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112 rISHFS AND FISHERIES OF NEVADA below Davis Dam, has lost many of its original features because it now represents drainage from a lake rather than unimpeded river flowage. Historically, the Colorado River was discovered and explored quite early. Captain Garcia Lopez de Cardenas, of C'oronado's force, is rcn- erally credited as the first white man to see the river, moving up to it with a small force from Cibola or Zuni in 1510. His brief contact with the river was in a portion of its Utah traverse. Padre Francisco Garct-%. the first man known to touch upon any portion of Nevada, saw the. State somewhere in the vicinity of its Colorado River border in 1776. (2) CHARACTERISTICS OF THE SYSTEM ? f -. t ?i fi Since river conditions typical of the original Colorado no longer exist in that portion of the channel impinging on Nevada, its one-time lim- nology is only of academic interest in showing how the water environ- ment has changed and how this has been reflected iu alterations in the, fish fauna. PHYSICAL AND CHEMICAL CONDITIONS WATER MOVEMENTS Strong, continuous water motion is a dominant property of the Colo- rado River as a habitat. As stated, the only river section left is t.l?e 14-mile stretch below Davis Dam, but originally the entire length of the river adjacent to Nevada represented water which contained species unique to it. The effects of this swiftly-moving water are reflected in the rather bizarre forms of some of the dominant native fishes-the Gila Chub, Colorado Squawfish and Razorback Seeker all have flat. sloping heads which tend to hold them against the bottom when pointed upstream. The chub and sucker in turn have conspicuous dorsal kei•Is which have a stabilizing effect in the current. The most peculiar fish ill this regard is Miller's Gila "cypha," an extremely streamlined form of the Gila Chub. While movement of water is the channel below Davis Dam has not changed particularly, other river peculiarities have altered consider- ably. TEMPERATURE AND TURBIDITY This, of course, still shows the familiar pattern of ;gradually increas- ing as the river flows southward, but is significantly colder in the lower levels and warmer in the upper reaches. In addition to cooling the river waters, Lake Mohave behind Davis Dam is also a silting basin and water emerging from the dam is clear for many miles downstroaut. 'Actually, most of the silt is taken out of the river by Lake Mead. su that the once conspicuously opaque river is now colder and clearer-n much changed environment. 0XYGEN-IIISs0IXF,D SOI.II S-10 As would be expected, these are variable. The two oxygen readin-^ the writer took in the Fort Mohave area gave results of 6.8 and 7.:1 parts per million, indicating the anticipated sufficiency of the gas.