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-.Ii . 1721 ..' r ... .t~~.:)~~:,.:;l; . .."'" , . ~, .... the selected bank.full dischat:ge identifies the peak. n"" of the Chapter 30 channel maintenance hydrograph. The second step in the Chapter 30 procedure is establishment of the rise and recession flows comprising the rest of the hydt:ograph. Information needed to construct the hydro graph graphically includes the flow duration curve, the average annual flow, the baseflow or thalweg flow, and the minimum flow. Following the Chapter 30 calculation procedure, tb~ peak bypass of 190 cfs would be required for 16 days and the rise and recession would occur over a period of 20 and 9 days, respectively. Total bypass volume required is about 13,200 acre-feet. . ~ I. . f. f,i to illustrate the Significance of channel. .maintenance flow require- ments obtained by a strict application of Chapter 30, the derived hydro- graph-.was _superimposed on the 1980 _water year hydrograph (Fig. 3). The 1980 water year was selected f~~ comparison since it pt:oduced a total fl"" of 23,630 acre-feet, approximately equal to the. average annual yield of 23,890 acre-feet at the gage ot:. about. 26,280 acre-feet at the point of quantification. The total volume of water requit:ed for channel maintenance is still 50 percent of the average annual yield. DISCUSSION OF CHAPTER 30 RESULTS A fundamental assumption of the USFS channel maintenance flow evalua- tion is that the bankfull discharge of a channel is the dominant or channel forming dischat:ge. A _ basic. condition for applying the procedure is no change in upstream sediment supply from pre- to post-pt:oject conditions. Consequently, strict application of the procedure to evaluate reservoir projects is questionable, since a reservoir will trap upstream sediment and t:elease relatively clear water downstream. A related concern is applica- tion of the procedure to a supply limited system. An implicit assumption in the procedure is that the channel is in equilibrium, that is sediment supplied to the channel approximately equals channel transport capacity. This may not always be true for high mountain watersheds. For example, results of the U. S. Geological Survey indicate that runoff in snowmelt dominated systems typically produces low suspended sediment concentt:ations due to the relatively low sedim~nt generating characteristics of snowmelt runoff compared to rainfall rainoff (7). As a result, transport of fine- grained material is typically supply limited and not in equilibrium with transport capacity (i.e., the channel has excess transport capacity). Under these conditions, aggradation of fine-grained material will not occur unless the discharge, and the corresponding transport capacity, are reduced below the level of the supply. ~ For the bedload portion of the total sediment load, the trend is typically the opposite; that is the transport rate of coarser particle sizes is often limited by transport capacity. Under this condition, applicability of the Chapter 30 pt:ocedure, relative to aggt:adation con- cerns, depends on the source of coarse material. Based on analysis of 24 gravel-bed rivers in the Rocky Mountain region of Colorado, Andrews (1) concluded that the primary source of coat:se material is the channel itself (i. e., not from the watershed) and that "the. bed material transport rate thus controls in large measure the quantity of coarse material supplied to a river channel." Other sources of coarse material (i.e., landslides, debris flow, frost creep, etc.) were considered by" Andrews, but were eliminated as significant sources based on historical activity and the fact