operation since 1945 and measures flow from a drainage area of 30.6 mi². The maximum recorded discharge is 6,220 cfs; it was recorded on November 24, 1990, and corresponds to a gage height of 6.56 ft. The highest physical discharge measurement of 3,430 cfs was recorded on February 2, 1951. There are no known data quality issues at this gage.

North Fork Tolt River Near Carnation, WA (12147500)

The North Fork Tolt River near Carnation gage began operation in 1952, and measures flow from a drainage area of 39.9 mi². The maximum recorded discharge of 9,560 cfs was measured on December 15, 1959. This peak flow estimate was based on a rating curve extrapolation above the highest physical measurement of 2,800 cfs made in 1953. The highest physical discharge measurement of 8,520 cfs was collected on January 7, 2009.

South Fork Tolt River Near Index, WA (12147600)

The South Fork Tolt River near Index gage began operation in 1959, and has been in continuous operation since November 1967. It measures flow from a drainage area of 5.34 mi². The USGS considers data in this gage record to be “fair” except for those measurements above 900 cfs, which are considered “poor.” The maximum discharge of 2,240 cfs was recorded on December 15, 1999, but may have been influenced by a debris dam breakup. A station note states that this station is affected by ice. The highest physical discharge measurement of 2,140 cfs was collected on November 23, 1986.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions A-1 South Fork Tolt River Near Carnation WA (12148000)

This site measures flow from a drainage area of 19.7 mi². The gage began operation in 1952, and has been in continuous operation since 1969. Since September 1963, Seattle Public Utilities has diverted an average daily discharge of about 77 cfs upstream from the gaging station. The maximum discharge of 6,500 cfs was recorded on December 15, 1959. The highest physical discharge measurement of 6,400 cfs was collected on November 24, 1990. Note that the reported peak for the November 24, 1990, event is 5,380 cfs, which is over 1,000 cfs lower than the physical discharge measurement that the USGS collected on that date. An explanation for why the reported peak flow is lower than the physical discharge measurement has not been provided by the USGS.

South Fork Tolt River below Regulating Basin Near Carnation, WA (12148300)

This is a stage-only gage and peak flood flows are not reported. The gage has been in operation since March 1982. The drainage area above the gage is 29.6 mi².

Tolt River Near Carnation, WA (12148500)

The Tolt River gage near Carnation has been in operation since 1928 and measures flow from a drainage area of 81.4 mi². There are no known data quality issues at this gage, which the USGS rates as a “stellar site” (D. Miller, USGS, personal communication, October 27, 2015). The maximum recorded discharge at this gage of 17,400 cfs, was reported on December 15, 1959. Since 1963 the site is affected by regulation of flow at the South Fork Tolt reservoir (the City of Seattle Water Department diverts an average daily discharge of 77 cfs).

July 2018 A-2 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

APPENDIX B

Summary of Discussion with CIG, USGS, and NWS

UNIVERSITY OF WASHINGTON CLIMATE IMPACTS GROUP

On October 29, 2015, WSE and King County met with Guillaume Mauger of the University of Washington Climate Impacts Group (CIG) to discuss recent and forthcoming work relating to climate change and flood impacts in the Pacific Northwest. Guillaume referenced a number of relevant studies relating to climate change or flood risk in the Pacific Northwest, and mentioned upcoming work to examine trends in climate and hydrology within the Skagit River basin and explore links between those trends and regional climate change using a regional climate model. This will include two model runs to dynamically downscale global climate model (GCM) results to the regional level (as opposed to statistical downscaling). Guillaume noted that statistical downscaling is more commonly done due to the computational capacity (and expense) required to dynamically downscale the GCM data.

Guillaume also discussed the need to consider how anthropogenic changes in a river basin can impact flood characteristics. Work completed by Dennis Lettenmaier (for example) suggests that the impacts of development and logging can be much more significant than the impacts of climate change. This may not be the case in Snoqualmie basin, which is relatively undeveloped.

The CIG is also working with King County (Jeff Burkey) to assess climate change data to better understand potential changes in hydrology of creeks and storm drainage systems in lowland areas. This work is more focused on (combined sewer overflows) CSOs and storm drainage as opposed to large river systems, and will not include any river routing. This work is scheduled to be completed by the 3rd quarter or 2016.

UNITED STATES GEOLOGICAL SURVEY

On October 27, 2015, representatives from WSE and King County met with Mark Mastin and Darrin Miller of the USGS. Mark is a Surface-water Specialist at the USGS, with a history of working with the Snoqualmie River basin. He no longer works directly with stream gaging. Darrin Miller is a Supervisory Hydrologic Technician at the USGS. The Ferndale USGS field office where Darrin is based has taken over responsibility for gaging in the upper Snoqualmie basin in the last year and a half.

This meeting was intended to discuss streamflow gaging, and get USGS’ perspective on reliability of current gaging and flood reporting, known issues at existing gage sites and any future work planned by the USGS. King County relies on USGS data for flood information, background, planning, and their early warning flood system.

A summary of key issues discussed during the meeting included the following:

Flow has been noted to bypass the South Fork Snoqualmie gage at Edgewick during large flow events (>8,000 cfs). The USGS gaging program does not currently account for this flow.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions B-1 The 1959 flood peak reported at the Middle Fork gage at Tanner is an extremely high historical peak that was estimated based on high water mark survey. The 1959 flood occurred 2 years before the gage was installed in 1961. WSE asked whether a report is available to explain how the 1959 peak was estimated. (Documentation of indirect discharge estimates was later provided for this event).

The 1990 flood peak of 78,800 cfs reported at the Snoqualmie River at Snoqualmie gage appears high when compared to flows reported at other Snoqualmie River gages (Carnation in particular) during the same event. The gage was destroyed during the event, and the peak stage was determined based on a high water mark survey after the flood. The USGS noted a recent investigation that has determined that a 0.4-foot reduction to the high water mark elevations may be appropriate to correct for leveling errors that are believed to have occurred during HWM collection following the 1990 event. If this change is applied, it would bring the 1990 peak flow estimate down to 74,300 cfs.

Issues at the Carnation gages include a large portion of flood flows that leave the main channel upstream of Carnation Farm Road and therefore do not pass the gage site and may not be directly accounted for in the current USGS rating curve. For this reason, the accuracy of flow estimates is uncertain above 50,000 cfs. Unsteady flow conditions also produce a hysteresis effect at this site beginning at flows as low as 9,000 cfs, resulting in under-reporting of flows during the rising limb of a flood hydrograph and over-reporting on the falling limb. The USGS is working on a new looped rating curve to apply at specific gage heights; however, this correction cannot be applied in real time, only after-the-fact.

