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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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 has 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
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 analysis, the
average annual suspended sediment load from the Little Snake is
1,110,100 tons per year and the mean annual suspended sediment load for
the combined rivers is only 1,517,300 tons per year.
Compared to 1982, the 1983 field season at Mathers Hole incorporated
improved data collection techniques, a greatly expanded sampling
program, and collection of samples on both the rising and recessional
limb. -The '82 and '83 data are combined where appropriate in the
analysis of sediment transport, but the 1983 data is generally used in
obtaining sediment discharge versus water discharge regression
relationships. These sediment regfessions are presented in Table VI.
The coefficient of determination (r ) is higher for the recessional limb
than the rising limb and the correlation between the sediment load and
water discharge is excellent (see Figures 13 through 18). The Mathers
Hole and Deerlodge suspended sediment rating curves are nearly identical
(Figure 14). Excellent correlation was found with USGS measured
suspended load at Deerlodge Park and is shown in Table V and Figures 13
and 14.
With these regressions and a flow-duration curve, and a load-
duration analysis is performed on Mathers Hole data (Table V). The
total suspended load using all the data from 1982 and 1983 is 1.91
million tons per year and alone, the 1983 suspended load is 2.13
millions tons per year. Applying the load-duration analysis to the