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Swanson Hydrology+ Geomorphology

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Swanson Hydrology+ Geomorphology SWANSON HYDROLOGY + GEOMORPHOLOGY Chapter 2: Watershed Description and Hydrologic and Biologic Setting This chapter provides a brief background description of the hydrologic and biologic conditions in the Meeks Creek Watershed. Subsequent sections on the geologic and geomorphic setting (Chapter 3) and the land use and ecological changes at Meeks (Chapter 4) establish the framework for ecosystem evaluations and restoration recommendations developed in this report. More detailed information on vegetation and wildlife resources is provided in Appendices B and D, respectively. WATERSHED DESCRIPTION The Meeks Creek Watershed is located on the western shore of Lake Tahoe in El Dorado County, California (Figures 1.1 and 2.1). The “L” shaped basin drains approximately 8.1 square miles eastward from the crest of the Sierra Nevada at over 9,200 feet above mean sea level (MSL), into Lake Tahoe at elevation 6,625 feet. Meeks Creek is roughly 7.5 miles in length and fl ows northward from Rubicon Lake (elevation 8,300 feet) for four miles before turning sharply to the east and emptying into Meeks Bay. Meeks Creek Watershed can be separated into three distinct areas (see Figure 2.1): 1) upper watershed, 2) lower meadow, and 3) shoreline zone. The U-shaped valley of the upper watershed extends from the crest of the Sierra Nevada, passing through forests, small meadows, and numerous lakes. Meeks Creek then cascades down a glacial step to the wide fl at valley fl oor of the lower meadow then into the shoreline zone at Lake Tahoe. The predominantly granitic bedrock has been carved by the creation and recession of glaciers throughout the Tahoe Basin’s history, and now numerous alpine lakes dot the landscape, including Rubicon Lake, Stony Ridge Lake, Hidden Lake, Shadow Lake, Crag Lake and Lake Genevieve (Figure 2.2). North-south trending faults cross the upper watershed at numerous locations and a small belt of metamorphic rock is located in the southern end of the watershed, surrounding Stony Ridge Lake. The upper watershed is located fully within the boundaries of Desolation Wilderness which is managed by the LTBMU and El Dorado National Forest. No roads exist for vehicular access, though there are numerous trails for hiking and cross country skiing. The lower meadow shows abundant evidence of glacial activity. Lateral moraines bound the edges of the meadow rising to approximately 400 to 1,000 feet above the valley fl oor, and recessional moraines cross portions of the basin in several locations. This has created a complex environment 37 ecological system science hydrology + geomorphology restoration engineering regulatory compliance 0 3/8 3/4 1.5 Miles 1:47,520 LEGEND N Shoreline HIGHWAYS Zone STREETS / ROADS WILDERNESS BOUNDARY LAKE / POND PERRENIAL RIVERS / STREAMS Lower 89 Meadow INTERMITENT STREAM SEASONAL STREAM LAKE TAHOE Upper Watershed DESOLATION WILDERNESS LAKE TAHOE Y 89 EMERALD BAYBA SWANSON HYDROLOGY + GEOMORPHOLOGY FIGURE 2.1: Meeks watershed location map showing study areas. 500 Seabright Ave, Suite 202 Santa Cruz, CA 95062 GIS data source: USFS LTBMU. PH 831.427.0288 FX 831.427.0472 SWANSON HYDROLOGY + GEOMORPHOLOGY FIGURE 2.2: Photos of lakes in Upper Watershed Area. Lake Genevieve, (top), and small 500 Seabright Ave, Suite 202 Santa Cruz, CA 95062 pond with shoreline wetlands above Lake Genevieve, (bottom). Photo source: SH+G, Fall PH 831.427.0288 FX 831.427.0472 2003. SWANSON HYDROLOGY + GEOMORPHOLOGY of landforms and soils, including lacustrine (glacial lake) deposits, fl uvial deltaic deposits, glacial outwash, alluvial deposits, side valley alluvial fans, peat soils, and cobble/boulder lag deposits. The various soil communities have led to a mosaic of vegetation communities throughout the meadow, including conifer forests, lodgepole pine forests, riparian scrubs, vernal pools, dry and wet graminoid meadows, and obligate sedge meadows. As a constricted Meeks Creek fl ows through the culverts under Highway 89, it enters an area of intense human development of the shoreline zone. The area from Highway 89 to Meeks Bay has been a focal point of the Tahoe tourism industry since the 1920s, hosting resorts, campgrounds, and the Meeks Bay Marina. However, as a result of these developments, what was once a seasonal and dynamic lower creek, lagoon, and barrier beach environment has become a stagnant fi xed embayment structure surrounded by elevated fi ll areas and a deeply incised creek channel – in general, a total loss of geomorphic function has occurred. The lagoon system and nearshore habitat and marsh environments have been degraded, and the now fi xed lagoon requires dredging and remedial activities to accommodate boat use. Since federal acquisition in 1974, the shoreline zone has been within NFS lands and