2.0 BASIN SETTING. 2.1 LOCATION The Cache la Poudre River Basin is located in north central Colorado, as shown on Figure 2.1, on the eastern slope of the Continental Divide. The Laramie and Medicine Bow mountain ranges form the western boundary of the Basin. In the mountainous region, the southern boundary of the Basin is formed by the Mummy Range and Rocky Mountain foothills which separate the Cache la Poudre Basin from the adjacent Big Thompson River Basin. The Big Thompson River drainage bounds the Cache la Poudre to the south in the plains area. To the north, the Basin boundary is formed in the high plateau region of southern Wyoming. The Basin to the east is bounded by the Lone Tree Creek drainage. The Cache la Poudre River joins the South Platte River near the City of Greeley and has a total mainstem length of about 120 miles. About 80 percent of the Basin in Colorado is located in Larimer County with the remainder in Weld County. A small portion of the northern part of the Basin is located in Wyoming. Runoff from this portion of the Basin is small but was considered in the supply analyses. Water diversions, if any occur in the Wyoming portion of the Basin, are reflected in the gage records for stations in the Colorado portion of the Basin. 2.2 PHYSIOGRAPHY The Cache la Poudre River drains a total area of 1890 square miles (sq. mi.) in Colorado and Wyoming, above its confluence with the South Platte River near Greeley. The Basin is divided into two distinct geographical units -- the mountainous upper basin which has an area of about 1050 sq. mi. and the lower basin which has an area of about 840 sq. mi. Sharp hogback ridges separate the upper basin from the lower basin. The lower basin is within the Colorado Piedmont portion of the Great Plains. Most of the runoff in t~e Basin occurs in the upper basin comprising the mainstem Cache la Poudre River and its major tributaries the North Fork (570 sq. mi.) and the South Fork (90 sq. mi.). Most of the water use occurs in the lower 2-1 basin (pl~ins) for irrigated agriculture and for M&I supplies. Sub-basins are discussed further in Chapter 3.0. Basin elevations range from 4600 feet at the South Platte confluence to 13,560 feet at Hagues Peak. The elevations in the North Fork sub-basin range from 5550 to 11,000 feet (South Bald Mountain). Elevations in the South Fork sub-basin range from 6570 to 13,400 feet (Rowe Peak). The major drainage feature of the lower basin is Boxelder Creek which has a drainage area of about 290 sq. mi. 2.3 CLIMATE The climate of the Basin is extremely variable locally in response to variations in topography. The climate of the mountainous upper basin is characterized by colder temperatures and a short growing season typical of the Colorado mountain regime, as shown in Table 2.1. The lower basin, comprised of relatively flat plains, experiences warmer temperatures, less precipitation, and has a longer growing season. Most of the precipitation in the upper Basin occurs in the form of snow in the fall and winter. Scattered thunderstorms contribute a small amount to the total annual runoff. Precipitation in the lower basin is usually erratic and unevenly distributed. Based on Fort Collins records, maximum precipitation in the lower basin normally occurs in May, as shown on Figure 2.2. Average precipitation ranges from 12 to 40 inches per year, as shown on Figure 2.3. Precipitation in the lower basin is sufficient to support a light cover of native grasses and shrubs. Winter grains can be grown; however, successful agriculture is almost totally dependent on irrigation. 2-2 TABLE 2.1 Climatic Conditions in the Basin Lower Characteristics Basin Average Temperature (oF) 48 5(1) 35 Average Precipitation (inches) 12:7-14.5(2) 25 Growing Season (dt~') 90 Snowfall (inches) !~~4t85 100 (l)For 1951-80 per~od. Average at Fort Collins is 48.50 F and average at (2) Greeley is 48.6 F. (3)Greeley (12.7 inches) and Fort Collins (14.5 inches); 1951-80 period. Included in precipitation total and based on information in Tudor Study (1983). ~~~At Fort Collins (Tudor, 1983). Tudor, 1983. 2.4 GEOLOGY The bedrock in most of the Basin area is part of the Precambrian metamorphic basement complex forming the core of the Colorado Front Range. This bas~ment complex includes metasedimentary rock mixed with granitic rock, granite and biotite gneiss and schist, amphibolite, and large bodies of intrusive igneous rock, such as granite or granodiorite. The area is one of youthful topography consisting of deep, V-shaped canyons incised into an old erosional surface that forms the broad, upland foothills located between the plains and the main mountain ranges to the west. Along the eastern margin of the upper basin, a series of sedimentary beds have been upturned and folded by mountain building episodes that formed the Rocky Mountains. These sedimentary rocks consist of sandstone, siltstone, shale, and limestone ranging in age from Pennsylvanian to Cretaceous. The hard, resistant sandstone and limestone beds form sharp, hogback ridges that trend north-south along the edge of the Front Range. The interbedded shale and siltstone units usually form valleys or gentle slopes. 