Discussion also covered the USGS gaging program, including the following topics:

USGS visits gaging sites every 6 to 8 weeks to take flow measurements. High flow events don’t allow them to be at all locations at once, and they focus on “earmarked” locations including areas of concern such as Snoqualmie at Carnation, at the Falls, etc.

For peak flow estimation, the USGS will only extend a rating curve up to 2.5 times the highest measured discharge.

USGS is revising flood frequency and regional regression equations for Washington State (headed by Mark Mastin). This was scheduled to be released in late 2015, but no update has been provided.

Sedimentation tends to impact low flow rating more so than high flow rating. Areas of the Snoqualmie where sedimentation are a particularly known issue include the main stem below the mouth of the Tolt and Raging Rivers.

July 2018 B-2 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

NATIONAL WEATHER SERVICE (RE: NORTHWEST RIVER FORECAST CENTER)

On December 17, 2015, representatives from King County and WSE met with Brent Bower, a Senior Service Hydrologist with the National Weather Service (NWS) Forecast Office (WFO) in Seattle, Washington. This meeting was held to gather NWS input regarding hydrologic changes and observations for the Snoqualmie River basin, and to gain a better understanding of river flow forecasts produced by the Northwest River Forecast Center (NWRFC). Representatives from the NWRFC were unable to participate in the meeting due to ongoing flooding in Oregon and Washington that required their attention.

Hydrologic Trends in the Snoqualmie Basin

The NWS does not analyze hydrologic trends in their local office, and trends were therefore not discussed in great detail. Brent suggested a journal search for existing trends analyses. Available NWS data can be downloaded through the National Climate Data Center Website.

Reliability of Gage Date

Brent stated that data gaps do exist in precipitation gage records, particularly at sites that are difficult to access and maintain—such as Stampede Pass. The NWS is typically able to interpret data from such stations, but Brent could not speak to the exact process/method of interpretation.

NWRFC Forecasting for the Snoqualmie River Basin (NWRFC)

River forecast modeling is completed using the Sacramento Soil Moisture Accounting (SAC-SMA) model. SAC-SMA is run once daily to calculate runoff and complete river routing. Point flows are pulled from the model and reported at specific USGS gage locations including the Snoqualmie near Carnation, the Snoqualmie near Snoqualmie Falls, and for each of the Snoqualmie Forks).

The local NWS office completes precipitation forecasting twice a day and feeds that information to the NWRFC for input into the SAC-SMA river forecasting model. Brent noted that precipitation forecasts are provided on a gridded basis, which is different than the point locations used to pull flow forecasts. Brent thought that this difference may impact the sensitivity of the river forecast to changes in the precipitation forecast.

Streamflow is used for model calibration. The river forecast models are recalibrated every ~10 years based on precipitation, temperature, and streamflow data. The last calibration used records through 2003. The NWS’s goal is to move towards a 5-year calibration cycle.

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions B-3 The stream gage data can be used to update the river forecast during a flood event if the forecast begins to diverge from observed conditions. Updated forecasts can are developed by the NWRFC during flood events by re-running the SAC-SMA model using observed data in place of forecasted data. The resulting forecast hydrograph is then blended with the observed flow hydrograph to provide a seamless transition between observed and forecast flow.

River forecasts are issued one time a day during normal flow conditions. During a flood the forecast may also be updated if observed conditions diverge significantly from the forecast. The decision to update the forecast is subjective, and requires that staff are available to re-run the model and update the forecast. The NWRFC does not operate 24 hours a day, although staff will work long days during a flood as necessary.

The forecasts also incorporate planned reservoir operation based on communication with reservoir and dam operators, such as those at Tolt River Reservoir. If operators decide to spill without communicating with NWRFC staff, or when the NWRFC staff is not there, the spill will not be incorporated into the forecast.

River forecast data dissemination is not a “live feed.” There can be a lag in forecast updates and publication of that data on the website.

Which event types are hardest to predict? Rain on snow, particularly when the freezing level is near the elevation of the pass. In that situation a slight change in freezing level can significantly change model results. Brent noted that rain on snow has the greatest impact when there is more lowland snow, which tends to cover a greater area than snow near the passes. Rain on the upper elevation snow pack can often be absorbed with only a minimal increase in runoff.

Early season flooding can also be difficult to predict. This is partially attributable to uncertainty in soil moisture conditions.

July 2018 B-4 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

APPENDIX C

Trends for Water Year Discharge, 1962–2015

Tau – Kendall’s correlation coefficient S – Kendall’s S statistic Z – Kendall’s standard normal deviate p – p-value for significance of trend intercept – intercept of trend line (where year equals first water year in test series) slope – slope of trend line (units vary)

C1. Results of Mann-Kendall Trend Test on Annual Average Discharge at Gaging Stations Shown in Figure 12. Station Name Station No. Tau S z p Intercept Slope Middle Fork near Tanner 12141300 0.0029 4 0.023 0.9816 1117 0.05016 North Fork Snoqualmie near 12142000 0.0995 132 1.0337 0.3013 -1685 Snoqualmie Falls 1.106 Raging River near Fall City 12145500 0.0106 14 0.1026 0.9183 8.384 0.06333 South Fork Snoqualmie above 12143400 -0.0015 -2 -0.0079 0.9937 297.5 Alice Springs near Garcia -0.00185 Snoqualmie near Snoqualmie 12144500 0.0261 36 0.2685 0.7883 -111.9 1.391 Snoqualmie near Carnation 12149000 -0.0116 -16 -0.1151 0.9084 5149 -0.7248

C2. Results of Mann-Kendall Trend Test on Annual Maximum Daily Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Middle Fork near Tanner 12141300 0.0573 79 0.5985 0.5495 -5.25E+04 32.4 North Fork Snoqualmie near 12142000 0.1297 172 1.3495 0.1772 -4.00E+04 22.52 Snoqualmie Falls Raging River near Fall City 12145500 0.0988 126 1.0156 0.3098 -1.05E+04 6 South Fork Snoqualmie above 12143400 0.0792 105 0.8207 0.4118 -1.27E+04 7.817 Alice Springs near Garcia Snoqualmie near Snoqualmie 12144500 0.0203 28 0.2071 0.8359 -1.49E+04 18.98 Snoqualmie near Carnation 12149000 0.0706 101 0.7462 0.4556 -1.63E+05 96.43

