Hydromodification Assessment

August 31, 2015

Cover: Oswego Creek below the outlet to Sucker Lake with remnants of an early mill dam. Photo courtesy of the Lake Oswego Library.

Table of Contents 1.0 Introduction ...... 1 2.0 Background ...... 2 2.1 Hydromodification Causes ...... 2 2.2 Hydromodification Impacts ...... 4 3.0 Watershed Conditions ...... 7 3.1 Physical Attributes ...... 7 3.1.1 Geology and Soils ...... 7 3.1.2 Drainage and Physiography ...... 12 3.1.3 Hydrology ...... 14 3.2 Development History ...... 14 3.2.1 Urbanization History ...... 14 3.2.2 Riparian Land Ownership ...... 20 3.2.3 Drainage Infrastructure ...... 20 3.3 Hydromodification Characterization ...... 20 3.3.1 Desktop Assessment ...... 21 3.3.2 Field Assessment ...... 23 4.0 Hydromodification Management Strategies ...... 25 4.1 Collect Information On Waterbodies ...... 25 4.2 Mitigate Hydromodification Impacts and Reduce Hydromodification ...... 26 4.2.1 Current City Code and Draft Stormwater Manual ...... 28 4.2.2 Retrofit Strategy for Hydromodification ...... 31 5.0 Conclusions ...... 36

List of Tables

Table 1. Estimated flow frequencies for selected Lake Oswego Creeks ...... 16 Table 2. Available hydromodification reduction strategies ...... 27 Table 3. Hydromodification risk ranking for Lake Oswego sub‐watersheds ...... 30 Table 4. Water quality retrofit projects with substantial hydromodification benefits as prioritized for FY15‐16 to FY20‐21 ...... 35

ii List of Figures

Figure 1. Typical changes in runoff hydrographs as a result of urbanization ...... 3 Figure 2. Hydromodification impact risk as a function of hydrologic changes and channel susceptibility ...... 6 Figure 3. Geologic map with Missoula flood elevation (400 ft) and overlay of hydrologic soil groups ...... 10 Figure 4. Soil erodibility as measured by "K‐factor" ...... 11 Figure 5. Physiography and major watersheds in Lake Oswego ...... 13 Figure 6. Rainfall‐runoff for a typical storm, March 13‐16, 2015 ...... 15 Figure 7. History of urban development in Lake Oswego, 1954‐2011 ...... 17 Figure 8. Annexation history of Lake Oswego ...... 18 Figure 9. Current land use in Lake Oswego ...... 19 Figure 10. Expected hydrologic impacts from urbanization for Lake Oswego subwatersheds based on the screening method of Leopold (1968) ...... 22 Figure 11. Stream channels within the City locally exhibit both stability and instability in the face of hydromodification ...... 24 Figure 12. Stormwater standards proposed for adoption in the Lake Oswego Stormwater Management Manual [August 2015 Review Draft] ...... 32 Figure 13. Locations of Lake Oswego’s ongoing and future CIP prioritized retrofit projects with hydromodification components, identified by CIP rank...... 34

Appendix

Appendix A: Composite Profiles & Map of Profiled Streams

iii 1.0 Introduction The City of Lake Oswego is responsible for managing stormwater and surface water quality through both a National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) permit and through implementation of total maximum daily loads (TMDLs) on the Tualatin and Willamette Rivers. This document represents compliance with the City of Lake Oswego’s MS4 permit, Schedule A.5, which requires completion of a City‐wide hydromodification Assessment. Per Schedule A.5, the assessment must:

… Examines [sic] the hydromodification impacts related to the co-permittee’s MS4 discharges, including erosion, sedimentation, and alteration to stormwater flow, volume and duration that may cause or contribute to water quality degradation. The report shall describe existing efforts and proposed actions the co-permittee has identified to address the following objectives:

a. Collect and maintain information that will inform future stormwater management decisions related to hydromodification based on local conditions and needs;

b. Identify or develop strategies to address hydromodification information or data gaps related to waterbodies within the co-permittee’s jurisdiction;

c. Identify strategies and priorities for preventing or reducing hydromodification impacts related to the co-permittee’s MS4 discharges; and,

d. Identify or develop effective tools to reduce hydromodification. The remainder of this document is divided into the following sections:

2.0 Background: what is hydromodification? what are its impacts?

3.0 Watershed Conditions:

 Conditions of geology, soils, physiography, hydrology, development history and land use that impact hydromodification  Characterization of City‐specific hydromodification

4.0 Hydromodification Strategies:

 Collection and management of hydromodification‐related information  Existing and proposed strategies to address hydromodification impacts

5.0 Conclusions.

1 August 31, 2015 2.0 Background The City of Lake Oswego is located in the greater Portland metropolitan area, immediately west of the Willamette River and north of the Tualatin River. Portland and a small portion of unincorporated Clackamas County border the city’s urban services boundary to the north, while the southern border is comprised of the cities of Rivergrove and West Linn along with unincorporated Clackamas County both inside and outside of the urban growth boundary. The area within the City’s urban services boundary and city limits is approximately 11.2 square miles. Current population of the city is approximately 38,000, resulting in a population density of 3400/mi2. This population density is lower than most of the Portland metropolitan area, although it is similar to values found in the Portland Hills to the north and the Tualatin Mountains to the west and south.

2.1 Hydromodification Causes Hydromodification results from the changes in land cover due to urban development that either directly or indirectly result in elevated streamflow volumes and flood peaks over undeveloped conditions.1 Development or redevelopment typically creates impervious surfaces that shed rain as runoff rather than allowing rainfall to infiltrate into the soil. Construction activities over soil that is later covered with vegetation reduces the infiltration capacity of the soil as well. Together, these conditions increase the volume of runoff for a given rainfall event. Because runoff can result in an urban flooding nuisance, it is typically further routed off of the landscape in stormwater drainage infrastructure. This infrastructure consists of curbs and gutters on streets, roadside ditches, and storm sewer pipes to locally collect runoff and deliver it to receiving waterways. Such infrastructure reduces the time over which runoff is delivered to receiving waterways (lag time or “time of concentration” in runoff models), resulting in higher peak flows for a given runoff event. Figure 1 shows the typical changes in runoff hydrographs as a result of urbanization. Finally, to aid conveyance of floodwaters, many urban stream reaches have been straightened and cleared of wood or other obstructions. While this isn’t always a direct cause of hydromodification, it can exacerbate the hydrological impacts of urbanization.

1 Leopold, L.B. 1968. Hydrology for urban land planning – a guidebook on the hydrologic effects of urban land use. U.S. Geological Survey Circular 554. 18 p.

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Figure 1. Typical changes in runoff hydrographs as a result of urbanization (after Leopold [1968])

3 August 31, 2015 2.2 Hydromodification Impacts If the change in hydrology represents the risk of occurrence, the response of stream channels to the changes in hydrology are the focus of the impact risk from hydromodification. Stream channel erosion is a function of stream power, which is the product of discharge (runoff volume/unit time) and the channel slope. Channel beds and banks are eroded when stream power exceeds the local threshold for erosion; this threshold is a function of the erodibility of the bed and bank materials. Alluvial channels that can fully adjust to hydrologic inputs form such that the erosion threshold of bedload material occurs at the flow that just begins to inundate the floodplain (termed “bankfull” flow), which typically occurs or is exceeded in two years out of three.2 Increased peak flows in urban environments are more likely to exceed the erodibility threshold of bed and bank materials, resulting in enlargement of stream channel cross sectional area to the point at which a new stable geometry develops.3 Research by MacRae suggests that in urban environments, runoff events below bankfull levels are also significant for eroding channels, both because these events occur so much more frequently in an urban setting than in undisturbed basins and because urban environments tend to have higher concentrations of finer grained (more mobile) sediment in the fluvial system.4

Eroding stream channels result in elevated concentrations of suspended sediment downstream, and concentrations of the many pollutants that can be associated with those suspended sediments (e.g., metals, nutrients, hydrocarbon compounds, many pesticides). Materials eroded from stream channels that are too coarse to move in suspension may not have water quality impacts, but may still result in downstream impacts: habitat is reduced if bedload sediments fill pools; deposition of coarse material that causes local channel aggradation can result in streambank erosion that further de‐stabilizes the stream channel.

