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. J~pth~ and velocities in the m~jn ch,mncl at overbank flow" Thi~ overestimation may aruflClally depres~ the estimate of suitable habitat at high flows, The combination of these ~rrors may lead to an erroneous conclusion that high flows provide little or no habitat. Another management problem associated with the omis- sion of floodplain analyses relates to channel morphology. It is generally recognized that high flows are important for maintaining the existing channel structure and dimensions. and for cleansing the substrate by removing accumulations of fine materials. A commonly used estimator for a channel maintenance discharge is the bankfull discharge. which is taken as the maximum instantaneous flow with a recurrence interval of about 1.5 yr (USFS 1986). This flow is, by defi- nition. the discharge that is equalled or exceeded within that recurrence period. The probability that the flow will remain exactly at bankfull for a significant period is much smaller than the 2 out of 3 yr implied by the recurrence interval. Bv recommending that the bankfull discharge be delivered f~r several days or weeks as a channel maintenance flow, the investigator may inadvertently design a water manage- ment regime that is detrimental to fish that could otherwise move onto the floodplain during high flow events. Inclusion of floodplain habitat usually reveals that flows slightly above bankfull provide high-flow refuge microhabitat. Temperature and Water Quality The issues of water quality and temperature are some- times complex when one analyzes specialized habitats in large rivers. Most water quality and temperature models used in river analyses are one-dimensional; they provide an estimate of the average concentration of water chemistry constituents or the temperature for the entire cross section of the stream. Complete mixing is assumed and the output variable is assumed to have the same value at all locations across the cross section. Habitat models incorporating tem- perature and water quality variables based on one- dimensional descriptions are therefore usually considered as "macro" models. Threshold criteria based upon mean daily water temperature are typically used in conjunction with such macro models (Brungs and Jones 1977; Coutant 1977). A typical output from such models is a longitudinal profile showing temperature along a specified length of water- course. As illustrated in Fig. 3, it is possible to obtain fami- ~o ----+TO '" ,ee ---- r- : 10'" 15C '5 ~ " 10 ; ~ 1 1 I ~I ::1 01 c 1 ~I ~I ~I '1 1 I r I I .-Green R._r , I YamDa R. o 700 650 600 550 500 Distance (kilomel res) FIG. 3. Longitudinal temperature profiles from the Yampa and Green Rivers for nonnal July hydrometeorological conditions (Theurer et al. 1982). 450 lies of 10ngituclinal profiles represenllng different hydrok)gical. mct~orologlcal. thermalloadlOg. or chemical loading conditions, The use of one-dimensional water quality and temperature models may be appropriate in most situations. However. there may be occasions when it would be desirable to be able to predict these characteristics in a more nearly two- dimensional fashion. For example, the average water tem- perature may be several degrees higher in backwater areas than in the main channel. This difference may have impor- tant implications about the growth of young fish. The mouths of tributaries may serve as local refuges during epi- sodes of low DO in the larger river. where prediction of the average main channel condition is not entirely adequate. In addition, low gradient rivers with long deep pools interspersed with shallow riffles or bars may stratify ther- mally during low flows in summer. Cooler, denser upstream water may slide beneath the warmer pool water, forcing cool-water species (in the absence of groundwater dis- charge) into a narrow band along the bottom. In other situa- tions, the inflowing water from upstream may be warmer than the pool water and consequently flow over the top of the pools. If this condition persists long enough, DO may become depleted in the pools. Two-dimensional water qual- ity models commonly used in small reservoirs may be ade- quate for predicting these occurrences. Data needs for model calibration become large, and computer time for long-term simulations become prohibitive, due to the com- plexity of these models. Prediction of these variables in a specialized habitat type may not require the use of a two-dimensional model, but rather a nontraditional use of one-dimensional models. Two basic approaches are suggested: in habitat types such as side channels, it may be possible to subdivide the reach and use a one-dimensional model to predict the localized average side channel condition; and in small specialized habitat types a combination of one-dimensional modeling and site specific empiricism may provide the answer. Certain specialized habitat types, such as connected sloughs and oxbows, have relatively low water exchange rates with the main channel. The hydraulic connection with the river is usually through a small inlet, or by groundwater inflow and outflow. In either event, water may flow directly through the slough area only during floods. A simple equilibrium temperature model applied to the slough area may be completely adequate - as a one-dimensional dis- solved oxygen model would be. A similar approach could be taken to estimate the dis- solved oxygen concentr~tion in backwater areas formed at the mouths of tributaries. Here, it would be necessary to define the area of the backwater and estimate the relative contributions of the main channel and the tributary to the volume of the backwater. One-dimensional DO models could be applied to both the tributary and the main stem to predict their respective DO concentrations and a simple dilution equation used to compute the mixed DO concentra- tion in the backwater. Where the hydraulic connection is more direct, and the main channel is the sole water source, a combination of models may be more appropriate. An example of this type of habitat is a side channel backwater, where the tempera- ture of the water at the inlet to the side channel is the same as that of water in the river. However, depending on the 19