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H~ 00 ~ N C c.. Plants of the United States as an endangered species. Today it is restricted to the Virgin River and the lower reaches of Littlefield Springs. Its continued existence within the Virgin River system is directly linked to the amount of physical, chemical, and biological alteration of the mainstream habitat. Because of the local opposition and this concern, the Littlefield Springs Unit as a location for the project was moved downstream to the Lower Virgin River Unit and was so named. Virgin River Flow An analysis of the effects of a water quality improvement program on the Vi rgi n River requi res an understandi ng of the ri ver from a point above the Virgin River Gorge to its point of discharge into Lake Mead (Figure 1). The Virgin River drainage is largely a semi desert area receiving less than 15 inches of annual precipitation. Flows in the river are irregular. The greatest part of the total annual flow occurs during the winter and spring corresponding to greater precipitation and snowmelt runoff over these months. However, high peak flows (flash floods) primarily associated with thunderstorm activity, are known to occur at any time. Records of the U.S. Geological Survey (USGS) show flood peaks as high as 22,800 ft3/s at Virgin, Utah (a 100-year flood is computed to be 23,500 ft3/s at this location), and 35,200 ft3/s at Littlefield, Arizona. High flows carry high sediment loads. Sediment loads at the Virgin River gage vary from a minimum of 8 tons per day to about 360,000 tons per day. A few miles below St. George is a reach of the river where water percolates into the riverbed. This reach extends about 6 miles from Bloomington, Utah, into the Virgin River Gorge, and causes the river to lose up to a maximum of 100 ft3/s of water. The loss has been observed during high flow winter months as well as low flow summer 7