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'\ J nn?Q')5 conditions are favorable fbr fish and to se- lect appropriate year-round flows. The primary purpose of PHABSIM is to describe the relation between streamflow and usable quantities of physical watercol- umn space (discharge versus habitat), Such relationships represent the space in a stream that can be used by a specific species during its life stage, PHABSIM is partic- ularly useful during late summer, which corresponds to the period when most hab- itat suitability index (HSI) curves are developed, PHABSIM-derived fish main- tenance flows carry fish mainly through low-flow conditions, Seldom would PHABSIM flows be a restriction during moderate to high flow regimes, PHABSIM is necessary to determine base flow needs, particularly in late summer and falL Thus, PHABSIM will adequately handle some phases of an instream flow assessment. However, other analytical methods are needed to address channel maintenance flows, and especially riparian and valley maintenance flows. Channel Maintenance Flows Channel maintenance flows consist of moderately high flows that are expected to prevent vegetation growth in the channel and remove sediments (Reiser et aL 1989), Most channel-maintenance flow methods suggest that bankfull discharge is a simple discriminator for differentiating between channel-forming and floodplain-forming processes (Wesche and Rechard 1980), Le- opold and Emmett (1983) suggest L5-year recurrence intervals for bankfull flows. However, Chorley et aL (1984), in studying 36 active floodplains, showed that bankfull recurrence intervals vary between 1 and 32 years, Therefore, bankfull flow must be evaluated for a specific stream. For certain snowmelt stream channels much of the fluvial process response occurs during the ascending limb of the peak flow hydrograph (Rosgen et aL 1986), Thus, the entire range of the hydrograph may be du- plicated by short timed releases of flows at appropriate intervals. These observations were made in reference to channel pro- cesses but not to those associated with ri- parian habitats and floodplains, To illustrate complex flow regimes, Ros- gen et aL (1986) used simplified relation- I~I 202 ships. The relationships are based on the assumption that flows on the ascending hydrograph mimic the range of frequently occurring discharges that form and main- tain channels over time. It was further as. sumed that sediment loads are not changed appreciably by those factors controlling flows. Given these assumptions, then three basic flow components are required in snowmelt streams: (1) a snowmelt peak flow that is defined as bankfull discharge, (2) a low flow that is defined as base flow dis- charge, and (3) snowmelt rising and re- cession discharges (flow regimes over time) (Rosgen et aL 1986), Apparently, an intermediate range of discharge transports most of the sediment load over the long term and thus deter- mines channel form and condition, Rosgen et at (1986) further determined that a sed- iment rating curve and a frequency curve of daily discharges, based on the work of Andrews (1980), could be used to define effective flow, Andrews determined that effective discharge was nearly equivalent to the discharge at bankfull stage, This sim- plifies the estimation of channel mainte- nance flows because only those measure- ments that determine bankfull flows are needed, In contrast. Platts et al. (1985) found that bankfull flows were not always an adequate indicator of channel form and condition. This was the case for certain types of basin-range streams, especially those that tend to experience lateral shifts in channel location when stressed by man- agement activities. Overall, the interme- diate range of discharges (i.e" approxi- mately bankfull stage) is, in our opinion, an important hydrological benchmark that is related to the shape of many alluvial channels. Base flow discharge is determined from low-flow statistics. Rosgen et aL (1986) sug- gested that 7 -day low flow at the L5-year return interval be used for calculating base flow, The difference between base flow and bankfull flow is large in most streams, Therefore, there is a need to prevent rapid changes in the rise or fall of streamflow so as not to accelerate bank erosion unduly. Duration of each increment on the rising curve is determined from hydrograph analysis to approximate the normal rate of rise of the ascending limb (Figure 2), The duration of recession flows gener- July 1991 I Rivers' Volume 2, Number 3 l