Flood profiles and inundated areas along the Lower Nisqually River, Washington

Nisqually River flood profiles, covering the reach from near the river mouth to river mile 6.4, were developed in a 2-year field study (1970-72) as part of a continuing program with the State of Washington Department of Ecology. The main channel of the reach will convey without overflow discharges as large as about 21,000 cubic feet per second, which have a 6-year recurrence interval. The banks in some areas will be overtopped at 25,500 cubic feet per second, which has a 13-year recurrence interval. The areas where overbank flooding will first occur, and water-surface profiles of a 3.4-year and 100-year flood were determined for six flood-profile stations. The largest flood for which data are available was that of December 1933; the approximate area that would be presently inundated by such a flood is shown. Alder and La Grande Reservoirs can reduce the magnitude of lower annual flood peaks downstream to some extent, but insufficient data are available to predict their effects on very large floods.