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I I I I I I I I I I I II I I I I I I I I SECTION I SUMMARY WATERSHED The South Boulder Creek Watershed encompasses 135.7 square miles (sq. mi.) of area starting from 13,294 feet elevation at the Continental Divide to its confluence with Boulder Creek at 5,160 feet elevation. As depicted by Drawings 1, 2, and 3 at the end of this report, the watershed is 27.2 miles long on a generally east west axis, and up to 6.3 miles wide. There are about 93.2 sq. mi. above Gross Reservoir and 42.5 sq. mi. below. The watershed is heavily forested between elevations 6,400 feet and 11 ,000 feet, particularly on the north facing slopes. There are significant areas of rock outcroppings and steeply sloped, narrow streams and incised stream banks which can encourage flashy runoff response. The foothills are steep, have far less ground cover and higher peak rainfall than the mountain watershed, which leads to high runoff response. The lower valley and floodplain generally have higher soil infiltration characteristics, but have substantial rainfall runoff response because of the development of impervious areas and street drainage conveyances. Gross Reservoir, completed in 1954 as a water reservoir for the Denver System, inadvertently provides a substantial flood control function for the Boulder community. Its purpose is to store up to 43,060 AcFt of water for domestic water supply. It is also authorized to generate power. Under current regulations, and due to the lack of any formal agreement to provide flood storage, this study can only assume storage above the crest of the spillway. The modeling herein estimates that during a 1 OO-year flood, assuming the reservoir was full to the spillway crest, 1,780 acre feet of dynamic storage would occur, which attenuates the peak flow and delays the peak about 3 to 4 hours. The total 1 OO-year rainfall volume tributary to the reservoir would be approximately 20,400 AcFt, while the total Basin Rainfall Volume would be 29,000 AcFt. Runoff would be roughly 5600 AcFt to the reservoir and 8000 AcFt for the total watershed. Therefore, Gross Reservoir, even if full to the spillway crest, has a significant effect on hydrology. In the May 1969 Flood event, all runoff (5485 AcFt) from the mountains above Gross was captured with no discharge over the spillway. Reservoir statistics indicate that the average storage available for the months of May through August is 20% of the reservoir volume (8362 acre feet) with an average of 7.7% of the reservoir volume (3219 acre feet) available in June when the maximum volume of water is stored. However, modification of the reservoir operating practices could change these statistics. I - 1