Laserfiche WebLink
<br />1. Stations were chosen from regions where streams have experienced high <br />peak flows of both snowmelt and rainfall origin. <br />2. Stations with natural streamflow or where peak flows were only minimally <br />affected by regulation or diversion were used in the data base. If diversion above <br />the station was used for irrigation of an area greater than 10 percent of the to- <br />tal drainage area, the station was excluded. <br /> <br />Although many stations met the selection criteria, the sample of stations <br />from the South Platte River and the Arkansas River basins were limited to those <br />stations with a period of record including at least 10 years. Most stations in the <br />sample from the Colorado River basin included at least 20 years of record and were <br />selected in order to cover a broad geographic distribution. <br /> <br />DIFFERENTIATION BETWEEN SNOWMElT- AND RAiNFAll-PRODUCED PEAK FLOWS <br /> <br />Peak flows in the foothills region of Colorado are caused primarily by one of <br />two meteorologic sources--snowmelt or rainfall. In a majority of the basins in <br />this region, severe floods are generated by intense rainfall. However, in many <br />watersheds at higher elevations, the yearly peak flow is produced from snowmelt. <br />Previously no distinction was made between snowmelt- and rainfall-produced peak <br />flows in the same year. <br /> <br />To establ ish the data base for this study, streamflow records were examined <br />for 69 gaged sites, as shown in table 1. Gage heights were determined from <br />continuous-stage recorders to calculate peak discharges from snowmelt and rainfall <br />runoff. Indirect measurements of peak flow, if available, were used to supplement <br />station records. For a given water year, the published annual runoff peak was <br />checked against the streamflow chart and weather records and identified as either <br />a snowmelt- or rainfall-produced event. Then the chart was examined for a second <br />peak--the highest peak of the other runoff category. <br /> <br />Because the results of the study are to be utilized in flood-plain manage- <br />ment, interest was focused on the frequency with which portions of a flood plain <br />were inundated. Therefore, instantaneous discharge was identified rather than the <br />total volume of an event, regardless of the duration of the event. For example, <br />when choosing between two rainfall-produced peaks, the peak exhibiting a higher <br />instantaneous discharge was selected rather than one which may have had a greater <br />volume during a given event but had a lower peak discharge. <br /> <br />Precipitation, ground cover, aspect, soils, and geomorphology vary from basin <br />to basin. Similarly, the hydrologic character of each watershed is unique, but <br />patterns of runoff exist that enable an investigator to differentiate between <br />snowmelt runoff and rainfall runoff. <br /> <br />Snowmelt-runoff peaks result from the seasonal ablation of the snowpack. in- <br />spection of streamflow charts revealed that typically the snowmelt hydrograph had <br />a diurnal pattern in which daily peaks rose and fell over a period of several days <br />or weeks, as shown in figure 5. At elevations above 6,000 to 7,000 ft, these peaks <br />had a regular, well defined appearance. Daily snowmelt-runoff peaks for a given <br />basin often occurred at the same time 'of day, and the annual snowmelt-runoff peak <br /> <br />7 <br />