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396 Robert Rush Miller Irrigation and dams.-Consumption of water is the chief way in which irrigation operations affect fish life. Once diverted from the river it is expended on the land and lost by evaporation. Fishes may be carried out into the canals and ditches at the point of diversion and later perish when the water is cut off or as the habitat becomes un- suitable. Dams prevent upstream migration and in years of low water all fish below them may be killed as the stream bed becomes dry. The prevention of upstream movement may particularly affect large fishes, such as the Colorado squawfish, that develop in the lower courses of rivers but spawn in tributaries. The greatly altered habitat produced by impoundments may prove to be wholly unsuited to the native species-especially when, as so frequently happens, they are placed into competition with introduced species. Mining operations.-The sediments from concentrating mills rap- idly fill up river channels and destroy fish habitats-in much the same way as do floods., Thus streams become barren for considerable dis- tances below the point of entry of such wastes. Cyanide and other poisons resulting from ore processing also may denude a stream of all life if such chemicals are permitted free access to surface waters. Al- though chemical pollution is very destructive, its effects have gen- erally been local in the Southwest and have not seriously threatened the native fish fauna. It has had more serious consequences outside of the area under discussion (Ellis, 1914, pp. 128-129; Sumner and Smith, 1940). Industrial pollution has risen in importance with in- creasing population trends. Familiarity with powder in mining op- erations has also led to the use of dynamite as a means of obtaining fish-a-destructive practice since only a small proportion of the ani- mals killed can usually be recovered. Such a fishing method, if widely used, may lead to streams nearly barren of fish life. Depletion of ground water.-Pumping of water and the deep trenching of the valley floors has gradually lowered the water table in the Southwest. Springs and cienegas went dry, streams ceased to flow or diminished in size, and wells had to he drilled to greater depths. Continuing demands for increased water supply following the population explosion since World War II have brought about an even more drastic lowering of the water table. For example, in 1950, one of the few remaining perennial flows of the Santa Cruz River went dry near San Xavier Mission for the first time in recorded his-