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30 portions of the lower Green River subbasin had extensive outcrops of Mancos Shale. The upper basin areas exhibited a similar increase in vegetation and loss rates with elevation rise. Many of the loss rates used in the 1951 study were found to still be appropriate, although adjustments were necessary for some subbasins. 3.2 SUBBASIN DELINEATION The Colorado River basin above Hoover Dam includes some 167,000 square miles. Of this total, 59,000 square miles are between Hoover Dam and Glen Canyon Dam in the lower basin, and 108,000 square miles are above Glen Canyon Dam in the upper basin. The lower and upper basins were divided into smaller subbasins for ease in meteorologic and hydrologic analysis. In general, subbasin delineation was made by following major tributary basin boundaries. Subbasins that had similar characteristics of elevation, slope, and stream development were combined where possible. The size of the subbasins was limited to areas of less than 5,000 square miles. The lower basin was divided into 34 subbasins and the upper basin into 65 subbasins. The average subbasin size was about 1,700 square miles. Figure 3.1 shows the subbasin boundaries along with their identification nunlbers. The base maps used for subbasin delineation were the "hydrologic unit" maps prepared by the Geological Survey for the six states. These maps were published at a scale of 1:500,000. The maps provided a consistent set of basin boundaries for both the meteorologic and hydrologic studies. For the hydrologic investigation, subbasins were delineated from the river basin boundaries outlined on these maps. Some additional boundaries were established to divide basins at dam site locations, and some small basins were combined to facili.tate computer modeling. Basin identification numbers were abbreviated from the hydrologic unit numbers supplied by the USGS. 3.3 BASIN PARAMETERS The basin parameters of channel length, slope, and distance to the centroid were used in calculating basin factors and lag times. The stream length (L) was measured along the longest stream course from the basin outlet to the watershed divide. The basin centroid (Lea> was measured from the basin outlet to a point on the stream nearest the centroid. The subbasin's low elevation and high elevation along the longest stream course were used in calculating the stream slope (S). The basin factor (BF) was determined using the formula, LL 0.33 BF=(~) 80.5 These parameters are sUn\n\arized in table 3.1 along with the drainage areas of each subbasin.