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-. 17Q4 ~.. .. . , '. .._ _._~ c - .~,,;.. . ~, .. o.! rill .a. ~nt u.s. Y. JESSE Colo. 499 CUe .744 ..2.1 49. (Cola. 1917) Ilo fiter eour( granUd the motion for issue of reserved water rights to maintain ~ judgment, and concluded that the instream water flows had been determined _teD8Dce because the COUrt iDdicaltd that . rcsuh of significant lone-term. .stream flow fIberies maintenance was DOl considered a reductions include: primUY purpose of the National Forests. a. Accelerated delivery of channel iedi- llqiDJlina: in Iatc 1980. and in conjunction ment aDd channel bank erosion. I'ilh IP adjudication in the Big Horn Basin in b. Reduced channel c.pacity due to sedj. W,.min& [the United States Fo..... s..v;"'1 ment deposition. II:pn to formulate. proc:edurc and melhod- c. l.a1ermittcnt or subten'1lnCaD. flow as a '*cY for assessiDg the quantity, I'aIlgC. and result of channel agradBtion. which can re- aming of stream flows necessary for main- sult in reduced availability of surface water lliainl the iategrity and stability of stream during low flow periods. . dllnDcls. Until this time. we bad assumed d. Latera.I movement of the c:ha.nnel and dill a fisheries maintenance flow would also flood plain encroachment. ICCOmplish most of the channel maintenance c. Accelerated erosion of lands adjacent to fuDction. the strcaI1\, with attendant incn:.a.scs in sed.i- ['Ibe United States Forest Service] c:onclud. ment supplies. cd thai iPStream flow l"aIu.iremcnts for cban. f. Enaoacb.ment of riparian vegetation DCI maintenance must be based 00 fundamen- into the channel area. with subsequenl reduc- III principles of fluvial geomorphology which tions in channel capacity. IIold thal natural stream channels are formed I. An inCl'ea$e in flood impacts or damage md maintained by frequently recurrina flows resultins from transmission of flood flows as rJ Wiler and sedimenL Consequently, if such well as flow, of a lower IJl8ID.itudc. Thus, Bows are DOl available on a frequent basis, flows thal previously would have been re- (be natural equilibrium of the channel system tained within the stream banks would, under will be changed. with . rauhing Joss in the the reduced channel capacity, become flood flows. capacity of the channels to ClllT)' albscquent flows of equal or greater magnitude. This led h. An increase in sediment deposition with. ItS to conclude thal insrrmm flows are reo in the containment areas of water storage and fIliml to rruWrtain IN fI/I.tJlrtd du2nneb in a diversion facilities, resulting in accelerated .te of rcIIJtiw equilibrium in order to deliver loss of water storage capacity in reservoirs IIIJer to IN ultiwuzu user IUI.tkr favorahle and increased mainlcIWlce of headgates and ditches.. mrulitiom. 1be bases for this conclusion an: Tbe maiDlenance of iDstream flows within outlined in the paragraphs that follow. the national forests in Water Division No.2 Sediment movement or transpon by stream would, in nre instances, if at all, cunail the ~n~S~ ~? ~e:n~~ :=~o:o~ni~ operation of existing water uses and develop- ments. It is nol our intent to use such in. essential to maintaining the balance of equi- stream flows to reverse the development of librium Of the totaJ waler delivery system. present channel conditions that exist below FoUowiDa: a significant reduction in the natu. established facilities. Nor do we intend that rally occurring rmIIe of Itream flOW5, stream all such impacted or aJtered channels should channels dOwnslream of the point of flow be considered necessarily unsatisfactory. We rafuction begin a process of alteration which do believe that. in some cascs. adverse chao. WlI1 be described as a sublle process of insta- Del conditions exist. or may yel develop into bWly. In genera.l tenns, sodiment from side problem situations. which may well be avoid. drainqres which prmously was tTansponed ed in Ihe future, to the benefit of both the through the syslem. DOW begins to accumulate waler user and the natural delivery system. within the main channel, resulting in a grad- Generally, instream flow qualification. points ual elevation of the origiuJ cbannel bottom would be located above c:xistins significant or reduction in origiDaJ channel capacity. points of diversion, and, as such. we believe When high flows do return to the oriJinal the application of insInam flow,requirements c:hannc:1 system aD an infrequent basis, they on and within the Natioul Forest would Mve may I1lC.lYe IOme of the accumulated sediment; Iinle, if any. affect on the existing system of but moct remains., with the result that thf: now water uses and development. ele\'ll&ed flows .ttempt to reeslBblish the origi. One of the primary purposes of . National naI channel capacity, workins apin.sc former. Forest is to provide wate for beneficial use Iy stable sicfe..slope materials. The end resuh by pri....tc and public users. Tbe provision of O\Ier time is instability chantc1erized by signif. water for beneficial use from a National For- icant on-she disturbaDoe wilhin various reach. . est under f.vorable conditions depends upon a of the cbaa..Dd and accelerated sediment the continued intelrity aDd efficienl function. delivery to the d0WD5tream reaches of the ing of the n.atural stream cbanne.I conveyance I)'SICID. Of major conccrn is thar once these system. WlStable processes are initiated. they are sel- An in.stream flow is DOnconsumplive. It dam reversible. IrI"YeS to maintain the stream channel deliv- Some specific responses or unfavorable ery systems so that water can be delivered to conditions that can be expected to dcYelop as appropriaton under f.\'Orable conditions. ,!," ~ If 'If > ,. . < ~: