River Regulations as Influence on Peak Discharge

River regulation materially affects the magnitude of peak flood discharge in a river basin. Peak flow in the upper Colorado River basin is primarily from snowmelt runoff. The first significant regulation occurred in 1936 with the inauguration of the Moffat Tunnel and in 1942 with the completion of Green Mountain Dam. Three analytical approaches were used to determine the amount of reduction in peak flow resulting from regulation. Utilizing river flow data during the years 1900-1968, a 50-yr flood flow of 31,000 cfs was computed whereas the series 1942-1968 yields 22,800 cfs, a 30% flow reduction. Regional analysis of adjacent watersheds indicated a 10% reduction attributed to climatic conditions. An analysis of tributaries in the basin indicated a 20% reduction in unit contribution resulting from controls. These approaches gave support for the recommendation of a design flow reduced 20% from the peak discharge computed over the longer period of record.