7196 - Laserfiche WebLink

Home Browse Search

15 The river contacts the Weber sandstone in the lower portion of the canyon. In many areas the river flows directly on the sandstone bedrock which is soft and easily eroded. As the Green River confluence approaches the river slope becomes more mild. Reaches of cobble substrate have evolved into a riffle-pool. sequence in this section. The very mild sloped portions have a sand and gravel substrate. In contrast, the initial twenty-five miles of canyon, are uniformly steep punctuated by rapids and corresponding backwaters formed by side canyon flood events. The river is characterized in the cobble substate reach as a wide bend, point bar stream with a distinct riffle-pool sequence. The wider bends are the remanents of the meandering paleochannel whose pattern was partially preserved in the structure of incised bedrock canyon. The Yampa riffle-pool sequence is not typical of a natural meandering river because of the bedrock controls. Riffle spacing is normally five to seven times the channel width, but riffles have developed in the Yampa wherever channel expansions have occurred; most often just upstream of wide bends. The long term development of the shape, size and orientation of cobble bars and islands in this reach has been a gradual process starting with initial formation of meander incision. Over geologic time a sequence of large, infrequent disharges contribute to the progressive downstream movement of the cobbles. The riffle-pool. sequence evolves as a function of the large discharges; the cobbles tending to pile up in the flow expansions upstream of bends. The riffle is initiated by the leading edge of the depositional region. In this fashion, the bar or island stability has been established on a quasi-permanent scale with lesser discharge events inciting limited cobble motion. Maintenance of channel dynamic equilibrium requires progressive adjustment of slope and spatial variation. Bars tend to be energy dissipating structures that promote overall channel stability. Energy expediture per unit bed area is equalized with mobilization of cobbles and localized width and depth adjustment. Channels around cobbles bars are reformed with failing side slopes and changing widths. At peak flows vertical accretion of the cobble bar is an example of depth adjustment. Such accretion, forces additional flow to impinge on the banks and create side channels of high velocity and unstable beds. The large sediment load in the Yampa River is supplied principally by the Little Snake River. Sufficiently steep slopes and velocities insure that the sediment is transported without significant deposition in the canyon. Pool reaches often have a sand substrate which degrades during peak discharges. Cobble substrate riffles remain essentially free from sand deposition in spite of the large sediment load. The processes which insure a sand free cobble substrate are closely linked to a seasonal hydrograph in terms of shape and duration. Historical Flow and Flood Freauencv Analvsis USGS gaging stations are located at Maybell on the Yampa River and Lilly on the Little Snake, approximately 40 and 15 miles upstream of Dinosaur National Monument, respectively. No substantial tributary