7075 - Laserfiche WebLink

Home Browse Search

Exceedance probability hydrographs were developed for the Little Snake near Lily and Yampa River near Maybell stations, retaining the historic shape of the annual hydrograph. A suspended sediment budget analysis was preformed, summing the Little Snake and Yampa River loads and deeming this as the supply available at Deerlodge Park. Supply at Deerlodge Park was compared with Mathers Hole and the surplus or deficit determined on an annual basis. The budget showed an approximate equilibrium (5%), between upstream suspended sediment supply and transport through Yampa Canyon. The exceedance hydrographs were also input to the sediment budget, along with baseflows determined from historic record. Results of the various reduced hydrographs showed a sediment balance would only be maintained if streamflow of each tributary (Little Snake & Yampa) was reduced by equal proportions. Also, if the Little Snake River remains essentially undepleted, the Yampa River i should not be depleted over 100,000 acre-feet/year if equilibrium is to be 'maintained. DISCUSSION OF PREVIOUS STUDIES The results of the mathematical modeling effort for the cobble reach should be viewed in terms of trends, and not in absolutes. This is because there are a number of assumptions, theory and estimations employed, as stated previously and in O'Brien's report. It is my opinion that the modeling indicates some depletion can occur without affecting the cobble substrate, but depletions ?hould not be 'pushed' towards 288,000 acre-feet/year based solely on this ffort. The manner in which depletions occur, or the resulting distribution of high and low flows, will greatly affect the volume which may be depleted. O'Briens later budget indicated only 100,000 acre-feet/year could be depleted _nd still maintain an equilibrium in annual suspended sediment transport. so, in any budget study, if the prevailing system is stressed too greatly, the sediment rating curves which the sediment budgets utilize may no longer be valid. More importantly, the location (Little Snake v. Yampa) and the type of project (dam and/or diversion) will have major bearing upon the tons of sediment and acre-feet of water that may be safely depleted without affecting the Yampa River channel morphology. The relation of the Yampa River's sediment supply to -the Green River should also not be overlooked, as reductions in the Yampa River's sediment load will also affect the channel morphology of the Green River. For these reasons, it is impossible to define one single 'minimum `hydrograph' which can maintain Yampa River sediment transport for all possible scenarios of development and also not induce further channel responses in the Green River.