Since the flooding of Banks Lake (the equalizing reservoir for the Columbia Basin Irrigation System) in 1953 and widespread local irrigation, Soap Lake began to change. Based on the research from the NSF Soap Lake Study of 2002, the upper layer of Soap Lake has freshened to about 46.3% of its former pre-irrigation mineralization. Soap Lake is also one of only a few special lakes composed of two layers. This condition is known as meromictic. The lake is ninety feet deep with the lower fifteen feet of the lake many times more saline, alkaline and sulfate than the upper portion. The baseline for Soap Lake was established before any large scale irrigation came to the area by Dr. W. T. Edmondson of the University of Washington in his studies of 1949 when he arrived in the area to find one of the most "interesting lakes" he had ever worked with.
Soap Lake is situated on the upper edge of one of the worlds largest irrigation systems built in the 1950's. There is no question that the recent freshening of Soap lake is due to the surrounding irrigation and the necessary underground and surface pumping which prevents the flooding of the lakeside town of Soap Lake. As the upper layer of the lake becomes fresher due to the dilution of the lake from the surrounding irrigation, minerals are robbed from the lower, denser layer. The line of meromixis, the margin between the upper and lower layers descends into the lake a few inches per year. Eventually, if nothing is done, this lower denser layer will disappear entirely. Here are two articles explaining the pipes and pumps of the irrigation system and the declining condition of the lake.
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