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~e GCZ8 Deter.ained ~at 80.e Aspects Of The Operation Of Glen canyon D.. Kave Substantial Adverse Iffects On DoVDstre.. Bnvirouaental ADd aecreational ae.ource. Construction of the dam and subsequent regulation of .river flows have chanqed downstream resources in many ways. Some of these chanqes, such as the increase in riparian veq.tation, the development of an exceptional trout fishery, and the extended white-water boatinq season are b.neficial. How.ver, two aspects of current op.rations, flood rel.as.s and fluctuatinq releases. were found to have substantial adverse effects on down- stream resources. Impacts were assessed by comparing current operations, which include floods and fluc- tuations, to operations which would avoid flood rele- as.s . and which would convert fluctuatinq r.leases to steady r.l.ases. Plood a.l..... Caus. D4a&qe To a.aches ADd T.rr..trial a.sourc.. A flood r.l.as. is d.fined in this report as a discharge greater than the maximum powerplant release. Durinq the course of the GCES, maximum powerplant rele- ases wera 31,500 cubic feet per second (cfs). During flood releases, substantial quantities of riparian veg- etation are seour.d away, drowned, or buried by re- d.posited sand. As a r.sult of the flood releases of 1983, veqetation loss in some areas reached 50 percent. and 95 percent of the marshes and 75 percent of the n.sts of some riparian bird species were destroyed. Because the dam cuts off the main pre-dam source of sediment to the river downstream. flood releases of sedim.nt-free water cause significant and irreversible degradation of the environment by eroding a substantial portion of the sand deposits. These deposits provide substrata for riparian vegetation and wildlife habitat and are highly valued as campsites by boaters. Signif- icant loss ot sand beaches would reduce by approx- imately 50 percent the recreation benetits (no~ com- mercial revenues) associa~ed with white-water boating.