The degree to which channel cross‐sectional enlargement occurs due to higher flood peaks, and the mix between erosion of the streambed to cause channel incision or erosion of the stream banks to cause channel widening, depends on the strength (erodibility) of bed and

2 Dunne, T., and L.B. Leopold. 1978. Water In Environmental Planning. WH Freeman. San Francisco, CA. 816 p.

3 Henshaw, P.C., and D.B. Booth. 2000. Natural restabilization of stream channels in urban watersheds. Journal of the American Water Resources Association. 36(6): 1219‐1236.

4 MacRae, C.R. 1997. Experience from morphological research on Canadian streams: is control of the two‐year frequency runoff event the best basis for stream channel protection? In: L.A. Roesner (ed.) Effects of Watershed Development and Management on Aquatic Ecosystems. Proceedings of an Engineering Foundation Conference, ASCE, New York, NY. p.4 14‐ 162.

4 August 31, 2015 bank materials (Figure 2). 5 Coarse grained materials and bedrock resist erosion where they are present, for instance. Stratified floodplain deposits are typically only as strong as the material at the base of the streambank. Vegetation roots can provide additional apparent cohesion to floodplain soils. In stable stream systems, the minimum enlargement ratio, measured as cross‐sectional area post‐development/pre‐development, is close to the ratio of post‐ to pre‐development peak discharge. Enlargement ratios up to 6 have been observed for watersheds with highly impervious land cover.

Stream channels can remain somewhat resilient in the face of hydromodification if local bank and floodplain vegetation and instream materials such as log jams are present that can dissipate stream power. Once stream channels have been cleared of wood or channelized to increase flood conveyance, or riparian vegetation has been removed by erosion or reduced in density due to streamside development, or the channel has incised below the elevation where the vegetation interacts with streamflows, inherent instream mechanisms to mitigate hydromodification impacts are substantially eliminated.

Another hydrologic impact associated with hydromodification is a reduction in baseflow, those flows that occur in the absence of active runoff. Rainfall that infiltrates into the ground re‐emerges in stream channels as groundwater. While surface runoff velocity is measured in feet per second, water movement in even shallow groundwater is typically measured in fractions of feet per day. In undisturbed watersheds, a greater proportion of the rainfall is able to re‐charge groundwater. In areas where rainfall is limited during the summer months, groundwater discharge makes up most of the streamflow. Lower summer streamflow results in higher instream temperatures, with resulting impacts on aquatic ecosystems.6 Lower baseflows may also result in reduced vigor or density of riparian vegetation, with resulting secondary impacts on stream channel stability.

5 Caraco, D. 2000. Dynamics of urban stream channel enlargement: The Practice of Watershed Protection. Center for Watershed Protection, Ellicott City, MD. Pages 99‐104.

6 Poff, N.L., B. P Bledsoe, and C.O. Cuhaciyan. 2006. Hydrologic variation with land use across the contiguous United States: geomorphic and ecological consequences for stream ecoystems. Geomorphology 79:264‐285.

Xie, D.M. and W. James. 1994. Modelling solar thermal enrichment in urban stormwater. Journal of Water Management Modeling. R176‐13, p.5 20‐ 219.

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Figure 2. Hydromodification impact risk as a function of hydrologic changes and channel susceptibility (after Brown & Caldwell, 2015)7

7 Brown & Caldwell. 2015. Hydromodification Assessment. Prepared for the City of Oregon City. Brown & Caldwell, Portland, Oregon. 35 p.

6 August 31, 2015 3.0 Watershed Conditions Several key conditions within Lake Oswego inform this hydromodification assessment, as described below and in the City’s Clean Streams Plan.8 These include the physical attributes of the City’s watersheds, and the City’s past and current development patterns that affect both the extent of hydromodification and potential practicable solutions to hydromodification impacts.

3.1 Physical Attributes The geology, soils and physiography, and hydrology in Lake Oswego watersheds play an important role in this hydromodification assessment.

3.1.1 Geology and Soils Geology Even in the face of urbanization, the geologic history of Lake Oswego exerts considerable control over how water moves through the landscape (Figure 3). It controls the locations of stream channels, the infiltration capacity of soils, the stability of slopes, and the local seasonality of streamflow.

The Lake Oswego region is founded on volcanic bedrock of three different ages. The northeast corner of the City is underlain by the Waverly Heights basalt and associated marine sediments, approximately 35 million years old; these rocks are evidence of the volcanic activity associated with the arrival of the last pieces of the Oregon Coast Range that rafted against the North American continent.9 Fifteen million years later, the Puget‐ Willamette trough began to form as the Coast Range was uplifted in response to offshore subduction; a pronounced structural basin formed in the Portland area. Four to five million years later, the Portland basin began to collect many hundreds of feet of Columbia River Basalt (CRB) from the Wanapum and Grande Ronde eruptive phases, each of which consists of multiple discrete flows. Sediments deposited between these flows, iron‐enriched over millennia of weathering, formed the ores that first attracted the smelting industry to the Lake Oswego area. Groundwater has preferential pathways in these interflow zones, locally resulting in springs. While the CRBs were being deposited in the area, the Portland Hills were rising, eventually diverting the course of the Columbia River northward. Finally, within the last 2 million years, volcanic centers of Boring Lava erupted across the Portland

8 City of Lake Oswego. 2009. Clean Streams Plan. Prepared by Otak, Inc., Lake Oswego, Oregon. November.

9 Evarts, R.C., J. E. O’Connor, R. E. Wells, and I. P. Madin. 2009. The Portland Basin: a (big) river runs through it. GSA Today. 19:4‐10.

7 August 31, 2015 basin; Mt. Sylvania in the northwest corner of the City and Cooks Butte near the southern boundary are examples of Boring volcanoes.

Overlying the volcanic rocks across large areas of the City are sediment from four distinctive depositional episodes. The oldest of these consist of small patches of sediments shed off of the Cascades between the emplacement of the CRBs and the eruption of the Boring volcanoes are found in the upper Tryon Creek watershed. Contemporaneous with eruptions from the Boring volcanics, coarser sediments from the Cascades and the upper Willamette Valley were deposited in the Portland basin. Some of these, part of the Springwater Formation, are found on a terrace between Oswego Creek and Hallinan Elementary. By far the largest sedimentary deposits occurred as a result of the Missoula Floods. Floodwaters coursed down the Columbia multiple times between 16,000 and 12,000 years ago, rushing through the gap in the Portland hills now containing Oswego Lake. As flows expanded into the Tualatin basin, the coarsest materials dropped out. They now cover much of the western third of the City. Fine grained Missoula Flood deposits cover most of downtown, First Addition, and north and eastern portions of the North Shore/Country Club and Uplands neighborhoods. Only small patches of post‐Missoula alluvium can be found across the cityscape: on the Willamette River floodplain extending northward from the mouth of Tryon Creek, at the Hunt Club at the base of Iron Mountain, and on the Tualatin River floodplain in the southwestern portion of the City’s urban services boundary and in the City of Rivergrove.

Between the Missoula Flood deposits and the upper limit of Missoula flood inundation (approximately 400 feet asl)10, floodwaters removed much of the existing soil, along with bedrock, to form or enhance the steep slopes on the south side of Iron Mountain, around Lily Bay, and along the south shore Palisades. Above the inundation limit, deposits of Portland Hills silt cover the underlying volcanic bedrock.

