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............ ..-----.---,............. ."..... .......... .... . ",... ". .....-,.--."'......"'.----................ 1~\~~J7..... ..., ,.. '-'." ""-,,.. .... .-- ...,." ,... .."....,.----.,......."............ ............. . ....P!9\~PJ'l:....... .A~t\...~;A4...... -,"'--"'."',"',',",',"',',' ........... ........... .~ ." .. .... ., ./..IkAU>...) sb.tiRdI! of floodflows. Rain on snowlm;lt floods result from the augmentation of already high snowmelt flow volumes by the added runoff of rainfall. These types of floods can be characterized by the already moderate peak flows of snowmelt flooding followed abruptly by a short duration high flow. Historically, snowmelt has caused the greatest flooding around Delta and Grand Junction with augmentation from rain adding to some historical peak flows. The largest known snowmelt flood on the Gunnison Rivet was on July 2, 1884 with an estimated flow at Grand Junction of 50,000 cfs. Other significant flood events occurred in 1920, 1921, 1957, and 1984. The 1984 flood was at a frequency of slightly less than once in 50 years. TABLE 2 G1 G2 G3 G4 G5 G6 G7 DEL-TA GAGE BELOW UNCOMPAHGRE RIVER BELOW ROUBIDEAU CR BELOW.ESCALANTE CR BELOW DOMINGUEZ CR GAGE AT WHITEWATER (BELOW EAST CR) CONFLUENCE WIlli COLORADO RIVER 5628 mi' a 6760 mi' a 6998 mi' b 7274 mi' b 7532 mi' b 7928 mi' a 8010 mi" b Sources of Drainage Areas a Determined from published infonnation and USGS gage values b Detennined from upstream USGS gage infonnation combined with planimetering * FEMA published value at confluence (7870 mi~ 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 m?) 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 mi2) was used as a base value and planimetering downstream to the confluence provided the confluence value for this report (8010 mi2). 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. 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 7