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'- ~,.~ 58 '," ' HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT "", pIe: There are 61 years of record for the Umpqua River near Elkton where the lowest annual peak discharge prior to 1977 was 33,100 ft 3/s, recorded in 1915. But in 1977 the peak discharge was just 13,000 ft 3/s or 39 percent of that in 1915, On the other hand, record low minimum or daily flows were the exception rather than the rule throughout Oregon, Another anomaly related to the drought was the timing of the annual peak discharge, On the Applegate River near Copper in south- western Oregon, the maximum discharge, 860 ft 3/s, during the 1977 water year occurred on September 28, 1977, only 16 days after the minimum discharge, 19 ft 3/s, occurred, Gen- erally about 90 percent of the annual peak dis- charges occur between November 1 and March 31 near Copper and most of the others occur in October, April, or May, Reservoir storage in October 1976 was about average, but the heavy drafts needed to supplement the low runoff in 1977 reduced the storage in a rather mixed pattern. In Idaho, reservoir levels were lower in 1977 than any time since the reservoirs were first filled. Some reservoirs supplying irrigation water in eastern Oregon went dryas early as August 1, though 25 of the major reservoirs used for irri- gation in Oregon contained 773,000 acre-ft on October 1, 1977, which is 58 percent of the average for that date. In Washington, the rains in August and September prevented the irrigation reservoirs from being depleted as much as expected, In fact, they generally stored more water than they contained in 1973, the previous year with low runoff. Reservoirs used primarily for power generation were below normal. Ground-water Conditions Ground-water levels in western Montana were not affected by the drought except locally in shallow wells in the Bitterroot Valley south of Missoula. Domestic wells in Idaho near Carey and in the Wood River basin went dry early in April 1977. See figure 4 for the effect of reduced recharge in a well east of Carey. Many shallow wells in six counties in western Idaho went dry in June. Water levels in 400 wells in Idaho were monitored either on a bimonthly or semiannual schedule. Another 220 wells were measured in March and September 1977 and again in the spring of 1978. Declines ranged from 0,2 to 25 ft between the fall of 1976 and the fall of 1977. The larger declines occurred in the val- leys of tributaries to the Snake River from the south between the Raft River and Salmon Falls Creek in south-central Idaho. The declines are attributed to one or a combination of the following factors: Below normal precipitation caused reduced recharge, increased with- drawals of ground water to supplement dimin- ished surface-water supplies, and more effi- cient use of irrigation water because supplies were restricted or irrigators tried to keep the additional costs to a minimum. Water levels in wells respond to irrigation practices whether the water applied is surface water or ground water, The yield of wells in eastern Washington did not decline, but by early March 1977 water levels fell 3 to 10ft below the levels of the spring of 1976 to about the levels expected in the fall. Water levels in two wells in the Kitsap Peninsula in western Washington were still declining on June 28, 1977, whereas in June 1976 they were approaching their seasonal high levels. Though ground-water levels were low, few of them reached record low levels. The few new record lows were not very significant because the short records do not include the drought of 1944, Near Tacoma, 3 of 29 wells ina water district went dry, but supplies were ample. Ground-water supplies in Oregon were not generally deficient; however, the combination of less than normal recharge and greater than normal pumping produced abnormal water- level declines. In some areas record low levels were reached, In the dune area near Coos Bay, Ore., the additional pumping and low precipi- ta tion caused several sand-dune lakes to go dry, A drop in water level was not always bad news. The good news in some areas of the Willamette Valley in Oregon was that the lowered water table allowed good crops to be raised on fields that are too wet in normal years. Water levels in parts of the Willamette and Tualatin Valleys of Oregon dropped to all-time lows or nearly so by February 1977 because the usual recovery from November through JailU- ary did not take place. The water level in a well north of Eugene, Ore" was 9 ft below