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<br />. ,~ <br />.. - .. . _. <br />22 <br />The Virgin River has an average gradie-nt of about 7.6 m/km <br />(Figure 2). From the Markagunt Plateau (approximately 2,700 m) to <br />the base of the Hurricane Cliffs (1,676 m) it has an average gradient <br />of 15.2 m/km; in Parunuweap Canyon the gradient is about 13.3 m/km; <br />La Verkin Creek has a gradient of 20,8 m/km. The fall of many of the <br />tributaries to the Virgin River exceeds 37.9 m/km-and may, for short. <br />stretches, reach 94.7 m/km (Gregory 1950). <br />In some areas the river expends so much energy carrying vraste <br />materials from upstream sources that it literally chokes itself with <br />sediment. During low water the river is often observed meandering <br />about in braided channels or, just as commonly, it disappears from <br />the surface and flows underground (Gregory 1.950). - <br />Detailed discussions of the geology and structural history of <br />the Virgin River basin can be found in: Blackwelder 1934, Cook i957 <br />and 1960, Gregory 1950, Hunt 1969, Ives 1947, Longwell 1949, iongwell <br />et al. 1965, l.ucchitta 1972, McKee et al. 1967, McP;air 1951, "avian <br />1943, Utah Geological Mining Survey 1952. <br />._ ' ti_ .- <br />