LTBMU management. The Meeks Bay Resort and Marina and the Meeks Bay Campground are presently operated under Special Use Permits to the Washoe Tribe and CLM, respectively. The shoreline zone undergoes intense human use in the summer months, including heavy auto, pedestrian, and boating traffi c. HYDROLOGY The hydrology of Meeks Creek is driven by the seasonal precipitation patterns (rain and snow) and temperature conditions. During the summer months from July through September, little rain falls and when there is a storm, it tends to come in a short burst of a thunderstorm generating little runoff. The majority of precipitation falls as snow in the winter months from November to April, and the highest volume of runoff is generated by spring snowmelt of the upper watershed snowpack from April through June. Warm winter rains that falls on snow typical of El Nino year (e.g. 1955, 1964, 1986 and 1997) storms can contribute the highest instantaneous peak runoff. A USGS streamfl ow gage (#10336640) operated on Meeks Creek at Highway 89 from 1972 to 1975. In order to expand this limited period of record, streamfl ow data from adjacent watersheds with lengthier records were used to extend the Meeks Creek streamfl ow time series. Mean daily fl ow data measured from a gauge at Blackwood Creek (USGS # 10336660) and mean and peak daily fl ow data from General Creek (USGS #10336645) were used to establish a relationship regression with the limited Meeks Creek data (including data collected by the LTBMU in the 1990s) and to estimate an extended Meeks Creek streamfl ow record from 1960 to 2002 (Figure 2.3, part A). This fi gure gives a good visual indication of the cyclic nature of streamfl ow and the effects of drought years (mid-1970s and early 1990s) and extremely wet years (early 1980s and late 1990s). 40 ecological system science hydrology + geomorphology restoration engineering regulatory compliance SWANSON HYDROLOGY + GEOMORPHOLOGY An annual average hydrograph (Figure 2.3, part B) shows mean daily streamfl ow for each day of the calendar year over the entire 43 years of record. This shows that the seasonal magnitude and timing of runoff generated in the Meeks Creek Watershed. The average hydrograph shows that there is little runoff during the dry summer months, but runoff increases in November and December with the onset of winter rain. Through the cold winter months precipitation is stored in the snowpack, and then with rising temperatures the majority of Meeks Creek runoff is generated during spring snowmelt. Figure 2.4 shows the percent chance any given streamfl ow will be exceeded in any month and illustrates the monthly fl ow variance between wet and dry years. The 10% curve reveals a second peak in streamfl ow, during the month of January, which signifi es the periodic occurrence of warm, mid-winter rain-on-snow storm events that, owing to antecedent snowpack conditions often generate the highest peak instantaneous fl ows. In contrast, early winter rains often infi ltrate soils and recharges soil moisture depleted during the dry summers. A log Pearson-Type III fl ood frequency analysis using HEC-FFA (USACOE 1992) was performed on the Meeks Creek annual peak fl ow data to assess the magnitude and frequency of peak fl ows. Results are presented in Table 2.1. Due to the limited number of annual peak fl ows available, a partial duration series of peak fl ows was also used. Any event that generated fl ow greater than 50 cfs was added to the annual series, more than doubling the number of values used in the analysis. Table 2.1: Log Pearson Type III Flood Frequency Analysis for Meeks Creek Annual Series Partial Duration Recurrence Interval Results1 Series Results2 (years) n=43 n=94 13635 1.5 145 108 2 177 134 5 350 255 10 510 369 20 705 510 50 1030 750 100 1330 982 200 1700 1270 500 2290 1750 1 Annual Series derived from Blackwood Creek (USGS #10336660) and General Creek (USGS #10336645) streamfl ow data for WY1960-2002 and includes the peak fl ow for each water year. 2 Partial Series derived from Blackwood Creek (USGS #10336660) and General Creek (USGS #10336645) streamfl ow data for WY1960-2002 and includes peak fl ow for each water year, as well as any event runoff that exceeded 50 cfs. 41 ecological system science hydrology + geomorphology restoration engineering regulatory compliance 700 A 600 500 400 300 Mean Daily Discharge (cfs) 200 100 0 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 60 B 50 40 30 20 Mean Daily Discharge (cfs) 10 0 October November December January February March April May June July August September SWANSON HYDROLOGY + GEOMORPHOLOGY FIGURE 2.3: Mean daily discharge time series for Meeks Creek at Highway 89 (USGS #10336640, WY1972-1975) extended by using 500 Seabright Ave, Suite 202 Santa Cruz, CA 95062 streamfl ow record generated from General Creek (USGS #10336645) and Blackwood Creek (USGS #10336660).
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