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions C-1

C3. Results of Mann-Kendall Trend Test on Annual Peak Discharge at Gaging Stations Shown in Figure 12. Station

Station Name No. Tau S z p Intercept Slope Middle Fork near Tanner 12141300 0.079 113 0.8358 0.4033 -9.95E+04 58.26 North Fork Snoqualmie near 12142000 0.1426 204 1.5146 0.1299 -7.41E+04 41.11 Snoqualmie Falls Raging River near Fall City 12145500 0.081 190 0.9791 0.3275 -6864 4.495 South Fork Snoqualmie above 12143400 0.0741 106 0.7834 0.4334 -2.16E+04 12.86 Alice Springs near Garcia Snoqualmie near Snoqualmie 12144500 0.0498 74 0.53 0.5961 -1.01E+05 66.67 Snoqualmie near Carnation 12149000 0.0832 119 0.8804 0.3786 -2.43E+05 140

C4. Results of Mann-Kendall Trend Test on Frequency of 80 Percent of 2-Year Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope

Middle Fork near Tanner 12141300 0.0419 60 0.4503 0.6525 2 0 North Fork Snoqualmie near 12142000 0.1461 209 1.5822 0.1136 2 0 Snoqualmie Falls

Raging River near Fall City 12145500 0.1244 165 1.3244 0.1854 1.5 0

South Fork Snoqualmie above 12143400 0.0273 39 0.2933 0.7693 2 0 Alice Springs near Garcia

Snoqualmie near Snoqualmie 12144500 0.0559 80 0.6047 0.5454 2 0 Snoqualmie near Carnation 12149000 0.181 259 1.9519 0.0509 -58.26 0.0303

C5. Results of Mann-Kendall Trend Test on Monthly Average Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month Middle Fork near Tanner 12141300 -0.0443 -61 -0.4603 0.6453 7586 -3.037 1 Middle Fork near Tanner 12141300 -0.1001 -138 -1.0509 0.2933 1.17E+04 -5.398 2

Middle Fork near Tanner 12141300 0.1618 223 1.7029 0.0886 -9729 5.373 3

Middle Fork near Tanner 12141300 0.1023 141 1.0739 0.2829 -8230 4.808 4

Middle Fork near Tanner 12141300 0.1081 149 1.1353 0.2563 -7390 4.617 5

Middle Fork near Tanner 12141300 -0.0443 -61 -0.4603 0.6453 6073 -2.164 6

Middle Fork near Tanner 12141300 -0.1488 -205 -1.5649 0.1176 1.04E+04 -4.853 7

Middle Fork near Tanner 12141300 -0.2126 -293 -2.2399 0.0251 6184 -2.933 8

Middle Fork near Tanner 12141300 -0.201 -277 -2.1172 0.0342 8611 -4.128 9

Middle Fork near Tanner 12141300 0.111 153 1.166 0.2436 -9147 5.031 10

Middle Fork near Tanner 12141300 0.156 215 1.6416 0.1007 -1.67E+04 9.152 11

Middle Fork near Tanner 12141300 -0.1125 -155 -1.1813 0.2375 1.28E+04 -5.773 12

North Fork Snoqualmie 12142000 0.0256 34 0.2604 0.7946 -627.1 0.6676 1 near Snoqualmie Falls

July 2018 C-2 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

C5. Results of Mann-Kendall Trend Test on Monthly Average Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month

North Fork Snoqualmie 12142000 -0.0528 -70 -0.5445 0.5861 2606 -1.082 2 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.2127 282 2.2174 0.0266 -6211 3.342 3 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1599 212 1.665 0.0959 -4954 2.764 4 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1131 150 1.1758 0.2397 -4192 2.471 5 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.0196 26 0.1973 0.8436 -766.2 0.6791 6 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.1584 -210 -1.6492 0.0991 3401 -1.589 7 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.2217 -294 -2.3121 0.0208 2757 -1.334 8 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.1674 -222 -1.7439 0.0812 3681 -1.769 9 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1118 154 1.1736 0.2405 -4458 2.438 10 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.2061 284 2.1708 0.0299 -1.06E+04 5.662 11 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.1089 -150 -1.1429 0.2531 5112 -2.268 12 near Snoqualmie Falls

Raging River near 12145500 -0.1216 -155 -1.2508 0.211 3699 -1.719 1 Fall City

Raging River near 12145500 -0.1169 -149 -1.2021 0.2293 2363 -1.098 2 Fall City

Raging River near 12145500 0.0337 43 0.3411 0.733 -357 0.2706 3 Fall City

Raging River near 12145500 0.0478 61 0.4873 0.626 -220 0.1864 4 Fall City

Raging River near 12145500 0.0902 115 0.9259 0.3545 -759.8 0.426 5 Fall City Raging River near 12145500 0.0502 64 0.5117 0.6088 -279.3 0.1682 6 Fall City Raging River near 12145500 -0.1427 -182 -1.4702 0.1415 315.1 -0.1467 7 Fall City Raging River near 12145500 -0.2447 -312 -2.5263 0.0115 234.2 -0.11 8 Fall City Raging River near 12145500 -0.2541 -324 -2.6236 0.0087 591 -0.2857 9 Fall City Raging River near 12145500 0.0682 87 0.6986 0.4848 -502.2 0.2831 10 Fall City Raging River near 12145500 0.1467 187 1.5107 0.1309 -3076 1.657 11 Fall City

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions C-3

C5. Results of Mann-Kendall Trend Test on Monthly Average Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month Raging River near 12145500 -0.1529 -195 -1.5757 0.1151 3055 -1.414 12 Fall City South Fork Snoqualmie 12143400 -0.0332 -44 -0.3393 0.7344 1189 -0.4349 1 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0935 -124 -0.9706 0.3317 2752 -1.268 2 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1252 166 1.302 0.1929 -2359 1.312 3 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1403 186 1.4598 0.1443 -2843 1.606 4 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1056 140 1.0969 0.2727 -2636 1.605 5 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0211 -28 -0.2131 0.8313 1428 -0.5243 6 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.1614 -214 -1.6808 0.0928 1973 -0.9276 7 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.2097 -278 -2.1858 0.0288 867.5 -0.4061 8 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.1554 -206 -1.6177 0.1057 1260 -0.5986 9 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.0711 98 0.7441 0.4568 -1094 0.6424 10 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1495 206 1.5725 0.1158 -4453 2.408 11 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0972 -134 -1.0202 0.3076 3370 -1.549 12 above Alice Springs near Garcia Snoqualmie near 12144500 -0.0537 -74 -0.56 0.5755 1.87E+04 -7.605 1 Snoqualmie Snoqualmie near 12144500 -0.0972 -134 -1.0202 0.3076 2.11E+04 -9.339 2 Snoqualmie Snoqualmie near 12144500 0.1727 238 1.818 0.0691 -2.56E+04 14.04 3 Snoqualmie Snoqualmie near 12144500 0.1393 192 1.4651 0.1429 -2.25E+04 12.81 4 Snoqualmie

