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IJ River Hydrology General This section provide researchers and scientists using the staff gage and level logger data L base some general background on the river hydrology in Canyonlands. Amore detailed discussion is presented in FLO (1995b). The Green River hydrology is dependent on the spring snowpack melt of the Rocky Mountains in Wyoming, Colorado and Utah. The river drains approximately 44,700 miz. The principal tributaries include the Yampa and White Rivers in Colorado, and the Duchesne, Price, and San Rafael Rivers in Utah. Flow in the Green River has been regulated by Flaming Gorge Dam near the Utah-Wyoming border since October, 1962. In addition, other -water storage ' projects throughout the basin have effected the Green River flows including Fontenelle Reservoir on the Green River upstream of Flaming Gorge, Strawberry Reservoir on the Duchesne River, Taylor Dam on the White River, several small reservoirs on tributaries to the Yampa River, a transmountain diversion on the Little Snake River, and irrigation projects throughout the basin. The source areas of water and sediment in the Green River watershed are not distributed ' equally in the basins. The primary water source areas are located in the headwaters of the various tributaries throughout the Rocky Mountains rimming the basin. These areas have very little sediment yield. Most of the sediment load is delivered from the semi-arid regions in the ' middle to lower portions of the watershed (Andrews, 1986). ' Peak Flows, Mean Annual Flows and Base Flows The Green River generally peaks in the late spring with a base flow period extending from about September 1 to March 1. At the USGS Green River gage, the seasonal hydrograph is characterized by a snowmelt peak discharge usually occurring between May 15 and June 15. For the 95 year period of record, the mean date of peak flow occurrence was May 30. The highest peak on record at Green River, Utah was 68,100 cfs on June, 27, 1917. The lowest peak on record was 6,460 cfs on May 17, 1934. Both, the highest and lowest peak discharges at the 1 Green River gage occurred before any mainstem dam construction (Figure 2). For the first 29 years of record until 1930, the mean annual peak discharge was 39,700 cfs corresponding to the average bankfull discharge in the Canyonlands study reach (FLO, 1995b). The post-1963 mean ' annual peak discharge at the USGS Green River gage is 22,280 cfs. Base flows for the period of September 1 to March 1 were estimated. Flow regulation in the Green River system has affected the mean base flow in the river. The mean base flow has increased from 2,720 cfs pre-1963 to 3,380 cfs post-1963 at Green River, Utah. This increase in ' base flow reflects upstream flow regulation and the redistribution of the peak flows throughout the year. The pre-1963 ratio of mean peak discharge to mean base flow for the period Sept 1 to March 1 was 12.1. Following 1963, this ratio was reduced to 6.6 at the Green River gage.