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

Near the upstream end of the horseshoe bend, slightly more than 60 m up-
stream from station 434, station 502 exhibited a slightly different set of
conditions (Figure 18). A large gravel bar on the convex, inner side of the
bend was removed in f'1arch, 1975, restored by high flows in May, and again
removed in August. Width of the water surface was increased from 6 m in January,
1975 to 10 m in August. There was little change in profile until after the
spring runoff in 1976. The cross sectional profile in June, 1976 was very
similar to that of the previous June, with a water surface width of only 5.5 m,
0.6 m less than before the stream was altered. The cross sectional area had
increased by 33 percent from January, 1975 to April, 1976, but by June, 1976 it
was 28 percent less than in January, 1975. Maximum cross sectional depth was
about 0.4 m greater in June, 1976 than in January, 1975.

Station 545 was in the middle of a sharp bend about 30 m upstream from
station 502. As was typical for other locations, the gravel bar on the convex,
deposition side of the river was removed each time the stream was altered.
Stream width at the water line varied fro~ 7.6 m in January, 1975 to 13.4 m in
April after alteration, 7.9 m in June after spring high flow, 10.7 m in Septem-
ber after a second modification and 9.8 m in June, 1976 after high flow. In
each case the stream banks above waterlevel remained about the same. Differences
in width were caused by a gravel bar that was deposited along the inside of the
river bend.

The maximum depth of this cross section was increased about 30 cm by the
dredging in March, by an additional 5 cm during spring runoff, by 5 cm more by
dredging in August and by another 38 cm during the 1976 spring runoff. Part of
this 0.8 m increase in depth was due to normal deepening of the channel by the
spring runoff. During low flows, pools commonly decrease in depth because of
deposition of streambed materials. High spring flows remove this material again,
and deposit it along the convex sides of bends further downstream. The total
difference in depth between January, 1975 and June, 1976 is due to the combin-
ation of spring high flow and dredging.

Channel cross sectional areas changed from 3.25 m2 in January, 1975 to 7.5
in April, 4.9 in June, 4.9 in September, 4.2 in January, 1976, 6.4 in April and
8.8 in June, 1976. Water surface elevations changed very little during this
time, despite the 79 percent increase in cross sectional area.

Station 662 was midway in a straight reach of the stream about 77 m
upstream from station 545. The main water current flowed close to the eastern
or left bank. Dredging reduced the stream width from 12 to 9 m in spring,
1975, and other alterations upstream changed the thalweg course sufficiently to
cause erosion along the east bank and a large deposition of sand and gravel on
the west or right bank during spring runoff. Dredging in late summer did not
change the width, but shifted the stream channel to the right. Low flows
between September and April eroded 1.8 m of the left bank and deposited 60 cm
of gravel along the right bank. The spring runoff had little effect on the
left bank but eroded 1.2 m of the right bank and removed about 30 cm of the
streambed from the right bank. These cl1anges resulted in a more uniform depth
across the stream at this station. The lack of deposition on the right side was
probably the result of restoring the channel cross section that would occur
normally and which redirected the current at station 622. The maximum depth was

32