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114 National Water Summary 1987-Water Supply and Use: HYDROLOGIC PERSPECTIVES ON WATER ISSUES the effects of varying flow regimes on fish habitat, recreation, and other instrearn values. Several important research reports reflect past research and also serve as references for future methodological advances. Bovee (1982) addressed instream-flow assessments for fish habitat; his report has become the baseline for discussion of emerging technologies. Tennant (1976, p. 10) described the need for periodic flushing events to maintain certain hydrologic characteristics of the channel necessary to protect the environment for fish. Hyra (1978) analyzed the streamflow requirements of recreational activities. The technologies developed since 1978 fall into two categories-those appropriate to preliminary plan- ning and those designed for project impact assessment. The preliminary planning methods are related most closely to the traditional concept of minimum flow. These methods typically use a streamflow characteris- tic that represents the minimum flow for a particular instrearn use. Examples include 40 percent of mean annual flow, the point at which the size of wetted perimeter begins to fall sharply with small reductions in flow, flows equaled or exceeded 90 percent of the time, 10 percent of the mean annual flow, or the lowest flow on record (Trihey and Stalnaker, 1985). Project impact assessment requires a different approach. During the 1970's, instream-flow assess- ment methods that attempted to evaluate fish habitat in terms of changes in the environment were developed. These "incremental" methods estimate the quality of fish habitats at different increments of streamflow. Early investigators of these approaches used depth, velocity, and substrate criteria to evalu- ate the influence of incremental changes in stream- flow on the quality of spawning habitat for salmon in Washington streams (Collings and others, 1972). Waters (1976) applied weighted criteria for depth, velocity, and substrate/cover and introduced computer simulation to evaluate the response of rainbow trout habitat to streamflow in California. The application of hydraulic modeling methods in conjunction with streamflow-dependent criteria for fish habitat began with single transect methods in which the stream model was based on the measure- ments taken at a single cross-section of the stream. The U.S. Forest Service introduced one such method called R-2 CROSS (Isaacson, 1976). Single transect methods were followed by more sophisticated multiple transect methods in which the stream models were based on several representative cross sections of the stream channel. Fishery impact assessment methods were adapted from water-surface profile (wSP) simulation models that were used by the U.S. Bureau of Reclamation, the U.S. Soil Con- servation Service, and the U.S. Army Corps of Engineers. The multiple transect techniques support predictions of depth and velocity at points across a transect and changes in the wetted perimeter of the channel as a function of flow (Dooley, 1976). The development and refinement of hydraulic simulation models to facilitate evaluation of habitat conditions under a wider range of streamflow conditions has con- tinued to the present (Milhous, 1984). The Physical Habitat Simulation Model (PHABSIM) is an important analytical component of the Instream Flow Incremental Methodology (IF1M) described by Bovee (1982). In general, project impact assessment approaches are more labor and data intensive, and more costly, than preliminary planning methods. Thus, the first question facing the manager is which of the two approaches to use. The decision is based on the magnitude and nature of the problems being addressed. Generally, preliminary planning methods would be appropriate whenever a specific project has relatively benign effects; fisheries, recreational, and other instrearn values are limited; or development is not anticipated for several years in the future (Trihey and Stalnaker, 1985). The more complex and data-intensive project impact assessment methods are used when alteration of the streamflow, stream temperature, channel structure, or water chemistry is anticipated and there are concerns about the effects of these alterations on instrearn values. These methods can help answer the question, "What will happen if the minimum flow standards are violated?". These methods also might provide useful guidance to resource agencies seeking opportunities to improve existing fish populations or to alter the species composition of a stream. Once the decision is made as to the type of method to use, the manager has an array of specific methods available. The choice of method depends on the resource agency's management policy, the region of the country, the type of instrearn uses to be provided, and, for fishery uses, the species of concern. The more frequently used methods of determin- ing instrearn flows are listed in table 18. Two conclu- sions can be drawn from those data. First, they show the diversity of available methods. This diversity is Table 18. Methods for determining instream-flow requirements and number of States using method [Source: American Fisheries Society survey conducted by Dudley Reiser in 1987 (unpublished). More complete information on each method can be obtained from Lamb, 19891 Number Method of States using method Instream flow incremental methodology (IFIM) ................................................ 38 Tennant method ..................................... 16 Wetted perimeter .................................... 6 Aquatic Base Flow (ABF) ......................... 5 7-Day, 10-Year Low Flow (7010) ............... 5 Professional judgment ............................. 4 Single Cross Section (R-2 CROSS)............ 3 USGS Toe-Width ................................... 2 Flow records/ duration ............................. 2 Water quality ......................................... 2 Average Depth Predictor (AVDEPTH)......... 1 Arkansas ............................................... 1 Habitat quality index ................................ 1 Oregon ................................................. 1 Vermont fish-flow ................................... 1 U.S. Army Corps of Engineers Hydraulic Modeling (HEC-2) ................................ 1 the result of many people independently attempting to solve the technological problems associated with assessing appropriate streamflow levels for instream uses and also of the variation of instrearn uses. Second,