Floodplain Information Report Volume 3 Gunnison River from Delta to the Confluence with the Colorado River

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I 0 0 TABLE 2 Sources of Drainage Areas a Determined from published information and USGS gage values b Determined from upstream USGS gage information combined with planimetering * FEMA published value at confluence (7870 miZ) was found to be unverifiable based on the USGS gage value at a location approximately 9 miles upstream Only one problem was encountered during the drainage area determinations. The drainage area published by FEMA for the mouth of the Gunnison River in the Delta County FIS (7870 mil) was found to be lower than the value at the USGS gage location approximately 9 miles upstream at Whitewater. The value at the gage location (7928 miz) was used as a base value and planimetering downstream to the confluence provided the confluence value for this report (8010 miz). The FEMA value was assumed to be in error. Figure 4 shows the study reach and each of the hydrologic analysis points listed in Table 2. 1 2.5 Flood Characteristics Major flooding in the Gunnison River basin has been primarily the result of rapid snowmelt beginning as early as late April. Snowmelt flooding can continue into early July. Flooding can also occur from rainfall on snowmelt in late winter or early spring. Due to the small areal extent ' and limited duration of thunderstorms, they do not typically constitute a major flood threat on streams as large as the Gunnison or Uncompahgre Rivers. They do, however pose a problem for some of the smaller tributaries in the Gunnison River basin. Snowmelt flooding is characterized by moderate peak flows, large volume of runoff, long duration, and diurnal fluctuation of flow. Flooding from general rainfall alone, though uncommon on the Gunnison River, is characterized by high peak flows and moderate duration 6 u GCINNISCdN RIVER HYD .O.L IC ANALYSIS POINTS HYDROLOGIC LOCATION:; DRAINAGE ; D,A POINT AREA .(D A.} SOURCE GI DELTA GAGE 5628 mil a G2 BELOW UNCOMPAHGRE RIVER 6760 miZ a G3 BELOW ROUBIDEAU CR 6998 mil b G4 BELOW ESCALANTE CR 7274 miZ b G5 BELOW DOMINGUEZ CR 7532 miZ b G6 GAGE AT WHITEWATER (BELOW EAST CR) 7928 miZ a G7 CONFLUENCE WITH COLORADO RIVER 8010 miZ* b Sources of Drainage Areas a Determined from published information and USGS gage values b Determined from upstream USGS gage information combined with planimetering * FEMA published value at confluence (7870 miZ) was found to be unverifiable based on the USGS gage value at a location approximately 9 miles upstream Only one problem was encountered during the drainage area determinations. The drainage area published by FEMA for the mouth of the Gunnison River in the Delta County FIS (7870 mil) was found to be lower than the value at the USGS gage location approximately 9 miles upstream at Whitewater. The value at the gage location (7928 miz) was used as a base value and planimetering downstream to the confluence provided the confluence value for this report (8010 miz). The FEMA value was assumed to be in error. Figure 4 shows the study reach and each of the hydrologic analysis points listed in Table 2. 1 2.5 Flood Characteristics Major flooding in the Gunnison River basin has been primarily the result of rapid snowmelt beginning as early as late April. Snowmelt flooding can continue into early July. Flooding can also occur from rainfall on snowmelt in late winter or early spring. Due to the small areal extent ' and limited duration of thunderstorms, they do not typically constitute a major flood threat on streams as large as the Gunnison or Uncompahgre Rivers. They do, however pose a problem for some of the smaller tributaries in the Gunnison River basin. Snowmelt flooding is characterized by moderate peak flows, large volume of runoff, long duration, and diurnal fluctuation of flow. Flooding from general rainfall alone, though uncommon on the Gunnison River, is characterized by high peak flows and moderate duration 6 u