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published a Flood Blain Information Report on the Big Thompson River in the area south of Loveland in 1971 (Ref. 11). Both hydrologic and pertinent hydraulic data prepared as a part of that study were utilized in this report (Ref. 1). from 5,300 feet to 14,250 feet. Included in the 304 square miles of drainage area above the gage are two major tributaries, the North Fork of the Big Thompson and Cedar Creek. These subbasins drain approximately 83 square miles and 17 square miles. respectively. The U. S. Geological Survey and the Colorado Water Conservation Board jointly published a Flood Information Report of the Big Thompson River flood of July 31 - August 1, 1976 (Ref. 4). This report depicts the flooded arcas in the 1975 flood and also reports various computed flood discharges and measured high water elevations. The flood plain outline shown in this report is not to be interpreted to be the same as the 100-year flood plain outline delineated in Volume II of the Special FlOOd Plain Information Report. Below the mouth of the canyon, the river falls approximately 620 feet to an elevation of 4,680 feet at the mouth. The river drains a total area of 828 square miles at the mouth. The study area for this report generally includes that portion of the Big Thompson River between Loveland and Lake Estes near Bstes Park on the main stem and above Glen Haven on the North Fork. Table 1 lists the basin drainage areas and stream lengths at various locations within the basin. STUDY AREA DESCRIPTION Table 1 The Big Thompson River is a western tributary to the South Platte River with the confluence near LaSalle, Colorado. The river originates in the Rocky Mountains and the basin extends to the Continental Divide at an elevation of 14,250 feet above mean sea level on Long's Peak. From its headwaters in Rocky Mountain National Park, the river passes through the towns of Estes Park, Loveland and Millikan, ColoradO. The Big Thompson River Basin is bounded by the Cache LaPoudre River Basin on the north and the Little Thompson and St. vrain River Basins on the south. Figure 1, Vicinity Map, shows the location of the basin and general study area. BIG THOMPSON RIVER DRAINAGE AREAS ~~D MILEAGES Location Drainage Area (sg.mi.) Mileage Above Mouth Mouth, near LaSalle 828 o The Big Thompson River Basin is all mountainous terrain above the river gaging station just above the mouth of the canyon in "The Narrows". This portion of the basin 'laries in elevation Lower study limit, near Loveland 509 "The Narrows", mouth of canyon 304 Below Drake 274 Above Drake 1'11 Lake Estes, near Estes Park 156 North Fork above Drake 83 North Fork, below Glen Haven 51 31.3 3e .1 45.5 46.2 58.9 o 8.5 -5- -4-