OCR Text |
Show PART II DESCRIPTION Historic unit discharge on the main stem, as shown in the following tabulation, decreases rapidly as the streams flow from head- waters at high altitude into the less humid areas and finally into desert areas. Drainage area and historic unit discharge Upper Colorado Region Gaging Station Drainage area Record prior to 1965 Unit discharge Square Miles Years Cfs/Sq. Mi. Colorado River near Grand Lake, Colorado 103 46 0.922 Colorado River at Glenwood Springs, Colorado 4,560 66 .596 Colorado River near Cameo, Colorado 8,050 32 .484 Colorado River near Cisco, Utah 24,100 54 .327 Colorado River at Compact point, Lee Ferry, Arizona 109,580 1/ 52 .155 1/ Drainage area as measured in connection with this study; other area figures are from USGS Water Supply Papers. Part of the decrease in unit discharge as the river proceeds downstream can be attributed to exports from the region, depletions for irrigation and other consumptive uses, losses to ground water recharge, transpiration, and evaporation. However, most of the decrease is due simply to lower contribution from the downstream areas. Yet, as shown by the preceding map "Average Annual Basin Runoff," there is almost always an increase in discharge in the down- stream direction, except immediately below large diversions. Large variations in annual discharge occur also from year to year due to yearly variations in precipitation, and over periods of years due to long-term climatic trends. The average annual discharge of the Colorado River at Lee Ferry was 12,426,000 acre-feet for the 52- year period 1914-65, with extremes of 21,894,000 acre-feet in 1917 and 4,396,000 acre-feet in 1934. For the 17-year period 1914-30, the average discharge was 15,919,000 acre-feet per year, while for the 26- year dry cycle 1931-56 the average discharge was 11,183,000 acre-feet per year. These are residual flows reflecting upstream depletions approximating 1,800,000 acre-feet in 1914 increasing to about |