Multiple historic and pre‐historic landslides have been mapped across the City.11 Most of these are located in the Tryon Creek watershed, underlain by the weathered and fractured Waverly Heights basalts. Large pre‐historic landslides were less common elsewhere in the City but were mapped on the southeastern flank of Mt. Sylvania, along South Shore between Westview and Canyon drives, on the western side of the Iron Mountain scarp,

10 Minervini, J.M., J.E. O’Connor, and R.E. Wells. 2003. Maps showing inundation depths, ice‐rafted erratics, and sedimentary facies of late Pleistocene Missoula floods in the Willamette Valley, Oregon. Open File Report 03‐408. 1 plate.

11 Burns, W.J. and S. Duplantis. 2010. Landslide inventory maps for the Lake Oswego quadrangle, Clackamas, Multnomah, and Washington Counties, Oregon. Oregon Department of Geology and Mineral Industries Interpretive Map 32.

8 August 31, 2015 near the mouth of Springbrook Creek, along the east side of the Blue Heron canal across Blue Heron Drive, and on the south bank of Oswego Creek midway between the dam and the mouth. Large historic (100 years or less) slides occurred between Glenwood Ct. and Glenmorrie Drive, above Lakeview Blvd. west of Fir Rd., and in the South Shore Natural Area. Smaller historic landslides are scattered across the City; a cluster of them are present along the steep slopes across which Green Bluff Drive has been constructed.

The combination of uplift of the Portland hills and emplacement of the Boring volcanoes has resulted in a relatively dense network of faults within the City, most of which trend either NW‐SE or NE‐SW (Figure 3). Multiple reaches of the City’s major creeks, as well as Oswego Lake, occur on the traces of these faults. Faults can also be a preferential pathway for groundwater movement.

Soils & Vegetation Most of the soils in the City are silt loams, with low permeability.12 Over 80% are in hydrologic groups C and D, with infiltration capacity for bare soils of 0.15 inches per hour or less (Figure 3). These soils pose a substantial limit on the effectiveness of infiltration as a stormwater management measure. Dense, deeply rooted vegetation is required to maintain infiltration capacity in these soils. Because soils within the City are very silt‐rich, they are also highly erodible, as evidenced by the proportion of soils with erodibility (K) factors  40 (Figure 4).

Based on the regional interagency 1851 vegetation reconstruction, prior to Anglo‐ European settlement the Lake Oswego area was dominated by Douglas fir forest.13 Across the landscape that would become the current City, the forest had varying amounts of other conifers (particularly along the Willamette River), and hardwoods such as maple (on and adjacent to Cooks Butte) and oak (in the western portion of the city, particularly adjacent to Oswego [then Sucker] Lake). Forest soils under these conditions are typically quite permeable in the organic‐rich surface layers, regardless of soil texture.

12 NRCS. nd. Web Soil Survey: Lake Oswego area of interest. Compilation of Clackamas, Multnomah, and Washington County soil surveys, various dates. http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm.

13 Tobalske, C. 2002. Oregon historic vegetation. Compilation by Oregon Natural Heritage Program/Oregon Spatial Data Library. 1:100,000. http://spatialdata.oregonexplorer.info/geoportal/catalog/search/resource/details.page?uuid=%7B46B08A1F‐ FB7B‐43E4‐B9FD‐AFFDF9F48B94%7D

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Figure 3. Geologic map with Missoula flood elevation (400 ft) and overlay of hydrologic soil groups. Sources: DOGAMI (geology), NRCS (soils), USGS (Missoula Flood inundation limit)

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Figure 4. Soil erodibility as measured by "K‐factor"

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3.1.2 Drainage and Physiography The City of Lake Oswego extends generally from ridgeline to ridgeline north and south of Oswego Lake (Figure 5). Most of the northeastern portion of the City drains to the Willamette River through Tryon Creek. While approximately two‐thirds of the City drains to the lake, the remainder of the city drains west to Fanno Creek (a tributary of the Tualatin River) through Carter and Ball Creeks, south directly to the Tualatin River (Pecan and Wilson Creeks and several unnamed drainages), and east to the Willamette River (e.g., Tryon, Hallinan, Glenmorrie, Brookhurst/Stonebridge, and Convent creeks). Oswego Lake also drains to the Willamette River; the inlet to the lake is a man‐made channel from the Tualatin River. Other than the two rivers and Tryon and Springbrook creeks, surface water drainages within the City are dominantly small headwater streams.

Much of the City’s land area is sloping, and some of it is quite steep (greater than 25% slope). Most of the streams in the Tryon Creek watershed, and streams draining higher elevations (e.g., Mt. Sylvania, Cooks Butte, and slopes facing the Willamette River) are naturally well incised into the surrounding topography as a result of continued uplift of the Portland Hills and potentially erosion by Missoula floodwaters. Knickpoints—locations of very high localized channel slopes—are common in most streams that flow over Columbia River basalts. The sill that forms the outlet of Oswego Lake (and, before the dam, Sucker Lake) is one of the larger examples.

Due to the steep slopes, erodible soils and local bedrock conditions, a large proportion of the land area within the City has been recently mapped as being of moderate or high risk of slope failures.14 The steep terrain adjacent to streams results in efficient delivery of sediment and other pollutants to watercourses. At the same time, the type and distribution of stormwater best management practices (BMPs) that can be used in the City are constrained. Infiltration‐based BMPs cannot be used to maximum effect due to slope stability concern, stormwater cannot be transferred from one sub‐watershed to another, and few parcels with enough flat area for regional facilities are available in the locations where they could be effective.

14 Burns, W.G., K.A. Mickelson, C.B. Jones, S.G. Pickner, K.L.B. Hughes, and R. Sleeter. 2013. Landslide Hazard and Risk Study of Northwestern Clackamas County, Oregon. Open File Report O‐13‐08. Oregon Department of Geology and Mineral Industries.

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Figure 5. Physiography and major watersheds in Lake Oswego

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3.1.3 Hydrology Streams within the City are small: Tryon Creek, the largest, drains 6.96 square miles at the mouth, and 6.8 square miles at the USGS stream gauge. Springbrook Creek, the next largest stream, drains approximately 1.6 square miles. Flow patterns can be considered flashy across all of the basins, though more so in the smaller creeks. Figure 6 shows the runoff hydrograph for both Lost Dog (measured as stage) and Tryon Creek during a mid‐March 2015 event, along with the corresponding rainfall hyetograph from the rain gauge located at Portland Community College’s Sylvania campus, just north of the City.

Streamflow estimates for frequencies important to hydromodification impacts are shown in Table 1 below. These are based on 13 years of continuous flow data from the USGS stream gauge on Tryon Creek. The average daily streamflow estimates for Springbrook, Lost Dog, and Carter creeks are based on the measured value of 1.3 cfs/mi2 at the Tryon gauge. The flood flow estimates for Springbrook, Lost Dog, and Carter creeks are derived from regional regression equations (StreamStats) adjusted by the ratio of measured to StreamStats flows for the Tryon Creek gauge. It is an indication of the degree to which hydromodification has already occurred in the Tryon Creek basin (which drains much of southwest Portland as well as northeast Lake Oswego) that flood discharges calculated from the measured gauge record range from 2 to 3.4 times the values estimated by Stream Stats equations.

3.2 Development History As important as the natural features of the Lake Oswego cityscape are to developing a hydromodification strategy, the development history and conditions are equally significant.

3.2.1 Urbanization History The roots of urban Lake Oswego go back to early homesteading days in the Willamette Valley. Multiple homesteads were established in the early to mid‐1850s around Sucker Lake, and a sawmill was built along the creek at the lake’s outlet. By the late 1860s, iron ore was being mined in the midst of the homesteads, and smelted at facilities on the banks of the Willamette River. Trees on the surrounding hills that hadn’t been cut to facilitate grazing, farming or milling were cut to supply charcoal to the smelter. In 1872, a canal was built connecting the Tualatin River to Sucker Lake as an aid to commerce.

