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Changing Fish Fauna of the Southwest 395 n destructive floods may originate in a region of depleted soil and plant cover whereas adjacent, well-vegetated areas may at the same time receive no damaging runoff (Bailey, 1941, p. 245). The cata» strophic nature of stream erosion after ecological disturbance by man has been dramatically described.and illustrated for Kanab Creek, Arizona (Gregory, 1950, pp. 174-175, fig. 113). Bryan (1925, p. 342) stated that in only 10 years the channel of the San Pedro River of southern Arizona was incised from its mouth for 125 miles upstream. Destruction of the climax grassland by excessive grazing and trampling has, since about 1890, allowed the rapid spread of the ag- gressive mesquite tree beyond the bottom lands to which it was j formerly confined. The serious effects of this invasion are recog- nized by those concerned with the use and conservation of vegetative cover, land, and water. The increase and spread of mesquite has been accompanied by a rise in the population of certain small rodents which further disseminate the mesquite; thus a vicious circle has set in. Rodent pressure alone may prevent the revegetation of deterio- rated grasslands that are overburdened with mesquites (Glendening and Paulsen, 1955, pp. 45-47). The conspicuous arroyos of our semiarid regions, so prominent in Arizona and New Mexico, were largely cut between about 1880 and 1910. Their streams are mostly ephemeral, their channels have flat bottoms and vertical walls, and they range in size to 100 feet deep, many hundreds of feet wide, and 140 miles long (Antevs, 1952). That these erosive gashes were brought on by excessive numbers of live- stock has been fully demonstrated by Thornber (1910) and others (see Thornthwaite et al, 1942, pp. 120122). Cutting of timber in the forested headwater areas for use in mines, for fuel, for house construction, and in clearing land also promoted rapid runoff, and often a lowering of the water table, and resulted in damaging erosion from floods. Removal of protective vegetation led not only to depletion of perennial streams and lowering of the water table but also to the destruction of habitats for fish life in the sur- viving waters. The filling in of deep pools by sand and silt has elimi- nated. species that habitually seek and require such places for sur- vival, and the water has been rendered unfit for respiration when completely saturated with silt (Ellis, 1936; Wallen,1951). Reduction or elimination of aquatic plants and increasing water temperatures have also profoundly affected the survival of aquatic organisms. n