8141A

• UCREFRP Catalog Number: 8141A
• Author: Wydoski, R. S. and W. T. Helm.
• Title: Effects of Alterations to Low Gradient Reaches of Utah Streams.
• USFW Year: 1980.
• USFW - Doc Type: \
• Copyright Material: NO

about 30 cm greater in June, 1976 than January, 1975, but was about the same in
June, 1975 and June, 1976.

Station 689 was changed more drastically than any other station (Figure 19).
The origin of the profile on the baseline was located on a gravel bar on the
convex or deposition side of a sharp bend. Each time the stream was altered,
this gravel bar was removed and the deep pool on the outside of the bend was
filled. Every year, the spring runoff replaced the bar where it would be
expected to occur naturally and re-excavated the deep pool. These changes
resulted in the deposition of much bedload from upstream and moved the bedload
from the pool area downstream. In addition, the streambank on the north end of
this transect needed to be continually stabilized by the landowner. Dredging
in August had changed the channel sufficiently to redirect the thalweg, which
resulted in rapid erosion ofonepbrtion of the bank and created the threat of
cutting off an entire meander. This area of bank cutting was filled in with
gravel retained by logs on Nay 1, 1976. After spring runoff in 1976, the
outline of the stream in this vicinity was similar to that of January, 1975, but
the thalweg location was shifted. Changes in this reach appeared to have a
large effect downstream in deposition of streambed material and the location of
deep pools.

Shifts in the location of the bank on the north end of this transect
resulted in a deposition of 1.8 m of fill during dredging in spring. 1975,
erosion of 8.5 m during spring runoff, deposition of 8.8 m during dredging in
August, and an erosion of 8.2 m between August, 1975 and April, 1976. The bank
was finally relocated about 6.4 m north of where it was located in January,
1975. The maximum depth remained the same in January and June, 1975, and June,
1976, in spite of all the changes that occurred during this time.

Station 724, about 30 m upstream from station 689, and located between two
bends in the river, exhibited some great changes during the study. The strea~
width decreased by more than 50 percent as a result of dredging and sediment
deposition during spring runoff in 1975, and subsequently increased again to 82
percent of the original width. This increase was not due to the direct effects
of the August dredging, but rather to subsequent shifting of the streambed.

The stream depth changed little; however, it increased about 46 cm with
the first dredging, became 15 cm deeper with the 1975 runoff, and was mostly
refilled during the 1976 runoff. The final depth was less than 30 cm deeper
than it was in January, 1975. With one exception, the deepest part of the
cross section was always on the left or eastern side of the stream. Formation
of large gravel bars during the 1975 runoff, their removal in August, and the
subsequent reappearance of small but apparently stable bars by June of 1976
appear to be linked with changes in~ediately downstream. When the channel at
station 724 was narrow, severe erosion occurred'on the concave bank at station
689. A wide channel at 724 appeared to influence the location of the thalweg,
and reduce erosion at 689.

Cross sections were surveyed at the Blacksmith control site in November,
1975 and again in July 1976, following the spring runoff (Figure 20). No
alteration work was done during the period of study, and according to the land
owner it has been over 20 years since any was done.

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