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. . . ~ o O'J c.rJ CAUSES OF SALINITY (continued) acre-feet (380 x 106m3). The total salt inflow in this reach varied from a low of 498,000 tons (452 000 t) in 1962 to a high of 2.022 million tons (1834 x 103 t) in 1941 with an average inflow of 1.073 million tons (973 000 t) per year. Springs in the lower portion of the Little Colorado River contribute about half of the measured increase in dissolved-solids discharge in the Colorado River between Lees Ferry and Grand Canyon. Large amounts of dissolved solids are also added to the Colorado River between Grand Canyon and Hoover Dam. Some of this results from the solu- tion of materisl in the bed of Lake Mead, but like the reach above Grand Canyon, most is contributed by springs and tributary inflows. For the whole reach from Glen Canyon Dam to Hoover Dam recent studies have been made by the Geological Survey and the Water and Power Resources Service to provide information about the contribution of springs to the Colorado River. Major springs and spring-fed tributaries which could be measured were found to contribute about 757,000 tons (687 000 t) of dissolved solids annually to the Colorado River between Glen Canyon Dam and Lake Mead. Storm runoff in small tributaries in this reach of the Colorado River contributes an unknown, but probably much smaller, load to the river. The contribution of dissolved solids by these sources of inflow between Glen Canyon and Lake Mead equals about 10 percent of the average dissolved- solids load of the Colorado River at Lees Ferry. The average annual major springs, streams, between Glen Canyon Dam rized in Table B. dissolved-solids contributions of the measured and spring-fed tributaries to the Colorado River and Lake Mead and to the Virgin River are summa- The totsl average annual inflow of 1.403 million tons (1270 x 103 t) from these sources results in an increase of about 73 mg/L in the Colorado River on the basis of an average annual flow of 10.5 million acre-feet (13 000 x 106m3) at Hoover Dam. 2. Agricultural Sources of Salinity Irrigation in the Colorado River basin has increased the total dis- solved solids in the Colorado River. Return flows from the irrigated lands pick up salts from the soils and underlying shales and transport them to the river system. The development of future irrigation projects will further increase the salt load to the river. Studies prior to irrigation would be helpful to determine contribution from irrigation, but they have not been made in most areas. The amount of salt from this source must therefore be estimated or determined by detailed investigations possibly with the use of simulation models. 30