July 2018 C-4 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

C5. Results of Mann-Kendall Trend Test on Monthly Average Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month Snoqualmie near 12144500 0.0958 132 1.0049 0.315 -1.66E+04 10.23 5 Snoqualmie Snoqualmie near 12144500 0.0044 6 0.0384 0.9694 2743 0.3137 6 Snoqualmie Snoqualmie near 12144500 -0.1393 -192 -1.4651 0.1429 1.60E+04 -7.367 7 Snoqualmie Snoqualmie near 12144500 -0.1829 -252 -1.9253 0.0542 1.05E+04 -4.93 8 Snoqualmie Snoqualmie near 12144500 -0.1611 -222 -1.6952 0.09 1.52E+04 -7.191 9 Snoqualmie Snoqualmie near 12144500 0.103 142 1.0816 0.2794 -1.50E+04 8.414 10 Snoqualmie

Snoqualmie near 12144500 0.1684 232 1.7719 0.0764 -3.80E+04 20.72 11 Snoqualmie

Snoqualmie near 12144500 -0.135 -186 -1.4191 0.1559 3.34E+04 -15.22 12 Snoqualmie

Snoqualmie near 12149000 -0.0784 -108 -0.8208 0.4118 4.27E+04 -18.77 1 Carnation

Snoqualmie near 12149000 -0.0987 -136 -1.0355 0.3004 3.23E+04 -14.33 2 Carnation

Snoqualmie near 12149000 0.1118 154 1.1736 0.2405 -2.20E+04 12.87 3 Carnation

Snoqualmie near 12149000 0.0878 121 0.9205 0.3573 -2.32E+04 13.67 4 Carnation

Snoqualmie near 12149000 0.0522 72 0.5446 0.586 -8153 6.433 5 Carnation

Snoqualmie near 12149000 -0.0131 -18 -0.1304 0.8962 6997 -1.552 6 Carnation

Snoqualmie near 12149000 -0.1161 -160 -1.2196 0.2226 1.71E+04 -7.679 7 Carnation

Snoqualmie near 12149000 -0.1669 -230 -1.7566 0.079 1.26E+04 -5.797 8 Carnation

Snoqualmie near 12149000 -0.1742 -240 -1.8333 0.0668 1.91E+04 -9.052 9 Carnation

Snoqualmie near 12149000 0.0697 96 0.7287 0.4662 -1.44E+04 8.419 10 Carnation

Snoqualmie near 12149000 0.1364 188 1.4344 0.1515 -4.68E+04 25.81 11 Carnation

Snoqualmie near 12149000 -0.1335 -184 -1.4037 0.1604 5.15E+04 -23.57 12 Carnation

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions C-5

C6. Results of Mann-Kendall Trend Test on Monthly Maximum Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month

Middle Fork near Tanner 12141300 -0.0385 -53 -0.3989 0.69 4.52E+04 -19.32 1 Middle Fork near Tanner 12141300 -0.0697 -96 -0.7288 0.4661 2.43E+04 -11.06 2

Middle Fork near Tanner 12141300 0.1662 229 1.7492 0.0803 -3.54E+04 18.99 3

Middle Fork near Tanner 12141300 0.0871 120 0.9129 0.3613 -2.63E+04 14.61 4

Middle Fork near Tanner 12141300 0.0254 35 0.2608 0.7942 -5485 4.575 5

Middle Fork near Tanner 12141300 -0.029 -40 -0.2992 0.7648 1.27E+04 -4.762 6

Middle Fork near Tanner 12141300 -0.0878 -121 -0.9206 0.3572 1.60E+04 -7.303 7

Middle Fork near Tanner 12141300 -0.1662 -229 -1.7492 0.0803 1.36E+04 -6.527 8

Middle Fork near Tanner 12141300 -0.0994 -137 -1.0433 0.2968 1.98E+04 -9.348 9

Middle Fork near Tanner 12141300 0.0936 129 0.9819 0.3262 -4.33E+04 23.41 10

Middle Fork near Tanner 12141300 0.1343 185 1.412 0.1579 -8.36E+04 45 11

Middle Fork near Tanner 12141300 -0.0864 -119 -0.9052 0.3654 6.28E+04 -28.86 12

North Fork Snoqualmie 12142000 -0.0038 -5 -0.0316 0.9748 5299 -0.9881 1 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.0008 -1 0 1 1100 0 2 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1961 260 2.0439 0.041 -2.72E+04 14.28 3 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1124 149 1.168 0.2428 -1.46E+04 7.941 4 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1252 166 1.3024 0.1928 -1.16E+04 6.524 5 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.0513 68 0.5287 0.597 -4114 2.633 6 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.0558 -74 -0.5761 0.5646 3971 -1.7 7 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.1305 -173 -1.3573 0.1747 4621 -2.2 8 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.0317 -42 -0.3235 0.7463 4081 -1.732 9 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.098 135 1.028 0.304 -1.84E+04 10 10 near Snoqualmie Falls

North Fork Snoqualmie 12142000 0.1851 255 1.9485 0.0514 -5.39E+04 28.49 11 near Snoqualmie Falls

North Fork Snoqualmie 12142000 -0.0617 -85 -0.6444 0.5193 2.23E+04 -10 12 near Snoqualmie Falls

Raging River near 12145500 -0.0047 -6 -0.0406 0.9676 1583 -0.3125 1 Fall City

Raging River near 12145500 -0.0055 -7 -0.0487 0.9611 852.5 -0.1622 2 Fall City

Raging River near 12145500 0.12 153 1.2347 0.217 -5771 3.125 3 Fall City

Raging River near 12145500 0.1137 145 1.1697 0.2421 -4117 2.25 4 Fall City

July 2018 C-6 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

C6. Results of Mann-Kendall Trend Test on Monthly Maximum Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month