Based on several decades of research observations of logged watersheds elsewhere in the region (e.g., at experimental watersheds run by Oregon State University and the US Forest

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Figure 6. Rainfall‐runoff for a typical storm, March 13‐16, 2015. a. Lost Dog Creek stage; b. Rainfall at Portland Community College Sylvania gauge; c. Tryon Creek hydrograph

15 August 31, 2015 Table 1. Estimated Flow Frequencies For Selected Lake Oswego Creeks Tryon Creek Springbrook Lost Dog Carter Creek Creek Creek Drainage Area (mi2) 6.8 1.6 0.64 1.04 Estimated Q (cfs) Avg. Daily Streamflow 8.6 2.1 0.8 1.4 2‐Year Peak 400 55 50 70 5‐Year Peak 700 200 90 120 10‐Year Peak 1200 350 150 200

Service15), the large‐scale clearing of forest in the late 19th century had to have had an effect on streams in the area, although it was not well documented at the time. Runoff rates and volumes likely increased, and sediment loads likely spiked following clearing. Both runoff and sediment loads would gradually decrease, but the relative timing of recovery would have controlled channel conditions: channels would have aggraded where runoff patterns recovered before sediment loads, and incised in the opposite case.

Although the iron works closed in the early 20th century, Sucker Lake became a regional attraction for recreation. The City of Oswego incorporated in 1910 with industrial facilities along the Willamette River front and residential areas on the eastern side of the lake. An interurban trolley was built connecting the town with Portland in 1914. The Ladd Estate Company began developing residential properties on the remaining vacant lands owned by the Oregon Iron & Steel Company. A concrete dam was built in 1921 to permanently raise the level of Sucker Lake, giving rise to the current Oswego Lake. While residential development was occurring throughout the 1920s and into the post‐World War II period in Oswego, the urban center of Lake Grove (originally platted in 1912) was growing on the west side of Oswego Lake. The two urban areas merged in 1960 to form the City of Lake Oswego. Development since then has filled in the area between the two commercial centers, as well as undeveloped areas within the urban growth boundary in northwestern neighborhoods (Oak Creek and Westlake), farther up the flanks of Cooks Butte, and near Marylhurst University (toward West Linn). This development is shown graphically in Figure 7. The annexation history of the City is shown in Figure 8, and current land use is shown in Figure 9. Contrast development patterns in the northwestern portion of the City annexed in the 1980s and built out in the 1990s—medium density residential with open space and some stormwater management—with older portions of the City.

15 MacDonald, A. and K. Ritland. 1989. Sediment dynamics in Type 4 and 5 waters: a review and synthesis. TFW‐ 012‐89‐002. Washington Department of Natural Resources, Olympia, WA. 86 p.

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Figure 7. History of urban development in Lake Oswego, 1954‐1961

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Figure 8. Annexation history of Lake Oswego

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Figure 9. Current land use in Lake Oswego

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3.3.1 Desktop Assessment The desktop assessment of hydrologic impacts focused on evaluating the amount of impervious surface area and the areal distribution of storm sewer infrastructure, including curbed and guttered or ditched streets, following the methods outline in Leopold (1968). Leopold estimated the increase in mean annual flood before and after urban development for a 1 square mile watershed as a function of change in these two watershed characteristics. This screening method represents an appropriate level of analysis for the City given that future development is almost exclusively infill via 2‐ or 3‐lot partitions rather than larger infill subdivisions or greenfield development.

The City’s enterprise GIS system provided the required input data. Impervious surface, in the form of building footprints, driveways, and visible patios, is digitized from as‐builts and aerial photographs as development occurs. Most areas of the city have discontinuous stormwater pipes fed by curbed and guttered streets. Some portions of the City, particularly in the southwest, are served by dry wells, which infiltrate water rather than deliver it to surface streams. The existing public piped storm sewers, along with curbed and guttered streets and dry wells, were plotted and regions served dominantly by the combination of curbed and guttered/ditched streets and storm sewers were mapped and digitized. Areas served by dry wells were specifically excluded. This mapping was generalized in nature, and did not attempt to delineate pipesheds at the scale of individual parcels.

For each of the City’s mapped subwatersheds, the area percent impervious surface and the area percent drained by storm sewers and curbed and guttered/ditched streets was calculated in the GIS. Figure 10 shows each of these subwatersheds plotted on the diagram used by Leopold; the City’s subwatersheds are considerably smaller than 1 square mile in area, so the absolute values of predicted increases in mean annual flood are understated, but the relative hydromodification impacts remain the same.

The desktop assessment of geomorphic impacts focused on the vulnerability of streams to further hydromodification impacts. Based on the findings of the hydrology risk assessment and the age of most urban development in the City, it is to be expected that most streams have been subject to a reasonable degree of hydrologic impact. Booth (1990) notes four basin characteristics that pre‐dispose channels to geomorphic impacts in the face of hydromodification, of which two are common in the City:16

 Low‐order, high gradient streams

16 Booth, D.B. 1990. Stream‐channel incision following drainage‐basin urbanization. Water Resources Bulletin 26:407‐417.

21 August 31, 2015  Low infiltration capacities of upland soils.

In the absence of detailed observations of recent incision or the composition of bed and bank materials (particularly locations of bedrock outcrops), both of which require

Figure 10. Expected hydrologic impacts from urbanization for Lake Oswego subwatersheds based on the screening method of Leopold (1968). Watershed abbreviations are those shown on the City’s interactive surface water map at http://gis.ci.oswego.or.us/GeoNorth/Flexmap3/Flexmap.html

22 August 31, 2015 systematic field investigation, channels were evaluated relative to the following characteristics:

1. Channel slope 2. Geologic materials adjacent to which the stream flows, including landslides 3. Presence of piped reaches, which act as grade control to limit incision (see Figure 11a) 4. Presence of steep side slopes, which could fail as a result of channel incision at the toe of the slope; side slopes of 25% trigger additional riparian buffer width.

Annotated composite profiles for the major channel systems in the City are shown in Appendix A.

3.3.2 Field Assessment Detailed field assessments of stream channel morphology across the City have been performed in conjunction with periodic assessments of macroinvertebrate communities. Six sites were surveyed in 2004, and 10 sites in each of 2007, 2009, and 2013.17 Each of these sites had available access with minimal influence from artificial constraints such as road crossings. These surveys track pools, bank erosion and streambed substrate, all of which are indicators of hydromodification impacts. In 2004, recent channel incision was recorded as well. All of the sites indicate some hydromodification impacts; the impacts are lowest in the Springbrook restoration reach, which was designed based on post‐ development hydrologic conditions. Informal channel observations over the past 18 months suggest that hydromodification impacts are widespread but that channels have become relatively stable under the present hydrology due to the presence of bedrock and other grade controls. Figure 11b shows a typical situation from Ball Creek, where bedrock

17 Cole, M.B. 2014. Clackamas County NPDES MS4 Co‐Permittees 2013 Coordinated Macroinvertebrate Assessment. Prepared for the Cities of Gladstone, Lake Oswego, Milwaukie, Oregon City, West Linn, and Wilsonville. Prepared by Cole Ecological, Inc., Boston, MA. February.

Cole, M.B. and A.P. Harris. 2004. City of Lake Oswego 2004 Macroinvertebrate Assessment. Prepared efor th City of Lake Oswego by ABR Inc. Environmental Research and Services, Forest Grove, OR. November.

Lemke, J.L. and M.B. Cole. 2007. City of Lake Oswego 2007 Macroinvertebrate Assessment. Prepared for the City of Lake Oswego by ABR Inc. Environmental Research and Services, Forest Grove, OR. November.

Lemke, J.L. andB. M. Cole. 2009. City of Lake Oswego 2009 Macroinvertebrate Assessment. Prepared for the City of Lake Oswego by ABR Inc. Environmental Research and Services, Forest Grove, OR. December.

