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2365 content of the water. The per acre water use was a weighted aver- age based on the relative amounts of land above and below Palo Verde Dam. Water quality was based on the relative amounts of vegetation supplied with river water and with a combination of agricultural drainage and ground water. Average salt content estimates varied from 1. 41 tons per acre-foot in 1965 to 1. 62 tons per acre-foot in 1976. The resulting salt retention by natural vegetation is shown graphically by Plot A on Figure 5. Flushin~ of Ground Water. Subsurface irrigation drainage sets in motion a pattern of subsurface flow that tends to flush out ground water from beneath irrigated lands. It is assumed that the zone of active ground-water flow in response to irrigation occurs to a depth of about 130 feet below the flood plain in the Parker Valley, because the top of the Bouse formation, an extensive layer of clay, lies at that depth. The ground-water flushing effect of deep percolation from irriga- tion and canal seepage was estimated by comparing the salinity of the leaching fraction plus seepage with the quality of the underlying ground water. Based on apparent relationships in the Palo Verde Irrigation District, it was assumed that the drainage outflow was half ground water. The salt discharge from flushing estimated in this way was minor--about 20,000 tons per year between 1965 and 1975. The quality of the deep percolation was estimated to be slightly better than the quality of the underlying ground water. The effect of ground-water flushing is shown graphically by Plot B on Figure 5. Leaching of New Lands. The use of ground water by natural vegetation and possibly other factors cause an accumulation of soluble salts in the unsaturated soil. When natural areas are cleared and irrigated, most of the accumulated salts are flushed from the soil into the ground water and subsequently into the drainage system. The tonnage of salt involved was estimated as follows: The amount of salt per acre of new land has been influenced by flooding of the Colorado River. Records of riverflow indicate that much of the central and southern part of the Reservation was flooded regularly prior to Hoover Dam, and again in 1941. High water in the river prior to Hoover Dam was more likely to cause overbank flows 17