Raging River near 12145500 0.2424 309 2.5021 0.0123 -5451 2.848 5 Fall City Raging River near 12145500 0.0078 10 0.0731 0.9417 3.828 0.06897 6 Fall City Raging River near 12145500 -0.1153 -147 -1.1864 0.2355 811.5 -0.3824 7 Fall City Raging River near 12145500 -0.1224 -156 -1.2605 0.2075 440.4 -0.2083 8 Fall City Raging River near 12145500 -0.1271 -162 -1.308 0.1909 1154 -0.55 9 Fall City Raging River near 12145500 0.171 218 1.7627 0.078 -6357 3.31 10 Fall City Raging River near 12145500 0.1435 183 1.4783 0.1393 -1.19E+04 6.278 11 Fall City Raging River near 12145500 -0.0353 -45 -0.3574 0.7208 3776 -1.5 12 Fall City South Fork Snoqualmie 12143400 -0.0505 -67 -0.5208 0.6025 1.29E+04 -5.771 1 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0754 -100 -0.7812 0.4347 7473 -3.473 2 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1305 173 1.3573 0.1747 -8271 4.435 3 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.1267 168 1.3178 0.1876 -9318 5.093 4 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.0437 58 0.45 0.6527 -2114 1.589 5 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0747 -99 -0.7734 0.4393 5596 -2.45 6 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0943 -125 -0.9786 0.3278 3014 -1.394 7 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.1772 -235 -1.847 0.0648 2020 -0.9673 8 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0558 -74 -0.5761 0.5646 1555 -0.6703 9 above Alice Springs near Garcia South Fork Snoqualmie 12143400 0.0639 88 0.6674 0.5045 -5070 2.843 10 above Alice Springs near Garcia

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions C-7

C6. Results of Mann-Kendall Trend Test on Monthly Maximum Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month South Fork Snoqualmie 12143400 0.1183 163 1.2427 0.214 -1.76E+04 9.421 11 above Alice Springs near Garcia South Fork Snoqualmie 12143400 -0.0718 -99 -0.7518 0.4522 1.20E+04 -5.456 12 above Alice Springs near Garcia Snoqualmie near 12149000 -0.0522 -72 -0.5446 0.586 1.45E+05 -64.14 1 Carnation Snoqualmie near 12149000 -0.0327 -45 -0.3375 0.7357 3.98E+04 -15.86 2 Carnation Snoqualmie near 12149000 0.1197 165 1.258 0.2084 -7.53E+04 41.52 3 Carnation Snoqualmie near 12149000 0.0566 78 0.5907 0.5547 -3.25E+04 20 4 Carnation Snoqualmie near 12149000 0.0588 81 0.6137 0.5394 -3.10E+04 19.72 5 Carnation Snoqualmie near 12149000 -0.0327 -45 -0.3375 0.7357 2.07E+04 -6.983 6 Carnation Snoqualmie near 12149000 -0.0972 -134 -1.0203 0.3076 3.32E+04 -15 7 Carnation Snoqualmie near 12149000 -0.172 -237 -1.8103 0.0702 3.43E+04 -16.39 8 Carnation Snoqualmie near 12149000 -0.1081 -149 -1.1353 0.2563 5.07E+04 -24.07 9 Carnation Snoqualmie near 12149000 0.0849 117 0.8899 0.3735 -6.97E+04 38.33 10 Carnation

Snoqualmie near 12149000 0.1451 200 1.5265 0.1269 -2.13E+05 113.8 11 Carnation

Snoqualmie near 12149000 -0.0544 -75 -0.5677 0.5703 8.85E+04 -37.92 12 Carnation

Snoqualmie near 12144500 -0.0515 -71 -0.537 0.5912 1.16E+05 -51.89 1 Snoqualmie

Snoqualmie near 12144500 -0.0515 -71 -0.537 0.5913 3.73E+04 -15.86 2 Snoqualmie

Snoqualmie near 12144500 0.1597 220 1.68 0.093 -8.22E+04 43.94 3 Snoqualmie

Snoqualmie near 12144500 0.1009 139 1.0586 0.2898 -5.77E+04 31.87 4 Snoqualmie

Snoqualmie near 12144500 0.0755 104 0.7901 0.4295 -2.60E+04 16.52 5 Snoqualmie

Snoqualmie near 12144500 -0.0174 -24 -0.1764 0.86 1.17E+04 -2.847 6 Snoqualmie

Snoqualmie near 12144500 -0.0871 -120 -0.9129 0.3613 2.28E+04 -10.12 7 Snoqualmie

July 2018 C-8 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

C6. Results of Mann-Kendall Trend Test on Monthly Maximum Discharge at Gaging Stations Shown in Figure 12.

Station Name Station No. Tau S z p Intercept Slope Month

Snoqualmie near 12144500 -0.1669 -230 -1.7566 0.079 2.27e+04 -10.76 8 Snoqualmie

Snoqualmie near 12144500 -0.0776 -107 -0.8131 0.4162 2.70E+04 -12.37 9 Snoqualmie

Snoqualmie near 12144500 0.0893 123 0.9359 0.3493 -6.24E+04 34.36 10 Snoqualmie

Snoqualmie near 12144500 0.1343 185 1.4115 0.1581 -1.69E+05 90.19 11 Snoqualmie

Snoqualmie near 12144500 -0.0856 -118 -0.8975 0.3694 1.17E+05 -53.41 12 Snoqualmie

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions C-9

APPENDIX D

Trends for Water Year Precipitation, 1962–2015

Tau – Kendall’s correlation coefficient S – Kendall’s S statistic Z – Kendall’s standard normal deviate p – p-value for significance of trend intercept – intercept of trend line (where year equals first water year in test series) slope – slope of trend line (units vary)

D1. Results of Mann-Kendall Trend Test on Annual Total Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Buckley -0.0114 -14 -0.1087 0.9134 56.98 -0.004359 Cedar Lake -0.0217 -31 -0.2238 0.8229 203.2 -0.05279 Everett 0.1551 222 1.6488 0.0992 -140.4 0.08929 Monroe 0.0091 13 0.0895 0.9287 35.99 0.0062 Sea-Tac 0.0356 51 0.3730 0.7091 -16.89 0.0271 South Fork Tolt 0.3348 444 3.4957 0.0005 -925.6 0.5141 Snoqualmie Falls -0.0437 -58 -0.4498 0.6529 171.4 -0.05621 Startup 0.0524 75 0.5521 0.5809 -64.92 0.06472