23 August 31, 2015 and boulders in the channel bed limit incision, so channel enlargement is accommodated by bank erosion.

a. b.

Figure 11. Stream channels within the City locally exhibit both stability and instability in the face of hydromodification. a) Former Stone Bridge culvert provided grade control over a sewer line crossing Nettle Creek, but produced a fish passage barrier in the form of a 5‐ft drop. Bridge has since been replaced with boulder steps to provide the necessary grade control; b) Bank erosion of Ball Creek downstream from culverts under the access road to the LDS Temple

24 August 31, 2015

4.0 Hydromodification Management Strategies The City’s MS4 permit requires three categories of activities to be undertaken by the City to address hydromodification:

 Collect or identify strategies to collect information on hydromodification of waterbodies

 Identify strategies and priorities for preventing or reducing hydromodification impact

 Identify strategies and priorities for preventing or reducing hydromodification.

This section describes the City’s current and proposed future efforts regarding these activities.

4.1 Collect Information On Waterbodies The City went through a master planning process in the 2006‐2009 time frame, resulting in the Clean Streams Plan.18 This plan, supported by substantial contributions from the community, has as its governing principle,

To manage flooding and improve stormwater quality in a manner that recognizes the value and function of the natural surface water systems.

Out of this statement came the following surface water program goals:

To cost‐effectively implement and maintain a sustainable drainage system to:

 Promote public safety and minimize property damage

 Protect and enhance the quality of surface water

 Preserve and enhance fish and wildlife habitat.

Collection of structured geomorphic data on streams across the City has been limited to date, in large part because such a large portion of the access to stream channels is held privately. As noted above, detailed geomorphic information on stream channels is collected at macroinvertebrate sampling sites. The Clean Streams Plan described hydromodification‐ related impacts across the City, based on reconnaissance stream observations in 2008. Earlier surveys of riparian corridors, beginning in the mid‐1970s and continuing through to 2007, focused on upland conditions rather than instream conditions. As part of the

18 City of Lake Oswego. 2009. Clean Streams Plan. Prepared by Otak, Inc., Lake Oswego, Oregon. November.

25 August 31, 2015 development of the City’s 2015 TMDL Implementation Plan,19 channel canopy (derived from LiDAR and multi‐spectral data) was evaluated for the purpose of assessing stream shading. City staff attempted to map incised channel locations using the LiDAR bare earth model. However, there was insufficient resolution of conditions in the City’s small streams to allow this to be done using the 2014 LiDAR data.

In the future, the City plans to continue periodic habitat assessments at macroinvertebrate sampling sites. The City will also continue to acquire and analyze LiDAR topographic and forest canopy data, as included in the TMDL Implementation Plan. The City also has committed in the TMDL Implementation Plan by November 1, 2016 to “conduct a targeted field survey of waterbodies and riparian areas to implement hydromodification strategy.” This field survey will be focused on channels located on public lands and public rights‐of‐ way, in locations where evidence of hydromodification is not conflated with channel adjustments to culverts and bridges. Intent of this survey is to identify any reaches that might be particularly sensitive to further hydromodification, based on the characteristics listed on Figure 2 or observations from the desktop analysis.

The City is in discussions with staff at the Willamette Partnership to allow use of a “beta” version of the Oregon Function‐Based Rapid Stream Assessment Methodology currently in the final stages of development by the Willamette Partnership, Oregon Department of State Lands, the US Army Corps of Engineers, and the US EPA Region 10. This method is intended to be a stream‐channel equivalent of the Oregon Rapid Wetland Assessment Protocol, which is used to objectively assess wetland functions and values through measurement of specific attributes in a manner that is consistent and therefore understood and approved by multiple stakeholders. The assessment methodology can continue to be used to acquire additional stream channel information on private lands as opportunities allow, usually prior to development when riparian buffers regulated under the sensitive lands codes are formally delineated and mapped. Using an assessment method approved by multiple regulatory agencies increases the likelihood of ongoing training and technical support in its use and application.

4.2 Mitigate Hydromodification Impacts and Reduce Hydromodification Many preferred stormwater BMPs mitigate the impacts of hydromodification described in Section 2.2 above or reduce the magnitude of hydromodification. These are applied to upland locations. Several other approaches are available that mitigate instream impacts of

19 City of Lake Oswego. 2015. 2015 TMDL Implementation Plan. Lake Oswego, Oregon. June 30, 2015.

26 August 31, 2015 existing hydromodification. Approaches to mitigation and hydromodification reduction are summarized in Table 2.

Upland approaches are focused on one of two goals. The most important of these goals limits the amount of runoff from a site using infiltration BMPs and stormwater site planning approaches collected together under the category of “low impact development (LID).” These BMPs force rainfall into the ground either directly where it falls or very nearby (i.e., onsite), returning the landscape to a condition that is similar to pre‐ development infiltration conditions (Section 3.1.1). Because vegetation plays a central role in promoting infiltration, particularly in low‐permeability soils, many of the favored BMPs are also described as “green infrastructure.” BMPs in this category include rain gardens and planters, swales and filter strips, and green roofs. While somewhat less amenable to engineering analysis, properly maintained green infrastructure has been demonstrated in numerous studies over the past decade to be highly effective at reducing runoff.20 Permeable pavement—whether pavers, porous asphalt, or permeable concrete—is a type of engineered LID BMP that renders permeable what is typically an impermeable surface.

Table 2. Available Hydromodification Impact Reduction Strategies Location Strategy Status of Use by City Reduces Mitigates Hydromodification Hydromodification Impacts Upland Infiltration Current   Detention Current   LID Site Design In adoption process   Instream Channel Currently limited,  restoration to possible in future accommodate hydromodification Riparian buffers Currently active   and restoration Piped bypass Currently limited;  improbable in future due to topography and existing development  Primary effect  Secondary effect

Instream restoration and riparian restoration or buffers dissipate stream power and promote infiltration adjacent to stream channels by maintaining native vegetation and

20 For example, see studies collected by US Environmental Protection Agency at http://water.epa.gov/infrastructure/greeninfrastructure/gi_performance.cfm.

27 August 31, 2015 relatively undisturbed soil. Alternatively, runoff can be piped around sensitive stream segments and delivered directly to large water bodies where the additional runoff represents a negligible increase in flow.

The City has been subject to water quality regulations beginning with the 1988 Tualatin River phosphorus TMDL. Following the lead of other agencies within the basin, the City’s primary strategy for addressing these regulations has been for two decades detention and, where possible, infiltration. These strategies both reduce hydromodification. This hydromodification analysis, and the proposed strategy for continued data collection and analysis of instream conditions described above in Section 4.1, will be used to further inform conditions of site development under stormwater code (Section 4.2.1) and capital improvements of the surface water system (Section 4.2.2). For instance, the relative hydrologic risk shown in Figure 10 will inform future development review activities as shown in Table 3. Basins shown in green present less risk of hydrologic alteration with future development (assuming use of stormwater BMPs), while those shown in orange present a relatively high risk. The relative risk (indicated by basin rank and cell shading), coupled with basin and development area, can be used as part of the screening to determine whether a downstream analysis should be a required submission requirement for those projects proposing to discharge to the public stormwater system due to site conditions.

