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9 <br />Dams and Reservoirs -- There are three major impoundments on. the lower Colorado <br />River: lakes Mead, Mohave, and Havasu. These differ fundamentally. in morphology <br />and limnological characteristics (summarized in Table 1). <br />Hoover (Boulder) Dam and Lake Mead were created primarily to provide flood control <br />and storage of irrigation water. The dam also generates hydroelectric power. Primary <br />inflows to Lake Mead are the Colorado River at the outflow of Grand Canyon' and <br />Virgin River. The Virgin River is largely unregulated, although there are several <br />diversion dams, which under some conditions result in severe flow depletions. Lake <br />Mead also receives treated urban wastewater from the city of Las Vegas, which adds <br />substantial quantities of inorganic nutrients to the system. It is morphologically complex <br />with several basins separated by narrow, deep channels. As a result, different parts of <br />the reservoir exhibit unique limnological characters. In one study the Upper Basin (at <br />the Colorado River inflow) was oligotrophic, Boulder Basin was oligo-mesotrophic, and <br />Las Vegas Bay was mesotrophic (Paulson et al. 1980). Overall, the lake is <br />characterized as "mildly-mesotrophic" (LaBounty and Horn 1997). Outflow of Lake <br />Mead is a hypolimnetic discharge at Hoover Dam in Black Canyon. <br />Davis Dam and Lake Mohave were created primarily to re-regulate fluctuating <br />discharge from Hoover Dam. Hydroelectric power also is generated at Davis Dam. <br />Lake Mohave is best described as a "run of the river' impoundment. An upper reach <br />below Hoover dam flows for about 40 km through a steep, narrow canyon which <br />empties into a modestly-wider "Little Basin." Next is the relatively wide (6 km) <br />Cottonwood Basin, which narrows at its downstream end to form another canyon reach <br />s The Colorado River through Grand Canyon now is regulated by Glen Canyon <br />Dam on the mainstream about 16 km above Lees Ferry. The dam releases cold, clear <br />hypolimnetic water year around, with discharge regime determined largely by demands <br />for generation. Sediment inputs to the Colorado River in this reach are almost <br />exclusively via tributary streams, the two largest of which are Paria and Little Colorado <br />rivers, and the mainstream runs clear unless these other rivers are in flood.