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v. <br />limestone. These are often areas of constant width with sheer <br />wall (cliff) for shoreline. Portions of Reach 40 fall within <br />this classification. <br />The Colorado River between Lee's Ferry and Lake Mead drops <br />671 m in elevation. The river is a series of long runs and short <br />steep rapids. These long runs have a water surface gradient near <br />five 'feet (ft) in 10,000 (1.5/3048 m) (Leopold 1969). Rapids, <br />which account for most of the drop in elevation, fall 5-17 ft per <br />1,000 (1.5-52/304.8 m). Mean velocity through the rapids is <br />generally 11 to 15 feet per second (fps). <br />Tributaries to the Colorado River within this study area <br />generally fit one of two categories described by Hamblin and <br />Rigby (1968). The first category includes major streams with <br />large drainages, low gradients and well developed meanders. The <br />Paria and Little Colorado rivers and Kanab Creek fit this <br />description. In general, substrate in this type stream consists <br />of silt and clay. <br />The second type of tributary found in the study area is a <br />generally short, relatively straight stream originating from <br />springs and seeps issuing from various geologic formations within <br />Grand Canyon. Gradient-in these streams is generally steep, and, <br />as a result, substrates are predominantly cobble, gravel, and <br />sand. Nankoweap, Bright Angel, Deer, and Tapeats are good <br />examples of this type of stream. <br /> <br />-17-