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Erosion and Sediment - Field observations indicate that sheet, rill, gully and streambank erosion on approximately 40 percent of the study area is in excess of five tons-per-acre-per-year. Streambank erosion in excess of 1,000 tons-per-stream mile occurs on about ten percent of the stream channels. The badlands within the area are significant contributors of both salts and sediments to lower irrigated lands and water resources. Erosion damage on the rangeland areas is primarily geologic in nature. Erosion of channel banks along the Virgin River is rapid and a major source of sediment. About three million tons-per-year of suspended sediment are contributed from Virgin River to Lake Mead based on 20 years of record at Littlefield gage. This amounts to about 2,200 acre-feet of sediment per year. Additional sediment is contributed to the Overton Arm of Lake Mead from the Virgin Valley drainage area below the Littlefield gage. Muddy River, a smaller drainage also empties into the Overton Arm of Lake Mead. The 1935 to 1964 increase in sediment deposited in the Overton Arm of Lake Mead was 126,000 acre-feet or an average of 4,200 acre-feet-per-year (six million tons-per-year). ~ Badlands have developed in Muddy Creek Formation which is made up of interbedded clay, silt, conglomerate, and sand containing salt and gypsum. There is an abundance of limestone, gypsum and salt in other geologic strata exposed to the forces of 'geologic erosion which also contribute to the salinity of Virgin River. It is not economically feasible to control the geologic erosion of these formations. . Biotic Resources ~ The study area has a highly diversified environment consisting of a variety of biotic communities. Vegetation zones forming a basis for recognition are: Creosote Brush B1ackbrush Sa1tbrush Desert Riparian Riparian and Cliff Desert Springs and Marsh Streamside and Riparian ~ ~ Distribution of biota in terms of the numbers of species of vas- cular plants, fish, amphibians, reptiles, birds and mammals is given in Appendix C. ~ Numberous species of macroinvertebrates exist, but investigations under the salinity control program have not been conducted to identify them and to evaluate the effects of varying flows and salinity. Fisheries - Species endemic to Virgin River belong to two families Catostomidae and Cyprinidae. Eight endemic and five exotic species of fish have been identified in the study area. Nevada Wildlife Commis- sion has recently revised the listing and classification of endemic fish species to include 'sensitive species'. The sensitive species classifi- cation is designed to call attention to those groups of unprotected fish ~ ~~'~~9 ~~l~1 22