Ground water beneath the 6.6-square-mile Chehalis Indian Reservation, Wash., most of which lies on a flood plain, occurs in the alluvial and terrace sand and gravel deposits. Supplies available within the depth zone of water-level fluctuations are estimated to be between 20 and 60 times that being pumped at the time of the 1975 study. Seasonal water-level changes in wells obtained during the study ranged between about 3 and 10 feet and averaged 7 feet, and the average fluctuation of water levels in three oxbow lakes was about 6 feet. Chemical analysis of ground water shows it is suitable for most uses and that to date there are no serious problems of ground-water pollution. Surface-water quality also is considered generally good. Flooding of reservation land by the Chehalis and Black Rivers occurs annually, generally during periods of heavy rainfall in winter. The largest known flood of the Chehalis River, recorded on January 22, 1972, had an estimated discharge of 53,000 cubic feet per second. Flood-frequency analysis indicates that about 70 percent of the reservation would be flooded during a 50-year flood. Low flows generally occur in August and September. The lowest flow of Willamette Creek (the only stream entirely on the reservation) during 1975 was about 0.45 cubic foot per second, recorded in August. (Woodard-USGS).
