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

with base flows of 98 cfs for the Little Snake and 297 cfs for the Yampa
into the same sediment budget program which produced the historical data in
Tabl a 4. The summary resul is are shown in Tabl e 5 and al I the resul is are.
presented in Appendix B.
The exceedance hydrographs for each tributary are evaluated one at time
with the prescribed base flow (base flow or exceedance discharge whichever
is greater is used for that daily discharge) and these constitute the
minimum streamflow hydrograph In this analysis. A comparison is made
between the minimum streamflow hydrograph and the historic discharge on a
daily basis for each year on record and the smal I er val ue is used In the
analysis. This comparison Is made for the discharge at Mathers Hole (the
combined flow for the Little Snake and Yampa Rivers). The sediment load
predictions are then performed based on the f anal discharge val ues at
Maybel I, Lily, and Mathers Hol e.
The sediment budget array in Table 5 demonstrates that a sediment
balance or equi I ibrium in the canyon w it I only be maintained if the
streamf I ow s in each tributary are reduced by equal proportions. This table
also shows that if the Little Snake River flow remains essential ly
undepleted, the flows in the Yampa should not be reduced if the sediment
equilibrium is to be maintained. Within the range of error in the
measurements and the error introduced In this analysis some minor depletion
of the Yampa River as discussed later would not adversely effect the system.
To progress further with the sediment budget analysis, the NPS minimum
streamflow hydrograph (O'Brien, 1984) is tested assuming that the flows in
the Little Snake River are depleted according to criteria derived from Table
4 of the USFWS Stagecoach Biological Opinion (1986) This criteria
postulates monthly target flows for wet, dry and average years and results
in a minor depletion of the average annual volume of the Little Snake of
29,000 AF (6 percent). The concepts of a dry or drought year and a wet year
in the hydrologic record of a given river Is subject to interpretation. To
quantify these delineations for application of the FWS flow targets for the
Little Snake, the annual volume for the period of record was statistically.
analyzed for the combined f lows of the Yampa and Little Snake Rivers. Based
on the statistics, flows were divided into three categories using one-half
the standard deviation to identify wet, average and dry years:
Wet Years (18) > 1,800,000 AF
Average Years (25) 1,300,000 AF < Volume < 1,800,000 AF
Dry Years (21) < 1,300,000 AF
Af ter the minor depletions are subtracted from the Little Snake
historic flows, a comparison of the NPS minimum streamfIow (with an
alternative 340 cfs base flow) and historic flaw is made on a daily basis
and whichever discharge is less is assigned as the minimum hydrograph for
Mathers Hole. If this Mathers Hole minimum hydrograph is less than the
historic hydrograph, the difference is charged as a depletion from the Yampa
river at Maybel I, subject to a 297 cfs base flow at Maybel I.
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