4.2.1 Current City Code and Draft Stormwater Manual Current city code responds to both MS4 and TMDL requirements to manage stormwater quality and runoff volumes, as well as prohibit alterations of drainage patterns that could adversely affect other properties or sensitive lands districts on adjacent parcels. Stormwater quality requirements are based on treatment to the “maximum extent practicable” with a recognition that the City is responsible for treatment to remove TMDL pollutants phosphorus, mercury, and bacteria. Existing code favors infiltration wherever possible. City code related to managing flow volumes also responds to flood management and the capacity limitations For minor development (e.g., construction of an individual single family residential dwelling or remodel, even if in sites restricted by resource protection, flood, or slope stability overlays), the discharge standard is,

“Where conditions permit, individual lots shall be developed to maximize the amount of stormwater runoff which is percolated into the soil and to minimize direct overland runoff into streets, drainage systems, and/or adjoining property. Stormwater runoff from roofs and other impervious surfaces should be diverted into swales, terraces, and/or water percolation devices on the lot when possible.” LOC 50.06.006 (3)(a)(iv). For more extensive development, including subdivisions, partitions, and commercial construction, the City’s current discharge standard is (LOC 50.06.006 (3)(b),

28 August 31, 2015 ii. Standards for Approval (2) Stormwater Runoff Quality All drainage systems shall include engineering design features to minimize pollutants such as oil, suspended solids, and other objectionable material in stormwater runoff. (3) Drainage Pattern Alteration Development shall be conducted in such a manner that alterations of drainage patterns (streams, ditches, swales, and surface runoff) do not adversely affect: (a) Other properties; (b) RC districts on adjacent property; or (c) RP districts on adjacent property. (4) Stormwater Detention Sufficient stormwater detention shall be provided to maintain runoff rates at their natural undeveloped levels for all anticipated intensities and durations of rainfall and provide necessary detention to accomplish this requirement. (5) Required Stormwater Management Measures The applicant shall provide sufficient stormwater management measures to meet the above stormwater runoff requirements. The applicant shall provide designs of these measures taking into account existing drainage patterns, soil properties (such as erodibility and permeability) and site topography. iii. Standards for Construction (5) Secondary Uses Stormwater detention or retention areas may be designed to serve a secondary purpose for recreation, open space, or other types of uses that will not be adversely affected by occasional or intermittent flooding. (6) Release Rate Outlet The outlet opening controlling the release rate of detained stormwater runoff shall be: (a) Sized so as not to exceed the water conveyance capacity of the downstream drainage system; (b) Small enough to cause stormwater runoff to be detained from a storm of at least the undeveloped ten‐year frequency; (c) Designed to prevent siltation or clogging of the outlet opening; and (d) Provided with a means of adjusting the size of the outlet openings. (7) Required Detention Volume for Developments Detention volume shall be the maximum difference between: (a) The stormwater runoff produced from the proposed development site by a 50‐ year storm; and (b) The stormwater runoff produced from the pre‐development site area by a ten‐ year storm.

29 August 31, 2015 Table 3. Hydrologic Risk of Hydromodification Mean Annual Flood Post‐ Sub‐ Area Area development/ Mean Basin Basin Sub‐Basin Name Impervious Dominantly Annual Flood Pre‐ ID Area Surface Piped development for 1mi2 (mi2) Equivalent Eg Evergreen 0.15 63% 98% 4.7 Op Oswego Point 0.02 53% 100% 4.2 Hl Holy Names 0.04 51% 79% 3.5 Ct Carter Creek 0.77 46% 86% 3.2 Cc Country Club 0.20 11% 10% 3.2 Lb Lakewood Bay 0.08 41% 94% 3.2 Ph Palisades Heights 0.14 39% 90% 3.1 Pc Parelius Creek 0.04 37% 91% 3 Rs Reese Road 0.18 44% 69% 3 Bl Ball Creek 1.23 38% 82% 2.9 Pm Pearcy Maple 0.02 31% 90% 2.8 Rr River Run 0.06 33% 91% 2.8 Bc Blue Heron Canal 0.39 31% 89% 2.7 Fr Fernwood Creek 0.14 32% 86% 2.6 Jr Jean Road 0.26 45% 41% 2.6 Oc Oswego Creek 0.33 30% 91% 2.6 Bh Blue Heron Creek 0.14 28% 66% 2.5 Hn Hallinan Creek 0.10 30% 60% 2.5 Sb Springbrook Creek 1.96 32% 69% 2.5 Wb West Bay 0.15 29% 61% 2.5 Bf Boones Ferry 0.60 33% 54% 2.4 Er Eena Road 0.02 34% 41% 2.4 Lf Lake Forest 0.09 31% 60% 2.4 Lp Lily Pond 0.05 28% 60% 2.4 Mu Mulligan Creek 0.05 27% 58% 2.4 Ld Lost Dog Creek 0.68 25% 58% 2.3 Os Oswego Canal 0.88 28% 56% 2.3 Pv Pine Valley 0.03 24% 62% 2.3 Ot Oak Terrace 0.04 21% 47% 2 Ml Marylhurst Creek 0.05 15% 54% 1.9 Ol Oswego Lake Margin 0.37 28% 26% 1.9 Gm Glenmorrie Creek 0.20 20% 32% 1.7 Wr Willamette River Margin 0.17 16% 43% 1.7 Rd Riven Dell Creek 0.17 14% 32% 1.6 Ac Arbor Creek 0.76 15% 13% 1.4 Du Dunthorpe 0.23 18% 18% 1.4 Rg River Grove 0.37 30% 1% 1.4 St Stonebridge Creek 0.19 17% 17% 1.4 Tc Tryon Creek 5.31 15% 17% 1.4 Pc Pecan Creek 0.66 8% 18% 1.3 Tr Tualatin River Margin 0.18 7% 6% 1.1 Sf Stafford Creek 0.21 3% 0% 1 Wc Wilson Creek 2.03 2% 1% 1

30 August 31, 2015 The City is in the final stages of adoption of new stormwater code language that will provide more detailed guidance on management of stormwater quality, and further promote reduction of hydromodification and/or modification of its impacts (Figure 12). Flow control requirements are expressly aimed at substantially reducing hydromodification. Even if flow control requirements cannot be met on a parcel, water quality requirements favoring infiltration will provide some reduction of hydromodification.

Hydrological analyses required by the draft manual uses a design‐storm approach rather than continuous modeling. This represents a conscious decision on the part of the City because of the limited nature of new developments. Only a handful of parcels remain within the City’s urban services boundary that can be divided into more than three parcels under current zoning. The City would rather that site designers and developers concentrate their efforts on managing water quality than on developing hydrologic models for such small sites. A detailed analysis of downstream conditions may be required if the proposed development is likely to affect sensitive stream reaches.

4.2.2 Retrofit Strategy for Hydromodification The Clean Streams Plan recommended a number of capital surface water retrofit projects to compliment ongoing capital investments in public infrastructure. Surface water project ranking criteria outlined in the plan continue to be used to rank these and newly proposed capital projects:

1. Protects public infrastructure

2. Protects private infrastructure from runoff from City infrastructure

3. Reduces flooding or landslide risk

4. Required for City compliance with State and Federal regulations

5. Improves natural resource values

6. Improves water quality

7. Repairs existing infrastructure

8. Constitutes preventative maintenance.

Within this ranking scheme, the first three criteria are weighted double. Arguably, however, criteria #3, 5, and 6 combined provide for considerable weight being placed on goals related to watershed scale improvement of instream conditions for target resources.

31 August 31, 2015

Figure 12. Stormwater standards proposed for adoption in the Lake Oswego Stormwater Management Manual [August 2015 Review Draft].

32 August 31, 2015 In most cases, some of the benefit is achieved through mitigation of hydromodification impacts.

These priorities for stormwater projects were derived, in part, from priorities articulated in 1998 by the City Engineer for stream restoration projects. Instream projects would be selected based on these priorities:21

1. Water quality benefit

2. Protection of public facilities

3. Restoration of sensitive lands

4. Re‐establishment of historic streams

5. Protection of developed property from flooding.

Of the projects currently on the City’s Capital Improvement Plan (CIP) as shown in Figure 13, those in Table 4 are anticipated to locally reduce hydromodification or mitigate its impacts in receiving surface water systems.

