9331

• UCREFRP Catalog Number: 9331
• Author: Allred, T. M. and J. C. Schmidt
• Title: Channel narrowing by vertical accretion along the Green River near Green River, Utah
• USFW Year: 1999
• USFW - Doc Type: Geological Society of American Bulletin
• Copyright Material: YES

section dat:a obtained from discharge measure-
ments made between 1932 and 1984. Bed and
bank topographic data were extracted from those
measurements made when discharge inundated
the right bank. We synthesized these data into a
time series of the change in elevation of the sur-
face of the deposit near the right bank for the pe-
riod between 1956 and 1993 (Fig. 14). We deter-
mined the duration of time when this surface was
inundated by combining the stage-discharge rela-
tion and tbe hydrologic record. Everitt (1968)
used a combination of stratigraphic and den-
drochronologic techniques to produce a similar
plot of floodplain accretion rates for the Little
Missouri River in North Dakota. He demon-
strated that vertical accretion occurred, but he
could not provide a temporally-detailed perspec-
tive because his techniques had limited temporal
resolution.
Our data suggest that a channel bar had mi-
grated into the cross section, or otherwise estab-
lished itself, by 1956, and that the surface of this
bar rapidly increased in elevation during a period
when the bar was continuously inundated. This
bar was exposed for the first time after recession
from the 1959 flood. Thereafter, the bar was sub-
ject to deposition during each flood large enough
to inundate the surface. The bar was inundated 5
times between 1959 and 1966 and 9 times be-
tween 1966 and 1984, which was the last time
that deposition occurred at this location.
The elevation of the ground surface that would
have been exposed in the late 1960s and early
1970s is the same as the elevation of the organic
layers exposed in the excavation. These topo-
graphic data agree with the tree ring dates, be-
cause the lowest elevation root crowns exposed in
the trench are located at elevations equivalent to
the ground surface surveyed in June 1959.
Thus, sa1tcedar established itself on this de-
posit immediately after it was exposed following
recession of the 1959 flood. At the time of germi-
nation, the substrate was well-sorted fine and
very-fine sand. New root crowns were estab-
lished during the period when the bar was not in-
undated between 1965 and 1970, and during the
5 floods of the 1970s when inundation occurred
but when there was only minimal deposition.
We also determined if similar processes had
occurred before 1956 by developing a time series
of channel width at low discharge (Fig. 15). Sim-
ilar processes occurred in the past but did not nec-
essarily lead to channel narrowing. Between 1930
and 1936, the width of the channel at low dis-
charge varied between 90 and 106 m. Each time
the low-discharge channel narrowed, it was sub-
sequently rewidened. Between 1936 and 1948,
the low-discharge channel varied between 96 and
103 m in width. We interpret this temporal pattem

1764

ALLRED AND SCHMIDT

-t .

0.25
A > 0.15 m OF SCOUR SINCE PREVIOUS MEASUREMENT

0.20

0.15

Cf)
LU
()
Z
LU
a:
a:
:J
()
()
o
u.
o
z
o
~
a:
o
a..
o
a:
a..

0.10

0.05

0.00

0.35

_ Thalweg
c:::=J Mean bed

8 > 0.15 m OF FILL SINCE PREVIOUS MEASUREMENT
0.30

0.25

0.20

0.15
0.10

_ Thalweg
c:::=J Mean bed

0.05

0.00

co I'-- LO C\I C\I C') <:t LO <0 <0 I'-- co m 0 0 .,.... C\I C') C') <:t ~
C\I LO co <:t ..... co LO C\I m <0 C') 0 I'-- LO C\I m <0 C') 0 I'-- 0
..... C\I C\I C') <:t <:t LO <0 I'-- I'-- co m m 0 ..... C\I C\I
..... ..... ..... ..... :2
DISCHARGE
(m3/s)

Figure 6. Histograms of scour (A) and 6II (B) at the present cableway for both the thalweg and
the mean bed elevations. Note that for both scour and fill, the discharge causing the largest pro-
portion of bed adjustment is between 85 and 142 m3/s.

as reflecting the migration ofIow-elevation chan-
nel bars (Fig. 2) into and out of the cross section.
The width of the channel at low discharge de-
creased during those times when a bar migrated
into the reach along one of the channel banks,
and the channel was wider when those bars were
not present. The process of channel narrowing
that began in 1957 is apparent in the decrease in
channel width at low discharge. The magnitude
of this decrease in width is similar to that which
occurred in 1930 and 1931. However, narrowing
was reversed by spring floods of 1082 m3/s and
784 m3/s that occurred in 1932 and 1933, respec-
tively, and which eroded the low-elevation bar
that had been present.
In contrast, neither the low-discharge channel
nor the bankfull channel rewidened after 1959,
because this bar never migrated downstream be-

Geological Society of America Bulletin, December 1999

yond the cross section. The conversion of this bar
to a vegetated floodplain began during 3 years of
low annual floods, between 1959 and 1961. The
flood of 1 %2 was 920 m3/s, and it did not erode
this bar. Instead, this flood caused additional ver-
tical accretion. The magnitude of the 1962 flood,
which did not scour the bar, was 15% less than the
magnitude of the 1932 flood, which did produce
scour, but we cannot resolve the relative roles of
river hydraulics and more abundant saltcedar in
preventing bar scour in the early 1 %Os.

DISCUSSION AND CONCLUSIONS

Timing and Rates of Channel Change

Within the study area, the Green River was at its
widest prior to 1930. At the old cableway, the