7922 - Laserfiche WebLink

Home Browse Search

instream flow studies at any time. It may be advisable to defer an instream flow study in disturbed watersheds in the process of recovery until recovery is essentially complete. The decision on whether or not to delay a study must consider the estimated recovery time and the time frame in which water manage- ment decisions must be made. Short term water management decisions may require a prediction of future water supply, water quality, and channel characteristics in order to complete an instream flow study. Instream flow considerations may enter the decision process too late to have any influence when a study is deferred until watershed recovery is complete. Finally, a watershed distur- bance may be so pervasive that instream flow management would not be effec- tive, either in terms of ecosystem management or water management. Figure 2 illustrates the watershed evaluation process as it applies to the IFIM. Figure 2 expands Step 1 in Figure 1 (i.e., determine watershed influences on macrohabitat characteristics). An instream flow study may be initiated immediately if the watershed is in equilibrium and the project will not affect the watershed. The investigator has four choices if the watershed is not in equilibrium: (1) to defer the study until equilibrium is reestab- lished; (2) to predict the flow regime, channel structure, and water quality in the stream after a certain recovery period; (3) to recommend watershed treatment practices that will accelerate the recovery process; or (4) abandon the study and monitor the recovery. These options are discussed in detail in Chapter 2. a. Water yield. Water yield is important to an instream flow investiga- tion for two reasons. The most obvious reason is that the amount and timing of streamflow (the flow regime) is directly linked to runoff from the water- shed. Less obvious is the fact that the channel dimensions and the proportions of pools, riffles, and meanders are determined by the flow regime. Only a small portion of the annual precipitation falling on a watershed contributes to streamflow. Even less water reaches the stream as surface runoff. Runoff in an undisturbed watershed is impeded by vegetation, surface irregularities that create small storage basins, and soil structures that encourage infiltration.. These factors retard overland flow, allowing the water to soak into the ground. Eventually, percolating surface water enters the ground water reservoir, which becomes the source of streamflow during periods of little or no precipitation. The discharge during such periods is called base flow. Large scale disturbances on watersheds may alter the water yield by reducing resistance to overland flow, reducing or obliterating surface irregu- larities, and compacting the soil. This change in water yield is usually indicated by an increase in surface runoff during periods of precipitation and a reduction in the base flow during dry periods. The result is a flashy flow regime where the stream may run at nearly bankfull during a moderate rainstorm and practically dry up when the rain stops. The same type of flow regime is typical of agricultural lands which have been tile-drained or urban areas drained by storm sewers where infiltrating water is intercepted by a pipe and routed directly to the river. These types of alterations to the flow regime create two problems for the water or habitat resource manager. First, there is a problem with the timing 6