There are very high water depletion rates in the Great Basin 

 drainages in the Rocky Mountains. Storage ponds can supplement 

 water supplies. 



tion is no problem in any riffle areas. Water tempera- 

 tures are not expected to become limiting in any 

 reach of the stream. Invertebrate life forms should be 

 varied and abundant. Water quality and quantity 

 should be excellent for fishing and floating canoes, 

 rafts, and larger boats, and general recreation. Stream 

 esthetics and natural beauty will be excellent to 

 outstanding. 



"Thirty percent of the average flow (70 percent 

 depletion) is a base low recommended to sustain good 

 survival habitat for most aquatic life forms. Widths, 



depths, and velocities will generally be satisfactory 



The majority of the substrate will be covered with 

 water, except for very wide, shallow riffle or shoal 

 areas. Most side channels will carry some water. Most 

 gravel bars will be partially covered with water and 

 many islands will provide wildlife nesting, denning, 

 nursery, and refuge habitat. Streambanks will provide 

 cover for fish and wildlife denning habitat. Many 

 runs and most pools will be deep enough to serve 

 as cover for fishes. Riparian vegetation will not 

 suffer from lack of water. Large fish can move over 

 riffle areas. Water temperatures are not expected to 

 become limiting in most stream segments. Inverte- 

 brate life is reduced but not expected to become a 

 limiting factor in fish production. Water quality and 

 quantity should be good for fishing, floating, and 

 general recreation, especially with canoes, rubber 

 rafts, and smaller shallow draft boats. Stream esthetics 

 and natural beauty will generally be satisfactory. 



"Ten percent of the average flow (90 percent deple- 

 tion) is a minimum instantaneous flow recommended 

 to sustain short-term survival habitat for most aquatic 

 life forms. Channel widths, depths, and velocities 

 will all be significantly reduced and the aquatic habitat 

 degraded. . . . The stream substrate or wetted perim- 

 eter may be about half exposed, except in wide, 

 shallow riffle or shoal areas where exposure could be 



higher. Side channels will be severely or totally 

 dewatered. Gravel bars will be substantially de- 

 watered, and islands will usually no longer function 

 as wildlife nesting, denning, nursery, and refuge 

 habitat. Streambank cover for fish and fur animal 

 denning habitat will be severely diminished. Many 

 wetted areas will be so shallow they no longer will 

 serve as cover, and fish will generally be crowded into 

 the deepest pools. Riparian vegetation may suffer 

 from lack of water. Large fish will have difficulty 

 migrating upstream over many riffle areas. Water 

 temperature often becomes a limiting factor, espe- 

 cially in the lower reaches of streams in July and 

 August. Invertebrate life will be severely reduced. 

 Fishing will often be very good in the deeper pools 

 and runs since fish will be concentrated. Many fish- 

 ermen prefer this level of flow. However, fish may be 

 vulnerable to overharvest. Floating is difficult even in 

 a canoe or rubber raft. Natural beauty and stream 

 esthetics are badly degraded. Most streams carry less 

 than 10 percent of the average flow at times, so even 

 this low level of flow will occasionally provide some 

 enhancement over a natural flow regime." 



From the established criteria, it can be determined 

 that depletion levels in excess of 90 percent for sus- 

 tained periods usually will have serious adverse effects 

 on aquatic habitat. The monthly analysis in table 7. 1 1 

 indicates those subregions where the flow will be 

 reduced by more than 90 percent for long periods. 



Most of the major impacts of use on the volume 

 of water in streams occurs in the West (table 7.11). 

 These data, however, provide comparisons only of 

 total water consumption in a subregion with the aver- 

 age outflow of water from the subregion. Most regions 

 and subregions have main streams and tributaries 

 that have flows well below the "good survival habitat" 

 level at some time during a normal year, and many 

 also approach or go below the "minimum short-term 

 survival" flow level. In some cases, including some 

 in the western United States, natural streamflows 

 are augmented by reservoir releases to avoid such 

 problems. 



There are other cases, however, where streamflows 

 fluctuate widely during the day in response to reser- 

 voir discharges to meet varying demands for hydro- 

 electric power. Average flows seem adequate for 

 aquatic life in Water Resource Regions 1-9. High 

 depletions are causing the greatest instream impacts 

 on aquatic life in the Rio Grande region and the 

 Lower Colorado and the Southern California sub- 

 regions. Other areas under stress include the No/ So 

 Platte, parts of the Arkansas-White-Red region and 

 the Brazos, Colorado, San Joaquin, and the San 

 Francisco Bay subregions. In a dry year, additional 

 aquatic habitat areas that are likely to be greatly 

 impacted include the Southern Florida and Kansas 



295 



