Project Competion Report, OWRT Project No. A-055-ARIZ / Donor Reference Number: 14-31-0001-5003 / Period of Investigation: July - August 1974 / The work upon which this report is based was supported by the United States Department of Interior, Office of Water Research and Technology, as authorized under the Water Resources Research Act of 1964. / The rapid expansion in the development of real estate lakes in the Southwest has produced a somewhat haphazard use of clay soils or clays in attempts to seal these lakes against seepage losses. This situation is further aggravated by the fact that very little basic information exists at present on the equilibrium and movement of water in a swelling clay soil, which is the type of natural lining material of direct relevance to seepage control. This report presents new results in the theory of swelling clay soils, including a description of the equilibrium moisture profile and the steady flow of water in a submerged, saturated, natural clay soil liner. The theory then is applied to develop an equation for the rate of seepage (the rate of lowering of the water surface) through a swelling liner in a real estate lake of simple trapezoidal configuration. This equation is compared to the standard results for the seepage rate, as calculated on the classical theory of water flow through non-swelling soils, and is applied to estimate the seepage rate from an experimental reservoir studied by Rollins and Dylla. The principal conclusions are: (a) that the major effect of swelling in the liner, except for very shallow lakes, is to cancel the contribution of gravity to the seepage rate, (b) that the most important factor determining seepage loss is likely to be the soil water tension in the pervious soil surrounding the lake and liner, and (c) that the seepage equation can provide a useful estimate of the rate of loss when the important geometric and soil water parameters for the lake, the liner, and the surrounding soil are available.