21 Memo re. stream restoration capital improvement policy from Mark Schoening, City Engineer, to Douglas Schmitz, City Manager, February 19, 1998

33 August 31, 2015

Figure 13. Locations of Lake Oswego’s ongoing and future CIP prioritized retrofit projects with hydromodification components, identified by CIP rank. Scores are 0 = no hydromodification benefit; 1 = limited hydromodification benefit; and 2 = substantial hydromodification benefit. See Table 4 for key to additional details on projects with anticipated substantial hydromodification benefits.

34 August 31, 2015

Table 4. Water Quality Retrofit Projects With Substantial Hydromodification Benefits as Prioritized for FY15‐16 to FY20‐21

2

2 Existing

1 Landslide

With

Cost in FY15‐ Resource or CIP

16 to FY20‐21 Improvement

2 16 ‐

ID CIP, FY14‐15 Requirements

CIP?

Natural Facilities

CIP, or CSP Score 2015 15 Flooding

‐ Quality

(2015 $ for 2 In

Project

Unfunded FY14 Project Name/Location Projects)1 Basin Pollutants Targeted2 Notes Regulatory Risk Maintian Improves Retrofit CSP Source In Reduce Rank Water Value Tualatin & Project construction completed; now in plant establishment phase for LID Kerr Parkway Rehabilitation — $3,200,000 Springbrook Staff Y 1 1 2 P, Hg, T, DO/TSS, HC, M 6 0 swales and planters.

Cornell/Laurel/Bickner — $ 210,000 Willamette Staff N 2 2 P, Hg, T, DO/TSS, HC, M, B 6 2 Tie to Laurel Pathway WO 191, which has SW requirements that will use LID

D Avenue Storm System (includes 1st St, E Avenue) TC5 & TC6 $ 802,000 Tryon CSP Y 2 1 1 P, Hg, T, DO/TSS, HC, M, B 6 3 Includes E Avenue upgrade; LID pavement solutions contemplated Replaces pipe under Country Club and rehabilitates existing pond, assuming Country Club Road near Dolph Court TC1 $ 435,000 Tryon CSP Y 1 1 P, Hg, DO/TSS, B 4 7 access is granted. Project funding extends beyond approved CIP list. Tied to annexation and new Regional SW Facility ‐ Bergis/Stafford LD1 $ 671,000 Lost Dog CSP Y 2 P, Hg, DO/TSS, M 4 13 development. Also referred to as “Regional Storm Facility ‐ Lost Dog” Incorporate treatment into road rebuild; coordinate with Parks activities in Wembley Park/Crest Drive — $ 215,000 Springbrook Staff Y 2 1 1 2 P, Hg, DO/TSS 8 18 Springbrook Park

Operations Facility Replacement — — Oswego Lake Staff Y 2 P, Hg, T, DO/TSS, HC, M 4 F2 Stormwater improvement proportion of $13M budget not yet determined. P, Hg, T, DO/TSS, HC, M Southwood Park (Mtn Park) BL2 $ 135,000 Ball CSP Y 2 1 2 7 U02 Add LID. P, Hg, T, DO/TSS, HC, M Tanglewood Drive Open Space (Mtn Park) SB2 $ 140,000 Springbrook CSP Y 2 2 6 U03 Add LID. May accelerate with Nature in Neighborhoods Metro grant P, Hg, T, DO/TSS, HC, M Elizabeth M Gress Park (Mtn Park) SB4 $ 226,000 Springbrook CSP Y 2 2 6 U04 Add LID. P, Hg, T, DO/TSS, HC, M Preakness Court Hammerhead (Mtn Park) SB1 $ 140,000 Springbrook CSP Y 2 1 2 7 U05 Add LID.

Cherry Crest Detention & Municipal Golf Course LD2 $ 615,000 Lost Dog Staff Y 2 P, Hg, DO/TSS, HC, M, B 4 U07 Add parking lot/material management retrofit to CSP description

Melrose Street BL1 $ 265,000 Ball CSP Y 2 1 1 P, Hg, T, DO/TSS, M, B 6 U11 Regrade channel and revegetate riparian area

Water Reservoir Disconnect ON $ 721,000 City‐Wide CSP Y 1 P, Hg, DO/TSS, HC, M 3 U15 Improve options for contingency overflows at 16 City reservoirs Total $14,429,000 Notes: 1 Funded from Surface Water Utility unless noted. Rank preceded by U indicates project not funded in current CIP. Pollutants: P=phosphorus; Hg=mercury; T=temperature; DO/TSS=dissolved oxygen and total suspended sediment (DO surrogate); HC=hydrocarbons; note that treatments useful for hydrocarbons also are applicable for other organic compounds such as pesticides; M=metals, 2 primarily copper and zinc; B=bacteria CIP Capital Improvement Plan ON ongoing activity described in Clean Streams Plan

CSP Clean Streams Plan (City of Lake Oswego, 2009) F2 Rank in other CIP sections: F – Facilities. Not funded by surface water utility.

ND Not determined NR Not ranked

35 August 31, 2015

5.0 Conclusions This hydromodification assessment presents a current snapshop of the City’s long‐standing activities and direction in managing hydromodification. The City’s existing stormwater code, the new draft stormwater code/manual, and the City’s stormwater retrofit strategies favor infiltration wherever possible. However, watershed conditions within Lake Oswego present very real constraints on the use of infiltration, particularly in BMPs like trenches, infiltration‐based rain gardens, and dry wells that concentrate infiltration in a small space. These constraints include:

 Poor soil infiltration capacity

 Infiltration limitations due to bedrock or shallow groundwater

 Slope stability risk

Detention, while useful in reducing hydromodification impacts, can rarely eliminate them without the use of additional onsite BMPs. Detention is most effective for reducing flood peaks. However, to maintain capacity in the detention facility, it must be drawn down between storms, resulting in extended periods of elevated streamflow that is typically still above instream erosion thresholds.22

Finally, nearly all of the future development in the City is expected to be infill via remodeling/rebuilding on single lots or the partition of larger lots (i.e., “developable land” on Figure 9) with a single structure into at most two or three lots each with a single structure. Therefore, future hydromodification is likely to occur slowly and be mitigated by stormwater design requirements. For this reason, the City’s emphasis on addressing hydromodification is to retrofit portions of the City that developed with little or no stormwater treatment, as described in the City’s Retrofit Strategy,23 and as the City’s Habitat Enhancement Program24 and other funds allow and the need arises, address existing instream impacts using structural and vegetative restoration.

The City’s focus on instream restoration activities will be to reduce local stream power by forcing flood flows onto adjacent floodplains where possible, supplement natural features to improve grade control, and address erosion of the toes of unstable slopes by bioengineered or (if necessary) composite green‐grey bank hardening or other means to

22 Sovern, D. and A. MacDonald. 2002. Can in‐stream integrity be obtained through on‐site controls? IN: B.R. Urbonas [ed]. Linking Stormwater BMP Designs and Performance to Receiving Water Impact Mitigation. ASCE 2001 Engineering Foundation Conference, Snowmass, CO. p. 47‐59. 23 City of Lake Oswego. 2015. Retrofit Strategy. Lake Oswego, Oregon. July 1, 2015. 24 See the City’s TMDL Implementation Plan (2015) for more details.

36 August 31, 2015 safely direct flow away from sensitive locations. Project designs would be based on estimated peak and sustained flows using the assumption of full build‐out (i.e., densest allowed by then‐current zoning) in the upstream watershed.