2-3 The Precambrian igneous and metamorphic rocks are jointed and faulted to varying degrees of intensity, but in general the rock is hard, exceptionally strong, and fairly massive. No geologic features have yet been found that would have an adverse effect on any of the potential water project developments considered in earlier studies. More foundation exploration and treatment is expected to be needed for structures founded on sedimentary rock in comparison to structures founded on the igneous or metamorphic rocks. 2.5 LAND RESOURCES The U.S. Department of Agriculture, Soil Conservation Service (SCS), has identified and located the prime farmlands of Colorado which are considered to be of national importance. Prime farmlands in Colorado have adequate and dependable water supply for irrigation, favorable temperature and growing season, acceptable acidity or alkalinity, acceptable salt and sodium content, and few or no rocks. Categories of farmlands considered to have statewide importance include: (1) irrigated lands that are not prime because of susceptibility to wind erosion, high water table conditions and/or salt problems, and other factors; (2) irrigated land with inadequate water supply; and (3) high potential dry cropland that could become prime land with irrigation. Other land categories include land having soils that could become prime if irrigated, urbanized land, water-covered land (permanent water bodies such as lakes and reservoirs), and other land not fitting into any of the above categories. Mapping of farmlands was performed by the SCS in cooperation with the Colorado State University (CSU) Experiment Station. Data contained on the "Important Farmlands of Colorado" mapping for Larimer and Weld Counties is summarized in Table 2.2. 2-4 TABLE 2.2 Farmlands in Larimer County and Weld County(1) in 1979 Category Larimer County Weld County Total Prime 99,000 365,000 464,000 Irrigated (Not Prime) 15,560 118,000 133,560 High Potent~,l Dry Cropland 20,000 417,000 437,000 Prime If Irrigated 30,600 323,000 353,600 Urban Land 59,840 28,300 88,140 Total Land in County 1,689,600 2,581,120 4,270,720 (1)From mapping entitled "Important Farmlands of Colorado" prepared by the SCS and the CSU Experiment Station, 1979. Aerial photography of 1975-76; edited 1979. (2)Generally these lands meet the soils requirements for prime farmland and would become prime if irrigated. Note: Total land in County is not the sum of individual categories -listed above the total because other land uses have not been included as 1ine items. There currently are about 197,600 acres of land irrigated by ditch systems obtaining their water supply from the Cache la Poudre River. About 30 percent of this irrigated land is estimated to be located outside of the topographic limits of the Basin on its eastern boundary. An additional 21,400 acres of land within the Basin are irrigated from non-Basin water sources, primarily the Big Thompson River. The general location of lands currently being irrigated by ditch systems obtaining water from the Basin is shown on Figure 2.4. The extent of the irrigated lands shown on Figure 2.4 is based' on the irrigated area in 1980 (Tudor, 1983). Some land has gone out of irrigation since then, as described iri Chapt~r 6. Water, not land, has been the resource limiting the development of additional irrigated agriculture in the region. As described in Chapter 6, extensions of the North Poudre Ditch system could serve over 100,000 acres 2-5 of land that are categorized as prime lands if irrigation water were made available. Conversion from traditional surface irrigation methods to sprinkler systems also could increase the amount of irrigated lands because more steeply sloped lands could be irrigated. A variety of crops are grown on the lands irrigated from the Cache la Poudre River. Dominant are feedcrops, such as corn, alfalfa, and pasture, which support the local feedlot industry. Sugar beets were once an important crop but the sugar processing plants in the region suspended operations by the early 1980's. Winter wheat is the dominant crop on the non-irrigated lands of the Basin and pasture is a common interim land use on lands being converted from agricultural to urban uses.