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. a short time during a boating trip, and the remainder of the trip will be over water of greater depths and widths. An important assumption is that all water greater than the minimum is equally useful for the activ- ity (i.e., more is better until bank-full stage). A computer program (IFG-1) has been developed which predicts width and depth across the transect of any stage (water surface elevation). The output shows discharge and the width with depth equal to or greater than a specific depth. Different water surface elevations may be put into the computer model which are translated into flow in cubic feet per second. When a flow provides the minimum width and depth necessary for an activity, discharge may be considered minimum. Such a minimum indi- cates that significant losses, if not elimination of this activity, will occur if minimum flow is not equaled or exceeded. :;::=:;;::::: .: -~.',;.' . '. " THE lNCREMENTAL METHOD This method, more sophisticated than the single cross section method, describes a relationship between the amount of water in a reach of stream and the associated recreation potential. The incremental method can descri be the potential for any recreation activity at any streamflow. A major difference between the methods is that the single cross section method can only be used to identify low flow and cannot be used to assess the recreation potential at any other flow; the incre- mental method can be used to assess the potential at other flows or to calculate the change in receation potential caused by a change in stream flow. The incremental method i nvo 1 ves a mode 1 i ng procedure whereby the surface area of a stretch of stream is calculated. In addition to the total surface area of the reach of stream, the area which has certain depths and velocities is calculated. The usable surface area for each activity is then calculated by use of depth and velocity requirements. It is necessary to make three assumptions regarding the relation- ship between the quantity of water and the recreation uses of the water: (1) water depth and water vel oci ty are the two streamfl ow components , which are most important in determining whether or not a certain recre- ation activity may be safely and pleasurably engaged in1; (2) there are IOther parameters such as water quality and temperature are also very important in determining the amount of instream recreation use but in many cases are not significantly influenced by flow. Width is also important but is considered outside of the computer model (i.e., width is not a part of the calculation of usable surface area). --..-::-- 4