FLOOD00328

• County: Eagle
• Community: Eagle
• Stream Name: Eagle River
• Title: Hydrology Report Roaring Fork and Eagle Rivers
• Date: 12/1/1997
• Prepared For: Eagle County
• Prepared By: J.F. Sato & Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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two features that were not used in this analysis: two station comparisons and weighting of
independent estimates. A generalized coefficient of skew of - 0.3 was adopted for all three stations
based on the generalized skew map presented in Bulletin 17B.

A review of the peak flow data for the two Roaring Fork River stations and for the Eagle River
station shows that all recorded peak annual discharges occurred from late May through early July.
Ten out of the 212 peak annual discharges occurred during early July. Floods could occur as a result
of rapid snowmelt or rainfall alone, or a combination of those events. While floods as a result of
rainfall alone could be important for small drainage areas, peak flow records show snowmelt is the
primary cause of peaks at the three stations studied. Rainfall may have contributed some runoff to
a few of the annual peak flows, but it is very difficult to identify the amount of contribution from
available hydrologic records. Rainfall did not result in independent peak discharge events because
the peaks occurred during the snowmelt season and snowmelt would also have contributed to them.
This conclusion is reasonably consistent with results of computations for larger streams during
previous regional hydrologic studies. Computing independent flood frequencies for rainfall and
snowmelt events and combining those frequencies on a probability basis to obtain the all-season
curve is not a practical approach at these three gaging sites.

Roaring Fork River

Determining flood frequency values for the study reach of the Roaring Fork River is complex
because there are no streamgages near the study area and analyses must use an indirect method.
Flood frequency computations were made for the two existing USGS stations on the Roaring fork
River using their entire periods of record: station near Aspen (09073400) and Station at Glenwood
Springs (0908500).

The original Aspen station started recording stream flow data in 1911 and continued operating until
1964. Data for the year of 1913 and for the period of 1922 to 1931 were not recorded, A 43-year
peak flow series for the Roaring Fork River is therefore available for the original gage location. In
October of 1964 the gage was moved to a nearby location and improved, It is now a water-stage
recorder with satellite telemetry. A 32-year peak flow discharge series through 1996 is available for
the existing gage location. The total series of peak discharges at the Aspen gage is therefore 75 years
long.

The Glenwood Springs station started recording data in October 1905. The series of annual peak
flows includes the years 1906 through 1909 ( 4 years), 1912, 1914 through 1981 (68 years), and the
period from 1983 through the current year. The data available for this study includes the year 1996
(14 years). The peak flow series available for this study is therefore 87 years. A printout of the
results of flood frequency computations for the 75-year Aspen series and the 87 -year Glenwood
Springs series is presented in the Appendix. As shown in the Appendix printouts, peak flow data
recorded at a new gage location or data for years not yet included in the HYDRODA T A compact
disk file are added to the series by program FFFREAK as Historic Data. The new longer series is
then analyzed following the guidelines of Bulletin 17B.

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