7196

• UCREFRP Catalog Number: 7196
• Author: O'Brien, J. S.
• Title: Hydraulic and Sediment Transport Investigation Yampa River Dinosaur National Monument 1983 Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

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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. From the stage-discharge relationship 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.
In the cobble reach, the Yampa River has incised a channel below
the floodplain terrace. This incision has reduced the frequency of
overbank flooding, and the bankfull discharge Lhas a return period of 20
years. Bankfull discharge has also been determined to be the discharge
which moves the median size bed material. This evidence supports the
conclusion that the bankfull discharge is the channel forming flow.
Channel morphology in the canyon is not adjusted with every seasonal
variation in discharge. The dynamic nature of the cobble substrate
reach is maintained by bankfull discharge which reforms the bar shape
and orientation and reorders the substrate size distribution in local
areas. Bedload transport of the large cobble substrate occurs with
discharges in excess of bankfull discharge. The channel width will
adjust to form the most efficient section for the cobble bedload
transport. Rare floods, therefore, have the most significant effect on
channel morphology in the Yampa Canyon.
Sediment Transport
There is no corresponding period of record for daily sediment
discharge at the Maybell and Lilly gaging stations. Five years of daily
sediment discharge measurements were collected by the USGS for water
years 1960 through 1964 for the Little Snake and 1952 through 1958,
1976, and 1978 through 1982 for the Yampa, 13 years of record (see Table
IV). For these short records the mean annual suspended sediment load
was 1,341,300 tons for the Little Snake and 407,200 tons for the Yampa;
total of approximately 1,748,600 tons/year of suspended sediment
delivered to the Monument. The mean annual flow during these years was
1,418,400 or 4.4% less than normal for the 1941-83 period. During this
period, however, one extreme year of sediment discharge occurred for the
Little Snake. If 4 days in 1962 (1;156,000 tons) are excluded from the