7197

• UCREFRP Catalog Number: 7197
• Author: O'Brien, J. S.
• Title: 1983 Yampa River Cobble Reach Morphology Investigation Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

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The period of record is too short to discern any distinct cycles. The
sediment record for the Little Snake, however, occurs during a, drier
than average period. It is difficult to speculate on the relative
magnitude of sediment yield during dry or wet periods but generally, in
semiarid regions, a decrease in runoff retards erosion.
Instantaneous peak discharges are generally used to estimate
discharge frequency. Since the flows from the two gaging stations are
combined to determine the daily discharge at Deerlodge Park, the use of
instantaneous peak discharges is inappropriate. Discharges of a
specified return period are determined by the application of a
theoretical probability distribution. Richards (1982) reports that the
Gumbel Extreme Value distribution is a model which generates a linear
function on a transformed probability scale. It is a two parameter
model which seems to yield more representative values for the Yampa
River than the log-Pearson type III model. The results of the Gumbel
distribution and log-Pearson type III are shown in Table II. The Gumbel
distribution is plotted in Figure 11. The 1983 peak discharge of 20,300
cfs has a return period of approximately 13 years based on the Gumbel
analysis and over 20 years based on the log-Pearson type III. Four
discharges in 62 years have exceeded 20,000 cfs which is a return period
of about 15 years. The maximum recorded peak discharge was 21,750 cfs
in 1974 (see Table III). The one hundred year event is about 27,000
(Gumbel) and 22,000 cfs (Pearson). The bankfull discharge return period
is about 16 years based on the Gumbel distribution.
The 1983 Yampa aRiver hydrograph in the Monument had the fourth
highest peak discharge and the third largest annual volume in the 62
years of historical record. These values are 6.7% and 22.6° smaller
than the maximum historical discharge and volume respectively.
Effective Discharge and Bankfull Discharge
Effective discharge is the flow that transports the most sediment
over a long period of time. It is the product of the magnitude of the
sediment transported by a given discharge and the frequency of
occurrence of that discharge. The effective discharge is approximately
11,500 cfs as shown in Figure 12. The return period for this discharge
is about 1.5 years (Table II). From the stage-discharge relationships
and survey measurements at Mathers Hole, the bankfull discharge was
calculated to be approximately 21,500 cfs, which has return period of
about 20 years.
Bankfull discharge is usually afforded the status of a "dominant"
discharge event which controls channel morphology. It is responsible
for creating the channel morphological characteristics, changing
width/depth ratios, forming or destroying bars and islands, and changing
bends and meanders. For alluvial streams this is often an intermediate
magnitude flow with a return period of 1.5-2.0 years (Rosgen, 1982).
Such a discharge is sufficiently frequent to be an effective channel-
forming event. Large floods are too infrequent to control channel
morphology. The Yampa River, however, is not alluvial stream in the
canyon, but an incised river with an armored bed whose channel
adjustment flows are limited to infrequent events.