D2. Results of Mann-Kendall Trend Test on Annual Maximum Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Buckley 0.0727 89 0.7363 0.4616 -5.34 0.003571 Cedar Lake 0.0335 48 0.3507 0.7258 -2.209 0.002778 Everett 0.1614 231 1.7167 0.0860 -8.145 0.0048 Monroe 0.0992 142 1.0523 0.2927 -5.872 0.003871 Sea-Tac 0.1908 273 2.0296 0.0424 -18.86 0.0103 South Fork Tolt 0.3145 417 3.2828 0.0010 -49.94 0.02655 Snoqualmie Falls -0.0905 -120 -0.9393 0.3476 10.28 -0.003957 Startup 0.1335 191 1.4180 0.1562 -11.54 0.006977

D3. Results of Mann-Kendall Trend Test on Monthly Total Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Buckley -0.0327 -40 -0.3262 0.7442 22.94 -0.008333 1 Buckley -0.0906 -111 -0.9202 0.3575 47.03 -0.02158 2 Buckley 0.1722 211 1.7567 0.0790 -51.24 0.02818 3 Buckley 0.0318 39 0.3179 0.7506 -11.57 0.007895 4 Buckley 0.1894 232 1.9323 0.0533 -51.51 0.02756 5 Buckley 0.0057 7 0.0502 0.9600 -0.2649 0.001515 6

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions D-1

D3. Results of Mann-Kendall Trend Test on Monthly Total Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Buckley -0.1249 -153 -1.2716 0.2035 23.42 -0.01122 7 Buckley -0.0890 -109 -0.9035 0.3662 20.99 -0.01 8 Buckley -0.1771 -217 -1.8071 0.0708 52.97 -0.02537 9 Buckley 0.0686 84 0.6943 0.4875 -18.54 0.01133 10 Buckley 0.1069 131 1.0874 0.2768 -64.51 0.036 11 Buckley -0.1420 -174 -1.4472 0.1478 69.57 -0.03214 12 Cedar Lake -0.1237 -177 -1.3130 0.1892 170 -0.07893 1 Cedar Lake -0.1111 -159 -1.1787 0.2385 90.29 -0.04081 2 Cedar Lake 0.0797 114 0.8432 0.3991 -52.15 0.0312 3 Cedar Lake 0.0678 97 0.7162 0.4738 -31.75 0.02 4 Cedar Lake 0.2222 318 2.3650 0.0180 -83.99 0.04541 5 Cedar Lake 0.0056 8 0.0522 0.9584 -0.8245 0.003226 6 Cedar Lake -0.1237 -177 -1.3131 0.1891 36.13 -0.01706 7 Cedar Lake -0.0692 -99 -0.7313 0.4646 26.72 -0.01238 8 Cedar Lake -0.0328 -47 -0.3432 0.7314 23.92 -0.009592 9 Cedar Lake 0.0706 101 0.7461 0.4556 -31.9 0.02 10 Cedar Lake 0.0887 127 0.9400 0.3472 -76.89 0.04571 11 Cedar Lake -0.2187 -313 -2.3278 0.0199 183.1 -0.0861 12 Everett 0.0503 72 0.5297 0.5963 -13.32 0.008889 1 Everett 0.0419 60 0.4402 0.6598 -9.524 0.006333 2 Everett 0.0999 143 1.0594 0.2894 -23.99 0.014 3 Everett 0.1901 272 2.0219 0.0432 -34.35 0.01875 4 Everett 0.2048 293 2.1786 0.0294 -40.46 0.02158 5 Everett 0.0524 75 0.5522 0.5808 -13.42 0.007778 6 Everett -0.1244 -178 -1.3207 0.1866 17.97 -0.008571 7 Everett -0.0790 -113 -0.8357 0.4033 12.88 -0.005833 8 Everett -0.1027 -147 -1.0893 0.2760 27.8 -0.013 9 Everett 0.1202 172 1.2758 0.2020 -38.49 0.02091 10 Everett 0.1153 165 1.2235 0.2211 -40.45 0.02286 11 Everett -0.0881 -126 -0.9326 0.3510 37.17 -0.016 12 Monroe -0.0992 -142 -1.0519 0.2928 51.64 -0.02296 1 Monroe -0.0538 -77 -0.5670 0.5707 18.16 -0.007083 2 Monroe 0.1090 156 1.1565 0.2475 -34.75 0.02 3 Monroe 0.0545 78 0.5745 0.5656 -8.406 0.006 4 Monroe 0.2432 348 2.5888 0.0096 -51.68 0.0275 5 Monroe -0.0245 -35 -0.2537 0.7998 9.375 -0.003469 6 Monroe -0.1076 -154 -1.1415 0.2536 17.35 -0.008095 7 Monroe -0.1041 -149 -1.1043 0.2695 22.85 -0.01071 8 Monroe -0.0978 -140 -1.0371 0.2997 32.66 -0.015 9 Monroe 0.0999 143 1.0594 0.2894 -38.6 0.02146 10 Monroe 0.1013 145 1.0743 0.2827 -40.91 0.02409 11

