Streamflow Predictability in the Upper Versus Lower Colorado River Sub-Basins

Streamflow in the Upper Colorado River in the Western United States is always snowmelt dominated, while the Lower River’s perennial streamflows are snowmelt dominated only 50 % of the time. The magnitude and timing of peak flows is important for water resources management. In the Upper Basin the annual maximum daily discharge usually occurs in May or June, while in the Lower Basin this peak is observed to occur in any month except May or June. The timing of one-half of the runoff volume is used as a second measure of the variability in timing and magnitude of streamflows. For the Upper Basin, nine watersheds are used to illustrate streamflow trends, with the Yampa River used as a sample sub-basin. For the Lower Basin, five watersheds are used, of which the Salt and Little Colorado Rivers are used as sample sub-basins. The differences in monthly flow variation over 20-year time periods (1920–1939, 1940–1959, 1960– 1979, and 1980–1999) are substantial for the Salt and Little Colorado Rivers but not for the Yampa River. There is a good correlation between snow water equivalent (SWE) and winter runoff volumes for the three sub-basins. A weaker relationship exists between SWE and non-winter flows for the two sample Lower Basin watersheds