7916 - Laserfiche WebLink

Home Browse Search

An example of a two-dimensional matrix (depth, and velocity) is given in Table 2. In each cell of the matrix, the upper numeral refers to the surface area of stream having a certain depth-velocity combination. The numerals in parenthesis refer to the weighted usable area. Due to the large number of calculations required for each reach matrix, a computer program has been developed by the Cooperative Instream Flow Service Group to make all calculations for as many species as desired from a single set of hydraulic unput data (Main, 1978b). For each species and life stage, weighted usable area may be plotted against discharge and various flow regimes, such as median monthly flows or l-in-l0 year monthly low flows (Figure 3). Such plots can show critical time periods for a given life stage, limiting habitat availability for each life stage (i.e., physical carrying capacity), and limiting habitat availability for different species. Since changes in hydraulic characteristics will initiate differential species reactions, the incremental method is particularly useful in evaluating changes in species composition as inferred from changes in habitat usability. The quality of the physical habitat for a specific instream use is a deterministic function of the flows, and can be described as a stochastic process by converting the flows to some habitat index associated with the flows. In most controlled river systems a "minimum" flow is established for certain stream reaches and the system operated with this "minimum" flow as a constraint which is often violated under certain conditions such as low water yield (drought). Rarely are system operating criteria developed considering the stochastic variation of the production of benefits from use of the water instream (Milhous and Bovee, 1978). The IFG incremental methodology is designed to allow this stochastic approach. The IFG incremental methodology has been developed to quantify the impact on fish habitat resulting from incremental or alternative modifications in stream flow. Listed below are 12 most commonly asked questions of the IFG methodology which are also applicable to anticipated assessments of eastern streams impacted by coal developments. 1. What does the IFG incremental methodology do which previous in stream flow methodol- ogies could not do? Most instream flow methodologies in the past have specified minimum flow requirements for fishes. The IFG incremental methodology examines what will happen to the fish habitat with an increase or decrease in the amount of flow by any given increment. Further, it makes these predictions for each of several species of fish, each life history stage of those species, and for each month of the year. Thus, the IFG incremental methodology is no longer restricted to single value minimum flows. It is useful for predicting and quantifying impacts and establishing instream flow regimes for specified management objective (habitat maintenance). 2. What information does the IFG Incremental Methodology provide? The methodology provides information on physical impacts of altered stream flow on fish habitat. These impacts are expressed as an index of surface area of usable habitat for each reach of stream considered. The index relates a particular stream reach with optimal habitat of the same surface area. The methodology predicts the suitability of the stream habitat for fish of a given species and life stage as defined by combinations of depth, velocity and su bstrate for specified stream discharges. Based on information about a species and its habitat, an index termed "weighted usable area" is calculated. The index displays the optimal square feet equivalent of stream that provides suitable fish habitat, compared to the total square feet of surface area. Computer programs are available to calculate weighted usable area for each of several stream flows. Thus, the "best" str-eam flow for any desired management objective can be determined. These results help the fishery manager determine the potential impact of incremental changes in stream flow on fish habitat. This information can be used to make judgements about which stream flow quantities meet his management objectives. It can also be used to predict the environmental impact of water development activities on fishes by identifying life stages and times of the year which may become limiting to the continued well-being of a fish species. 3. Why is an instream flow methodology important? Under various laws (for example, the Fish and Wildlife Coordination Act), fish and wildlife agencies have made stream flow recommendations to water development agencies. Usually, the recommendation is for a minimum flow. A minimum flow is that stream flow below which there will be disastrous effects on fish. The minimum flow concept has not been very useful for predicting the impacton fish from modifica- tions in stream flow levels, such as reducing the flow from 200 to 150 cubic feet per second. The IFG incremental methodology, which allows such predictions, represents a breakthrough in water management by placing instream use of water on an equal status with offstream or consumptive uses. 5