37 August 31, 2015

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38 August 31, 2015

Appendix A

Composite Profiles & Map of Profiled Streams

39 August 31, 2015

Figure A‐1. Location of reaches shown in annotated composite stream profiles

40 August 31, 2015 Figure A‐2. Tryon Creek Composite Profile With Generalized Geology and Road Crossings 750

725 Tryon (1000) UNT1 (1100) 8th Street Trib (1200) 1300 700 1400 1410 675 Nettle Ck. (1500) Timberline Trib (1510) Partridge Ln Atwater Trib (1520) 1530 650 Walking Woods Dr UNT4 (1600) Hideaway Cr (1700) 625 UNT5 (1800) 1810 UNT6 (1900) 1910 600 1920 Forest Meadows Trib (2000) 575 Park (Paget) Cr (2100) 2010 Icarus Loop 550 Bonnibrae Trib (2020) 2030 SW 35th Pl Park UNT1 (2110) Park UNT2 (2120) Icarus Loop 525 Arnold Cr (2200) 2210 SW 33rd Pl SW 35th Ave 500 2220 Pipe SW 33rd Ave 475

SW 35th Ave 450 (Feet)

425 SW Boones Ferry Rd Hazel 400 SW 13th Ct Goodall Rd Elevation Goodall Rd 375

Atwater Ln Bridge Ct 350 Timberline Dr Country Club Rd SW Lancaster Rd 325 Forest Meadows Way

300 Atwater Rd SW 16th Dr

275 Boca Ratan Dr Boca Ratan Dr 250

225

200 SW Arnold St

175 Main stem of Tryon Creek 150

125

100

75 FAULT Landslides Landslide BoringVolcanics of Mt. Sylvania 50 Landslides 25 Hillsobro Fmn. FAULT CRB Sand Hollow CRB Ginko Basalt of Waverly Heights 0 Alluvium 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000 10500 11000 11500 12000 12500 13000 13500 14000 14500 15000 15500 16000 16500 17000 17500 18000 18500 19000 19500 20000 20500 21000 21500 22000 22500 23000 23500 24000 24500 25000 25500 26000 26500 Length (Feet) Upstream of the Mouth of Tryon Creek Figure A‐3. Springbrook Creek Composite Profile With Generalized Geology and Road Crossings 725 700 Springbrook Creek (2000) Iron Mtn Park Trib (2100) 2110

675 Glen Eagles Trib (2200) Three Sisters Trib (2300) Three Sisters Trib East (2310) 650 Springbrook Park Trib (2400) 2500 The Grotto Trib (2600) 625 600 Monroe Park Trib (2700) Brittan Court Trib (2800) Independence Trib (2900) 575 Monticello Trib (3000) Pipe

550 Tanglewood Dr 525 Cirque Dr Kerr Pkwy 500

475 Mountain Cir Kerr Pkwy 450

Kerr Pkwy

(Feet) 425

400 Kerr Pkwy 375 Kerr Pkwy Elevation 350 Rainbow Dr 325 Glen Eagles Rd Boones Ferry Rd Daniel Way 300 Sherbrook Pl Mercantile Dr Kruse Way

275 Parking Lot Parking Lot Kruse Way

250 Boones Ferry Rd Hallmark Dr 225

200 Spring Ln Twin Fir Rd Boones Way 175 Railroad

Iron Mountain Blvd 150 2110 1252000 Iron Mountain Blvd 100 Iron Mountain Blvd 75 Landslide Landslide Landslide FAULT 50 Alluvium Missoula Flood Deposits ‐ fine grained facies CRB Sand Hollow left bank/ 25 Boring volcanics of Mt. Sylvania BoringVolcanics of Mt. Sylvania CRB Ginko right bank 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000 10500 11000 11500 12000 12500 13000 13500 14000 14500 Length (Feet) Upstream of the Mouth of Springbrook Creek Figure A‐4. Ball Creek Composite Profile With Generalized Geology and Road Crossings 825 Ball Creek (7000) 800 Centerpoint Trib (7100) 775 7110 750 7120 725 Temple Trib (7200) 700 Mickle Pl. Trib (7300) 675 Peters Rd. Trib (7310) Jefferson Parkway and Cervantes 650 Peters Rd UNT1 (7311) 625 Sunbrook Trib (7400) 600 Mt. View Trib (7500) Abelard Jefferson Pkwy 575 Pipe 550 Bernini Ct 525 Mountainview Ln 500 Jefferson Pkwy Ableard 475 Fosberg Rd 450 Vermeer Dr (Feet)

Rogers Rd 425 Bay Point Dr Peters Rd 400 Deerfield Ct 375 Southwood Dr Cascara Ln Southwood Dr Elevation 350 Sunbrook Dr Fosberg Rd 325 Westlake Dr 300 Centerpointe Dr Melrose St 275 Kruse Oaks Blvd Hwy 217 Southwood Dr Westlake Dr 250 Suncreek Dr 225 Hwy 217 200 175 Kruse Oaks Blvd Boring Volcanics of Mt. Sylvania 150 I5 Ramp I5 Ramp 125 100 Missoula Flood Deposits ‐ fine grained facies 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000

Length (Feet) Upstream of Ball Creek at I5 Figure A‐5. Lost Dog Creek Composite Profile With Generalized Geology and Road Crossings 500 Lost Dog West (3000) Bergis Rd 475 Oak Meadow Ct Lost Dog East (3100) 450 Oak Meadow Dr UNT1 (3110) Bergis Rd 425 Pipe 400 375 FAULT Mcvey Ave Patton Rd 350 Sunny Hill Dr Greentree Rd 325 South Shore Blvd Wall St 300

(Feet) Laurel St

275 FAULT South Shore Blvd Oak St 250 Palisades Terrace Ct Palisades Terrace Dr Elevation 225 FAULT 200 175 150 125 100 Lake Front Rd 75 Crossing_Street

50 CRB Sentinel Bluffs right bank/ CRB Ginko CRB Ginko left bank CRB Ginko 25 CRB Sentinel Bluffs CRB Sentinel Bluffs 0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 4750 5000 5250 5500 5750 Length (Feet) Upstream of the Mouth of Lost Dog Creek Figure A‐6. Carter Creek Composite Profile With Generalized Geology and Road Crossings 325 Carter Creek (4000) 300 Parkview Dr Meadows Creek (4100)

275 Parkway Trib (4200) Kruse Way Pipe Kruse Way 250 Meadows Rd (Feet)

Meadows Rd Kruse Way 225

200 Bangy Rd Elevation 175 I‐5

150

125

100

75 Landslide (right bank) 50 Missoula Flood Deposits ‐ fine grained facies 25

0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 Length (Feet) Upstream of the Mouth of Carter Creek Figure A‐7. Glenmorrie Creek Composite Profile With Generalized Geology and Road Crossings

700 675 Glenmorrie Creek (5000) 650 Glenmorrie South Fork (5100) 625 600 Pipe 575 550 525 500 475 450 (Feet) 425 Cherry Ln 400 Hallinan St 375 CRB Ginko 350 Cherry Ln and Chapin Way 325 Glenmorrie Dr Elevation 300 275 Cherry Ln Pacific Hwy 250 225 FAULT Backyard Pool FAULT 200 FAULT 175 150 Ivy Ln CRB 125 Winter 100 SpringwaterF Landslide Water 75 FAULT mn. Landslide CRB Sentinel 50 Bluffs 25 CRB Umtanum 0 CRB Sand Hollow Missoula Flood Deposits fine grained facies 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 4750 5000 5250 5500 Length (Feet) Upstream of the Mouth of Glenmorrie Creek Figure A‐8. Brookhurst Creek Composite Profile With Generalized Geology and Road Crossings 725 Brookhurst/Stonebrook Creek (6000) 700 Brookhurst Trib (6100) 675 650 6110 625 6120 600 6130 575 6200 550 6210 525 6220 500 Pipe 475 450 425 400 (Feet) 375 350 Chapin Way 325 300 Elevation 275 250 Pacific Hwy (OR43) FAULT 225 Marylhurst College 200 175 150 125 100 CRB Sentinel 75 CRB Winter Water Bluffs 50 CRB Umtanum 25 Missoula Flood Deposits fine grained facies 0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 4750 5000 5250 Length (Feet) Upstream of the Mouth of Brookhurst Creek