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M':",":t. W;' ~j;l . . , . '.'~ The gelS Oe~eraiDed Th.~ So.. A.p.c~s or Th. Oper.tioD or Gl.D canyon 0.. a.ve Sub.~an~i.l Adver.. Brrec~. OD OOVD.~re.. BDViroDaeD~.l ADd a.cr..tion.l a..ourc.. Construction of the dam and subsequent regulation of .river flows have changed downstream resources in many ways. Some of these changes, such as the increase in riparian vegetation, the development or an exceptional ~rout rishery, and the extended white-water boating season are beneficial. However, two aspects of current operations, flood releases and fluctuating 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- ases and which would convert fluctuating releases to steady releases. ~lood a.l..... C.us. O...g. To Be.ch.. ADd Terre.tri.l ae.ource. A flood release is defined in this report as a discharge greater than the maximum powerplant release. During the course of the GCES, maximum powerplant rele- ases were 31,500 cubic feet per second (cfs). During flood releases, substantial quantities of riparian veg- etation are scoured away, drowned, or buried by re- deposited sand. As a result of the flood releases of 1983, vegetation loss in some areas reached 50 percent, and 95 percent of the marshes and 75 percent of the nests 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 sediment-free water cause significant and irreversible degradation of the environment by eroding a substantial portion of the sand deposits. These deposits provide substrate for riparian vegetation and wildlife habitat and are highly valued as campsites by boaters. Signif- icant loss of sand beaches would reduce by approx- imately 50 percent the recreation benefits (not com- mercial revenues) associated with white-water boating.