July 2018 D-2 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

D3. Results of Mann-Kendall Trend Test on Monthly Total Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Monroe -0.1139 -163 -1.2086 0.2268 58.2 -0.02583 12 Sea-Tac -0.0238 -34 -0.2462 0.8055 13.33 -0.004 1 Sea-Tac -0.0720 -103 -0.7610 0.4467 28.41 -0.0125 2 Sea-Tac 0.1579 226 1.6786 0.0932 -36.18 0.02005 3 Sea-Tac 0.1314 188 1.3952 0.1630 -29.62 0.01619 4 Sea-Tac 0.2117 303 2.2533 0.0242 -33.94 0.01789 5 Sea-Tac -0.0147 -21 -0.1492 0.8814 3.682 -0.001158 6 Sea-Tac -0.1230 -176 -1.3059 0.1916 12.15 -0.005789 7 Sea-Tac -0.0335 -48 -0.3507 0.7258 6.059 -0.002703 8 Sea-Tac -0.1181 -169 -1.2536 0.2100 25.93 -0.01231 9 Sea-Tac 0.0936 134 0.9923 0.3211 -23.42 0.01333 10 Sea-Tac 0.0419 60 0.4402 0.6598 -12.73 0.009082 11 Sea-Tac -0.1474 -211 -1.5668 0.1172 80.02 -0.03743 12 South Fork Tolt 0.1011 134 1.0495 0.2939 -104.2 0.05882 1 South Fork Tolt 0.0626 83 0.6471 0.5176 -25.17 0.01677 2 South Fork Tolt 0.3220 427 3.3617 0.0008 -214.4 0.1126 3 South Fork Tolt 0.2172 288 2.2647 0.0235 -128.3 0.06886 4 South Fork Tolt 0.1931 256 2.0123 0.0442 -86.49 0.04695 5 South Fork Tolt 0.0935 124 0.9707 0.3317 -42.15 0.02391 6 South Fork Tolt -0.1131 -150 -1.1758 0.2397 48.6 -0.02302 7 South Fork Tolt -0.0626 -83 -0.6471 0.5176 26.43 -0.01214 8 South Fork Tolt 0.0362 48 0.3709 0.7107 -13.72 0.009598 9 South Fork Tolt 0.1795 238 1.8703 0.0614 -139 0.07404 10 South Fork Tolt 0.2051 272 2.1385 0.0325 -204.7 0.1092 11 South Fork Tolt -0.0528 -70 -0.5446 0.5861 36.74 -0.01234 12 Snoqualmie Falls -0.0518 -66 -0.5280 0.5975 50.82 -0.02095 1 Snoqualmie Falls -0.0761 -97 -0.7797 0.4355 43.62 -0.01913 2 Snoqualmie Falls 0.1114 142 1.1453 0.2521 -43.86 0.025 3 Snoqualmie Falls 0.0204 26 0.2031 0.8391 -2.506 0.003571 4 Snoqualmie Falls 0.1631 208 1.6815 0.0927 -40.74 0.02226 5 Snoqualmie Falls 0.0008 1 0.0000 1.0000 2.065 0.000375 6 Snoqualmie Falls -0.1741 -222 -1.7953 0.0726 28.05 -0.01341 7 Snoqualmie Falls -0.0416 -53 -0.4224 0.6728 8.77 -0.00381 8 Snoqualmie Falls -0.0094 -12 -0.0893 0.9288 5.388 -0.001111 9 Snoqualmie Falls 0.0525 67 0.5361 0.5919 -26.56 0.01563 10 Snoqualmie Falls 0.0525 67 0.5361 0.5919 -35.75 0.0225 11 Snoqualmie Falls -0.1388 -177 -1.4295 0.1529 81.71 -0.03667 12 Startup 0.0021 3 0.0149 0.9881 6.724 0.001042 1 Startup -0.0818 -117 -0.8655 0.3868 37.68 -0.0163 2 Startup 0.1241 171 1.3041 0.1922 -50.19 0.02833 3 Startup 0.0769 110 0.8133 0.4161 -17.15 0.01136 4

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions D-3

D3. Results of Mann-Kendall Trend Test on Monthly Total Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Startup 0.1349 193 1.4326 0.1520 -36.85 0.0208 5 Startup 0.0218 30 0.2225 0.8240 -5.028 0.004305 6 Startup -0.1516 -217 -1.6115 0.1071 40.24 -0.01932 7 Startup -0.0894 -128 -0.9476 0.3434 32.85 -0.01563 8 Startup -0.0797 -114 -0.8430 0.3992 31.71 -0.01415 9 Startup 0.1440 206 1.5295 0.1261 -71.51 0.03868 10 Startup 0.1495 214 1.5891 0.1120 -89.08 0.04933 11 Startup -0.1335 -191 -1.4176 0.1563 82.23 -0.03704 12

D4. Results of Mann-Kendall Trend Test on Monthly Maximum Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Buckley 0.0008 1 0.0000 1.0000 1.155 0 1 Buckley -0.0416 -51 -0.4184 0.6756 4.863 -0.002 2 Buckley 0.1894 232 1.9331 0.0532 -12.98 0.006923 3 Buckley 0.0792 97 0.8033 0.4218 -5.194 0.003 4 Buckley 0.1037 127 1.0546 0.2916 -5.847 0.003333 5 Buckley -0.0514 -63 -0.5189 0.6038 4.695 -0.001935 6 Buckley -0.0629 -77 -0.6359 0.5248 4.87 -0.002188 7 Buckley -0.0767 -94 -0.7784 0.4363 6.469 -0.003 8 Buckley -0.1478 -181 -1.5061 0.1320 15.6 -0.0075 9 Buckley 0.0612 75 0.6193 0.5357 -3.827 0.002353 10 Buckley 0.0857 105 0.8702 0.3842 -8.823 0.005 11 Buckley -0.0033 -4 -0.0251 0.9800 1.08 0 12 Cedar Lake -0.0601 -86 -0.6342 0.5260 14.93 -0.006389 1 Cedar Lake -0.1146 -164 -1.2161 0.2239 20.17 -0.009286 2 Cedar Lake 0.0154 22 0.1567 0.8755 -0.1427 0.0009091 3 Cedar Lake 0.0133 19 0.1344 0.8931 0.4608 0.0005 4 Cedar Lake 0.2180 312 2.3208 0.0203 -17.98 0.009655 5 Cedar Lake -0.0650 -93 -0.6864 0.4924 10.25 -0.004545 6 Cedar Lake -0.0713 -102 -0.7536 0.4511 7.808 -0.003529 7 Cedar Lake 0.0769 110 0.8134 0.4160 -6.217 0.003571 8 Cedar Lake 0.0321 46 0.3357 0.7371 -3.12 0.002195 9 Cedar Lake 0.1034 148 1.0968 0.2727 -14.29 0.008 10 Cedar Lake -0.0196 -28 -0.2015 0.8403 4.358 -0.001111 11 Cedar Lake -0.1726 -247 -1.8356 0.0664 21.26 -0.0096 12 Everett 0.0342 49 0.3584 0.7200 -1.109 0.0009524 1 Everett 0.0098 14 0.0970 0.9227 9.26E-02 0.0002703 2 Everett 0.1293 185 1.3735 0.1696 -5.296 0.00303 3 Everett 0.0678 97 0.7165 0.4737 -3.008 0.001842 4

