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3.1 III. BASIN CHARACTERISTICS 3.1 General :<. Cascade and Portland Creeks drain the rugged volcanic and glaciated watersheds east of the town of Ouray. The boundaries of the basins are pre- sented in Figure 3.1. Elevations range from 7,685 feet at the, mouth of the creeks which empty into the Uncompahgre River to the headwaters of the cirques at 12,800 feet. The creeks convey runoff from rainfall and snowmelt and also channelize debris flows and avalanches. The debris flow hazards within these two watersheds are closely related to the pattern of glacial deposition and scour and postglacial landsliding of thick glacial material and erodible volcanics. 'I ". The headwaters of Cascade and Portland Creek watersheds both drain the Tertiary San Juan volcanic formation. These rocks are locally faulted and fractured and composed of tuff, breccia, andesitic and rhyodacitic flow, reworked bedded volcanic conglomerates and sandstone. The steeply sloping creeks then flow west and drain onto the Uncompahgre flood plain forming large debris fans upon which the town of Ouray resides. Other than these similarities, the two watershed and associated debris flow paths exhibit contrasting characteristics. First, Portland Creek drains the Amphitheater which is approximately twice as large an area as the Cascade watershed. Portland Creek drains an area of 2.8 square miles while Cascade Creek drains an area of 1.3 square miles. The Portland Creek basin's shape is more nearly circular as compared to Cascade's long, narrow shape. Much of the upper watershed and sediment source of the Amphitheater is more steeply sloping. The Portland watershed slopes at 30 to 100% compared to Cascade Creek at 30 to 70%. A graph of the two channel profiles is provided in Figure 3.2. As seen from the graph, the upper reach of Portland is much steeper than Cascade. As indicated on the attached map (Figure 3.1), the geologic hazard of this area is mapped as "rockfall" compared to parts of the Cascade head- waters where "talus slopes" are mapped on the less steep slopes. Although not shown on this map, another critical difference is that a majority of the Portland watershed below about 9,600 feet is overlain by thick accumulations of glacial drift and colluvium. This material is mapped as "cst," colluvial slopes with a thickness greater than 6 feet. Within Portland Creek itself, the colluvial slope and glaci~l drift material is relatively thick, e.g., a maximum of about 20 feet thick at point A and 10 feet thick at point B (Figure , 1 " f 'i ',...; 3.1) .