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13 <br />extensively developed. Indeed, in the former they estimated 40 % of banks in the <br />upper reach was comprised of riprap, while only about 3 % was so modified in the <br />lower. There was more than twice as much natural riparian and cattail habitat in the <br />relatively unmodified section. Their data cannot be directly extrapolated to other river <br />reaches of the lower Colorado River, but nonetheless provide a useful indication of the <br />range of conditions available along the lower river (see also Minckley 1979). Habitat <br />types vary, and except for those dictated by fixed structures, their location and extent <br />both are variable through time and in response to discharge changes (see Marsh and <br />Minckely 1985, 1987). There are only a few reaches of the river that persist un- <br />dredged and without structural modification. Eldorado Canyon, a steep-walled chasm <br />immediately below Hoover Dam, remains intact, although the bottom has been scoured <br />and armored, and thermal and discharge regimes are unlike the natural condition. <br />Likewise, but less spectacular, are short reaches below Davis and Parker dams, which <br />otherwise have been subjected to extensive shoreline development for housing and <br />recreation. <br />Off-Channel Habitats -- Some aquatic habitats adjacent to the lower river are remnants <br />of old river features such as oxbows and floodplain lakes, but these largely are <br />discontinuous with the present-day mainstream, or connected only at highest river <br />stages. Exceptions include Topock Marsh, created by sediment accrual following <br />closure of Davis Dam and filling of Lake Mohave, fitted with intake and outlet structures <br />to maintain desired water levels; several large backwaters in the reach downstream <br />from Blythe, CA; Palo Verde Oxbow and Cibola lakes, and extensive backwaters and <br />off-channel sites in the reach above Imperial Dam, that represent a small fraction of <br />those present historically. <br />Substrates -- Substrates of the lower Colorado River are unremarkable. Bedrock, <br />boulder and gravel predominate in armored reaches below major dams and in Topock <br />Gorge, while'sand is the typical bottom material in channelized segments. Depositional