7198

• UCREFRP Catalog Number: 7198
• Author: O'Brien, J. S.
• Title: Analysis of Minimum Streamflow and Sediment Transport in the Yampa River, Dinosaur National Monument.
• USFW Year: 1987.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

of the year In the historical record must be ranked according to the flow
magnitude from the lowest to the highest discharge. A probabiI ity for that
day based on the total number of years Is then assigned to each discharge.
Usi ng that probabi I Ity, the fl cw that i s exceeded (say 50 percent of the
time) can be determined on a daily basis to calculate the 50 percent
exceedance hydrograph. This was accompl i shed for 50, 75, 84, 90 and 95
percent exceedance hydrographs for each river. A 50 percent exceedance
hydrograph is equivalent to the median hydrograph for the flow period.
Smal I er exceedance hydrographs are represented by the I anger exceedance
percentages, 75, 84, 90 and 95. A 95 percent exceedance hydrograph is a
much smal i er hydrograph with substantial volume depletion. Examples of
exceedance hydrographs are shown in Figure 1. The hydrographs in this
figure are based on lognormal exceedance probabilities which would generate
sl ightl y different hydrographs than a W iebul I distribution. The hydrographs
shown In Figure 1 are very similar to those employed in this study.
To expand sediment budget analysis to years without measured sediment
load data or to analyze various altered hydrographs, the modified sediment
regression relationships were appl i ed to the given daily discharge to
predict daily sediment loads. The computed sediment load associated with
each day was summed for each gaging station and Mathers Hole to determine
the mean annual sediment for each exceedance hydrograph. An array of water
volumes and sediment loads was generated. In this analysis, the specified
exceedance hydrographs were analyzed as minimum streamflcw hydrographs with
the volumes of water in excess of minimum streamflow criteria assumed to be
withdrawn from the system and unavailable for sediment transport.
Variations on the NPS minimum streamf low hydrograph were analyzed by
incorporating additional peak flows from the Yampa River (MaybelI) in excess
of the proposed NPS minimum streamfl cw hydrograph. These peak flows were
added to the NPS minimum streamflow hydrograph in increasingly larger
percentages. The Little Snake River peak flows were not altered In the NPS
minimum streamf I ow analyses. For example, discharges above 90 percent of
the actual peak f I cw in any year were added to the Yampa River flows In one
set of runs those f I ows above 75 percent of the actual peak were added In
another. In this manner, the effects of decreasing the number of high f I ow
discharge days In the Yampa River could be evaluated.
The volume of the mean annual hydrograph of the Yampa River in Dinosaur
was increased from 1,510,000 AF to 1,540,000 AF as a result of adding two
very high volume years, 1983 and 1984, to the water discharge database
(Table 2). Addi ti onal I y, the mean f I ow for the per i od of September 1 to
February 28 was determined using the expanded database:
Little Snake:
Yampa:
Combined:
This compares with 367 cf s
These historic mean flows cor
period.
98 cf s
297 c f s
395 cf s
combined mean f I ow in O'Brien's 1984 report.
stitute the minimum base fIcw for the indicated
6