MetroSouthPlatteInterimBasinWaterSupplyNeedsReport (2)

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Section 3 Physical Environment of the South Platte Basin and Denver/South Metro Counties I l!l~! Water quality can be affected by geography and various land uses including runoff from point and non-point discharge sources. For example, mining in the mountainous regions, urbanization along the Front Range, and agriculture in the eastern plains and elsewhere can impact the quality of the state's waters and aquatic habitats. Habitat degradation, nutrient loading, soil erosion, and increased stormwater runoff are only a few examples of the concerns associated with rapid urbanization, particularly in the mountain recreational areas (Colorado Department of Public Health and Environment [CDPHE] 2000). Improving water quality and restoration and protection of water bodies in Colorado is occurring through programs such as the Total Maximum Daily Load (TMDL) process, Gold Medal fisheries establishment, instream flow programs, and federal and state listed threatened, endangered, and species of special concern. 3.2 South Platte Basin Physical Environment 3.2.1 Geography The South Platte Basin (including the Republican River Basin and the Denver/South Metro Counties) covers approximately 27,660 square miles in northeast Colorado, Figure 3-2. The Denver/South Metro Counties, which include Adams, Arapahoe, Denver, Douglas, Elbert, and Jefferson, make up approximately 5,606 square miles. The largest cities in the basin are Denver (population 560,882), Aurora (population 287,216), and Lakewood (population 144,150) (DOLA 2003). 3.2.2 Climate The South Platte Basin as a whole receives relatively low precipitation, which can be highly variable from year to year. The basin also has widely variable daily and seasonal air temperatures (Woodward-Clyde Consultants 1982). Figure 3-3 shows color-fill contours of the average annual precipitation. The plains region is characterized by small amounts of precipitation averaging between 7 and 17 inches per year. Greater amounts of precipitation accumulate in the mountain region, which receive upwards of 30 inches annually. The foothills of the Front Range, which provide a transition zone between the mountains and the plains, annually receive an average of 17 to 21 inches of precipitation. The potential evapotranspiration exceeds CONI 3-2 precipitation in the basin in all areas except for the mountain region (CGS 2003). 3.2.3 Topography The topographic characteristics of the South Platte Basin are diverse. Elevations in the basin range from over 14,000 feet at the headwaters near the Continental Divide to 3,400 feet at the Colorado/Nebraska state line (CDPHE 2002). The headwaters of the South Platte River originate at an elevation of about 11,500 feet. 3.2.4 Land Use Approximately one-third of the basin's land area is publicly owned, and the majority of these lands are forest areas in the mountains. Table 3-1 shows the square miles and percent of total by land cover type. Figure 3-4 summarizes the land cover by category for the entire South Platte Basin (U.S. Geological Survey [USGS] 1992). As the figure shows, western portions of the basin and its montane and subalpine areas are primarily forested, while the High Plains region is mainly grassland and planted/ cultivated land. Table 3-1 Land Cover Data for the South Platte Basin (Total) Basinwide Grassland 13956 505% 41051 395% Planted! 8,526 30.8% 13,737 13.2% Cultivated Forest I 3,372 I 12.2% I 29,577 I 28.4% Shrub land 866 3.1% 16,883 16.2% Developed I 586 I 2.1% I 923 I 0.9% .l)Ren Wati:lr 247 0.9% 590 0.6% Barren I 89 I 0.3% I 1,219 I 1.2% Wetland 18 0.06% 80 0.08% TOTAL I 27,659 I I 104,067 I Source: USGS 1992 NLCD 3.2.5 Surface Geology The mountains are comprised of Precambrian age metamorphic and igneous basement rocks. These rocks come into contact with Mesozoic and Paleozoic sedimentary rocks by a fault that runs north and south just west of Denver (CGS 2003). A well-known outcrop is observed along 1-70 just west of C-470 revealing the many layers of sedimentary rock that form the Denver Basin. S:\1177\BASIN REPORTSlSOUTH PLATTE\S3_S0UTH PLATTE. DOC