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Drainage Area and ~~in-Channel Length and Slope Drainage areas were determined from U.S. Geological Survey topo- graphic maps. Transfer of water from one drainage area to another by storm sewering is insignificant, except in the Turtle Creek Basin where the drainage from Mill Creek was diverted into Turtle Creek before the gaging station was established. Drainage areas are given in table 2. The main-channel length was calculated as the distance, in miles, along the longest watercourse from the gaging station to the basin di- vide. The average slope in feet per mile was obtained by dividing the difference in altitude between points located 10 and 85 percent of the distance upstream from the gaging station by the distance in miles be- tween these points (Benson, 1964). Length-slope ratios are defined as the quotient of the length and square root of the slope; this ratio is usually correlative with lag time. All length and slope data are given in table 2. Impervious Area The impervious area of each basin was obtained by: (1) Delineating the different city zoning categories (table 3) and the percentage of development on U.S. Geological Survey maps; (2) assuming that each zoning category had a constant average of impervious area; (3) planimetering the areas of each category and multiplying the areas by the constant av- erage of impervious area for each particular category; and (4) summing the impervious areas of all categories to obtain the total impervious area. The percentage of residential, nonresidential, and total impervious area in each basin (based on mean 1968 conditions) is given in table 2. Table 2 also gives the coefficients of imperviousness and topographic characteristics for each basin. The coefficient of imperviousness, K, was developed by Carter (1961) to adjust peak discharges due to urban development. The value of K was determined by the formula: K 1.00 + .015 I where I = the percentage of impervious area. -10-