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COAL SAlW-GRAVEL STONE _ PHOSPHATE liliiii GOLD liliiii I RON .. CLAY . OTHER _ o 50 10 20 30 40 DISTURBANCE BY SURFACE MINING (%) Figure 5. Disturbance of 1.3 million hectares (3.2 million acres) by type of surface mining in the United States - 1965. (r1odified from Kimmel and Sharpe 1976) being carried downstream annually (Utah Water Research Laboratory 1975). Salinity in this river system causes damage to ag- ricultural crops that are estimated to be between $110,000 and $230,000 for each mg/l increase at Imperial Dam, California (Wy- doski 1976). Salinity has been increasing in the Colorado River system and about one- third of the increase is due to man-made practices upstream (Utah Water Research Laboratory 1975). ;;, Although flood control projects are considered to be regulatory and do not consume water, the increased surface area required for leveling off the flood peaks increases the area exposed to evaporation so that consumptive uses do occur (BiShop 1971). The annual records of streamflow by the U.S. Geological Survey indicate that over half the water flow may be divert- ed to be used as irrigation water and, in some river systems, the streams are com- pletely dried in all but the exceptionally wet years due to diversions. Various methodologies have been developed to deter- mine the streamflow requirements of fish (Stalnaker and Arnette 1976) but these requirements or preferences by fish and invertebrates are not fully known (Orsborn and Allman 1976). Dewatering of streams, however, remains a critical problem in the arid and semi-arid regions of the United States that has tremendous impact on the fish and wildlife resources in this area. Feedlots and wintering areas for live- stock are often situated on streams so that the animals will have a ready source of water. The water quality in streams where such holding areas are located is degraded by feces, urine, and exposed soil that re- suI ts in sedimentation., Some gf these holdinq areas contain enough livestock to contribute as much waste as small to medium sized municipalities. Agricultural practices can significantly affect water quality and quantity and have great impacts on aquatic organisms. Since the human population is growing, these practices will intensify and be expected to have even a greater influence on aquatic life unless measures are taken to reduce the problems. Streamflow Any changes in natural watersheds from man-induced activities can influence stream- flows. Such impacts may be beneficial or detrimental to aquatic organisms, depending upon their requirements. In general, Shel- ford's ecological law of tolerance applies to the ability of an animal to cope with varying streamflows. Most aquatic organisms have a ,maximum, optimum, and minimum require- ment for almost any ecological variable. Furthermore, these requirements may differ for different species and for each life state of a species. Often the information that is collected on the streamflow require- ments of fish has not included the range of streamflows and accompanying water veloc- 63