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( 1 DESCRIPTION OF THE STUDY AREA Geology and Physiography _ The Yampa River 7 The Yampa River flows generally westward within Dinosaur National Monument. The river has cut a deep and often narrow canyon into the sedimentary rock of the east end of the Uinta uplift. From Deerlodge Park (elevation 5600 f t.) at the eastern end of the Monument to its conf luence with the Green River at Echo Park the Yampa River travels 46.5 miles and drops 540 ft. in elevation. The average river gradient from Deerlodge Park to the confluence with the Green is 11.6 ft./mile with a range between 1 ft./mile at Deerlodge Park (mile 46.5, measured from the conf luence) and above Big Joe Rapids (mile 23.5) to 31 f t./mile at Tepee Rapids (mile 36.4). The upper 23 miles of the river averages 17.0 ft./mile, while the lower 23.5 miles (below Big Joe Rapids) averages 7.4 f t./mile. The aerial distance between Deerlodge Park and Echo Park is 25.1 miles. The sinuosity of the river, or the ratio between the channel length and the down-valley distance, is 1.85. The sinuosity of the upper 23 miles is 1.51 and that of the lower 23.5 miles is 2.22. Leopold (et al., 1964) defined a meandering stream as one with a sinuosity greater than 1.50, thus, the Yampa can be described as meandering, with an increasing sinuosity toward the lower end. The Yampa River contacts six geologic units within the canyon. These are given below by increasing age and follow the nomenclature used in the USGS geologic maps (Hansen 1977a and 1977b, 1978; Rowley et al. 1979; Hansen and Carrara 1979; Hansen et al. 1979; Hansen and Rowley 1979): 1. Weber Sandstone (Upper Pennsylvanian) This cliff-forming, buff-colored sandstone contacts the river for less than