Streamflow gains and losses in the Snake River and ground-water budgets for the Snake River Plain, Idaho and eastern Oregon

Abstract

The Snake River is the regional drain for streams and aquifers in the Snake River basin upstream from Weiser, Idaho. The interaction between the river and ground water was quantified as streamflow gains from and losses to the aquifers. Upstream from Milner, Idaho, the Snake River both gains and loses water. Some reaches gain or lose throughout the year; other reaches gain during the irrigation season when ground-water levels rise as a result of application of surface water for irrigation but lose during the rest of the year. The largest continuous streamflow gain upstream from Milner is from springs between Blackfoot and Neeley, where, in 1980, the Snake River gained 1.9 million acre-feet of ground water. Downstream from Milner, the Snake River is a gaining stream. Gains arc largest between Milner and King Hill where numerous springs discharge to the river. In 1980, the Snake River gained 4.7 million acre-feet of ground water between Milner and King Hill. Although large springs were present in the Blackfoot-to-Neeley and Milner-to-King Hill reaches before irrigation began on the plain, the application of surface water for irrigation increased recharge to the Snake River Plain aquifer; therefore, spring discharge to both reaches also increased. However, despite changes in irrigated acreage and corresponding changes in the amount of water potentially available for recharge, spring discharge between Blackfoot and Neeley has remained relatively stable for 69 years (1912-80). During the same period, discharge from individual springs and total ground-water discharge between Milner and King Hill have increased and decreased substantially in quantities that can be attributed to changes in irrigation. The changes in ground-water recharge and discharge and ground-water storage generally are the net result of 100 successive years of irrigation on the Snake River Plain. Water-budget analyses indicate that the total volume of ground water in storage in the main part of the eastern Snake River Plain increased about 24 million acre-feet from 1880 to 1952, largely as a result of increased recharge in areas irrigated with surface water. The total volume of ground water in storage decreased about 6 million acre-feet from 1952 to 1980 as a result of several years of below-normal precipitation, increased pumping of ground water for irrigation, and other changes in irrigation practices. Ground-water storage in parts of the western plain increased about 3 million acre-feet from 1930 to 1972 but generally has decreased since 1972.