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because of conveyance losses between Grayrocks and Overton and evaporation from Lake McConaughy. In the preparation of the draft EIS for Grayrocks, conveyance losses were estimated by Ecology Consultants, Inc., (1977) to be 0.16% per mile; this means that over the 300 miles between the dam and the center of the critical habitat, the total conveyance loss is about 39%. The evaporation loss in Lake McConaughy is difficult to estimate because it depends on the incremen- tal effect on the area of the lake, which in turn depends on the reservoir operating rules. Other things being equal, it appears that consumption by the power plant and the Corn Creek irrigation project would reduce average annual flows in the critical habitat by about 22 000 acre-ft/yr.* In the past 25 years, the annual flow of the Platte at Overton has averaged one million acre- ft/yr, varying between 410 000 acre-ft/yr (1956) and 2 800 000 acre-ft/yr (1973). This means that the reduction caused by Grayrocks would be about 2% on average and about 5% during exceptionally dry years. The impact of Grayrocks on the critical habitat is likely to depend more on its effect on the pattern of flows rather than the total annual flow. As discussed below, the Platte River sandbars are believed to be kept free of veg- etation by the scouring action of the rushing water, which suggests that it may be more important to consider the effects of Grayrocks on peak flow, rather than on average flow. Unfortunately, however, estimating the effects of Gray- rocks on peak flows is considerably more difficult than estimating its effects on average flow. Expected peak flow in a stream below a reservoir depends on a number of variables, among which are the operating rules for the reservoir, the variation in inflows and withdrawals from the reservoir, and the size of the reservoir relative to inflows. Reservoirs attenuate peak flows because surplus water is put into storage. Therefore, after a reservoir is installed we would expect the drop in peak flow to be much greater than the drop in aver- age flow--in absolute terms. In relative terms, however, things can go either way. For example, four large reservoirs have been built on the North Platte River: Pathfinder Reservoir in 1910, Seminoe Reservoir and Lake McConaughy in 1939, and Glendo Reservoir in 1957 with respective storage capacities of 1.0, 1.0, 1.9, and 0.7 million acre-ft. Now consider Table I, which compares aver- age and peak flows during the intervals between the success~ve construction of *This is the outcome of a simple simulation of the operation of Grayrocks Reservoir, which is discussed below. 35