July 2018 D-4 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions

D4. Results of Mann-Kendall Trend Test on Monthly Maximum Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month Everett 0.1125 161 1.1941 0.2324 -3.789 0.002222 5 Everett -0.0818 -117 -0.8657 0.3866 6.008 -0.002667 6 Everett -0.0915 -131 -0.9701 0.3320 5.964 -0.002778 7 Everett 0.0370 53 0.3881 0.6979 -1.214 0.0008571 8 Everett -0.2145 -307 -2.2839 0.0224 14.22 -0.00684 9 Everett 0.1076 154 1.1424 0.2533 -5.825 0.003333 10 Everett 0.2355 337 2.5076 0.0122 -16.08 0.008571 11 Everett -0.0363 -52 -0.3807 0.7034 3.236 -0.001143 12 Monroe -0.0175 -25 -0.1791 0.8578 2.145 -0.0005556 1 Monroe -0.0105 -15 -0.1045 0.9168 1.277 -0.0002273 2 Monroe 0.1391 199 1.4775 0.1395 -7.662 0.004286 3 Monroe 0.0126 18 0.1269 0.8990 0.1422 0.0003333 4 Monroe 0.1964 281 2.0902 0.0366 -9.168 0.005 5 Monroe -0.0538 -77 -0.5672 0.5706 3.86 -0.001622 6 Monroe -0.0636 -91 -0.6717 0.5018 4.672 -0.002093 7 Monroe 0.0091 13 0.0896 0.9286 8.52E-02 0.0002941 8 Monroe -0.1146 -164 -1.2165 0.2238 10.51 -0.004878 9 Monroe 0.0419 60 0.4402 0.6598 -2.388 0.001667 10 Monroe 0.1104 158 1.1715 0.2414 -10.67 0.006 11 Monroe -0.0881 -126 -0.9328 0.3509 9.474 -0.004118 12 Sea-Tac 0.0070 10 0.0672 0.9465 0.1631 0.0004762 1 Sea-Tac -0.1908 -273 -2.0308 0.0423 13.05 -0.00619 2 Sea-Tac 0.0650 93 0.6867 0.4923 -3.694 0.002222 3 Sea-Tac 0.0377 54 0.3955 0.6925 -2.142 0.001389 4 Sea-Tac 0.0678 97 0.7165 0.4737 -2.223 0.001379 5 Sea-Tac -0.1055 -151 -1.1195 0.2629 6.576 -0.003056 6 Sea-Tac -0.0650 -93 -0.6868 0.4922 3.604 -0.001667 7 Sea-Tac -0.0203 -29 -0.2090 0.8345 1.289 -0.0005 8 Sea-Tac -0.0643 -92 -0.6792 0.4970 4.527 -0.002 9 Sea-Tac 0.0727 104 0.7686 0.4421 -3.536 0.002174 10 a-Tac 0.1349 193 1.4326 0.1520 -10.92 0.006129 11 Sea-Tac -0.1258 -180 -1.3358 0.1816 9.344 -0.004138 12 South Fork Tolt 0.1033 137 1.0733 0.2831 -15.27 0.008666 1 South Fork Tolt 0.1131 150 1.1759 0.2396 -12.53 0.007082 2 South Fork Tolt 0.3348 444 3.4964 0.0005 -30.99 0.01628 3 South Fork Tolt 0.1606 213 1.6732 0.0943 -18.86 0.01022 4 South Fork Tolt 0.1463 194 1.5236 0.1276 -9.868 0.005626 5 South Fork Tolt 0.0234 31 0.2368 0.8128 -1.341 0.001277 6 South Fork Tolt -0.0603 -80 -0.6236 0.5329 6.711 -0.002871 7 South Fork Tolt 0.0724 96 0.7498 0.4534 -7.247 0.00405 8 South Fork Tolt 0.0370 49 0.3788 0.7048 -6.156 0.00381 9

July 2018 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions D-5

D4. Results of Mann-Kendall Trend Test on Monthly Maximum Precipitation at Gaging Stations Shown in Figure 20. Station Tau S z p Intercept Slope Month South Fork Tolt 0.1478 196 1.5390 0.1238 -17.09 0.009282 10 South Fork Tolt 0.2225 295 2.3203 0.0203 -29.21 0.01568 11 South Fork Tolt -0.0158 -21 -0.1579 0.8746 2.986 -0.0005159 12 Snoqualmie Falls -0.0165 -21 -0.1625 0.8709 3.512 -0.0009524 1 Snoqualmie Falls -0.0816 -104 -0.8367 0.4028 10.53 -0.004762 2 Snoqualmie Falls 0.0471 60 0.4794 0.6317 -1.848 0.001463 3 Snoqualmie Falls -0.0235 -30 -0.2357 0.8137 2.472 -0.0007407 4 Snoqualmie Falls 0.0627 80 0.6419 0.5209 -3.021 0.001923 5 Snoqualmie Falls 0.0471 60 0.4794 0.6317 -2.219 0.001538 6 Snoqualmie Falls -0.1514 -193 -1.5599 0.1188 12.1 -0.005789 7 Snoqualmie Falls 0.0541 69 0.5525 0.5806 -3.416 0.001961 8 Snoqualmie Falls 0.0227 29 0.2275 0.8200 -1.279 0.001071 9 Snoqualmie Falls 0.0220 28 0.2193 0.8264 -0.8319 0.0009677 10 Snoqualmie Falls 0.0855 109 0.8776 0.3802 -11.14 0.006429 11 Snoqualmie Falls -0.0667 -85 -0.6824 0.4950 11.71 -0.005 12 Startup -0.0056 -8 -0.0522 0.9583 1.995 -0.0002439 1 Startup 0.0014 2 0.0075 0.9940 1.03 0 2 Startup 0.1988 274 2.0956 0.0361 -10.25 0.005729 3 Startup -0.0063 -9 -0.0597 0.9524 1.548 -0.0002857 4 Startup -0.0769 -110 -0.8134 0.4160 6.018 -0.002571 5 Startup -0.0581 -80 -0.6062 0.5444 5.373 -0.002265 6 Startup -0.0860 -123 -0.9104 0.3626 8.88 -0.004167 7 Startup -0.0014 -2 -0.0075 0.9940 0.675 0 8 Startup -0.1118 -160 -1.1863 0.2355 14.69 -0.006875 9 Startup 0.1628 233 1.7310 0.0835 -18.63 0.01 10 Startup 0.2180 312 2.3206 0.0203 -20.84 0.01133 11 Startup -0.0294 -42 -0.3060 0.7596 4.559 -0.001481 12

July 2018 D-6 Snoqualmie River Hydraulic Study—Hydrologic Evaluation of Flooding Trends and Current Conditions