A Model of the Hydrology of the Lakes of the Lower Grand Coulee, Washington

Abstract

The lakes that occupy the abandoned gorge of the Columbia River below Dry Falls, Washington, were formerly fed by groundwater and precipitation, and they lost water solely by evaporation. As a result the lower lakes became saline. Since 1951 an inflow of irrigation water from the development of the Columbia Basin Irrigation Project has caused the lakes to freshen. The lake levels are now controlled by pumping excess water directly from Lake Lenore and from protective wells drilled around the perimeter of Soap Lake. Data on the distribution of deuterium and chloride in the system, on measurements of the flow of water between two of the lakes, and on measurements of the pumpage have been used to develop models of the system for preirrigation and postirrigation periods. The models indicate the quantities of water that entered each lake from natural sources and from the irrigation system and that left by evaporation and by flow or seepage between the lakes, e.g., the quantities required to maintain the balance of water and chloride in the several lakes and in the system as a whole and to reproduce the distribution of deuterium and chloride in the lake waters. Annually prior to irrigation about 14 × 106 m3 of water entered the system and escaped by evaporation. About 20% of this amount was precipitated on the lake surfaces. The remainder was groundwater that entered in about equal quantities two areas separated by, the Park and Blue lakes; the groundwater seepage into these two lakes appears to have been negligible. Precipitation and groundwater have introduced 93 tons of chloride annually, and chloride has accumulated in Soap Lake at the same rate. At this rate 2350 years would have been required for the accumulation of the preirrigation chloride in Soap Lake. At present 20 × 106 m3 of irrigation water enters the system annually. The accessions of irrigation water, like those of natural groundwater, are limited to two areas separated by the Park, Blue, and Alkali lakes. The protective wells intercept much of the irrigation water and natural groundwater that would otherwise enter Soap Lake. The flow of water through the lakes is 1.7 of that prior to irrigation. The wells remove from the lakes 2550 tons of chloride annually, which is 25 times the annual inflow; this removal is rapidly reducing the chlorinity of Lake Lenore and Soap Lake. The history of the lake system in relation to changing climate is discussed.