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