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Dolores River at Bedrock (lower boundary of study area) and at Dolores (20 miles above the upper study boundary) are shown in Figure 7. Historically, flooding of the Dolores River has occurred from two different processes: (1) flooding during April, May, and June from snowmelt or snowmelt combined with rainfall, and (2) summer and fall flooding resulting from high- intensity rainfall events. Spring snowmelt floods are usually characterized by a large volume of runoff and extended peak flows occurring over a several week period. The basin above the study area can generally be divided into two snowpack zones which normally, during the snowmelt season, produce bimodal runoff peaks. The lower elevation zone includes an area dominated by gambel oak, ponderosa pine, and scattered aspen. The higher subalpine zone is characterized by engelmann spruce, subalpine fir, and aspen. The low-elevation zone melts off first, commonly producing a flow-peak in late April or 13,000 12,000 early May. Peak flows from the melting of the subalpine zone usually occur in late Mayor early June. The occurrence of flooding from rainfall events varies within the study area. The portion of the Dolores Basin within the study area is mostly semiarid, having sparse vegetation cover and signifi- cant acreage of rock outcrop. Consequently, surface runoff from rainfall events is much greater than in the basin above the study area, which is mostly forested. This is shown in Table 2 where, in a lO-year period from 1972-1981, the Dolores River at Dolores (above the study area) experienced all of the annual peak flows from snowmelt. However, the Dolores River at Bedrock had 3 (1972, 1974, 1977) out of the 10 years where summer or fall rainfall events produced the annual peak flow. Within the study area, annual peak flow events occurring from rainfall are most evident during years when the snowpack and subsequent snowmelt runoff are below normal. High Point on Watershed Divide (Tin Cup Basin) 11,000 Study Reach 10,000 i' 9,000 ~ c: 0 ~ 8,000 > a> iJj 7,000 6,000 5,000 4,000 o 50 100 250 150 200 Figure 6, Dolores River profile. Distance from Watershed Divide (miles) 13