8250

• UCREFRP Catalog Number: 8250
• Author: Lamarra, V. A.
• Title: Longitudinal Variation in the Trophic Structure of the Upper Colorado River.
• USFW Year: 1999.
• USFW - Doc Type: Logan, Utah.
• Copyright Material: NO

The Gunnison and the Colorado rivers had similar size distributions in the spring of 1994,
however, the temporal pattern for the data collected in 1994 and 1995 demonstrated the
opposite trend (Figure 3) in the Gunnison River. The data from fall 1994 and fall 1995
demonstrated a reduction in the percent of substrate less than 1 mm. In the spring of 1994,
9 percent of the bed material was less than or equal to 1 mm, however, in the fall of 1994
the value had decreased to 3.5 percent. A slight increase (to 5%) was observed in the fall of
1995.
COLORADO RIVER
RIVERBED SUBSTRATE SIZE
,00
u
9L +o

e
,
.
0 .01 10 ,00 1000
BED SUBSTRATE Siff (mm)
=- SPRING 96- FALL 94 - SPRING 98 -s- FALL 95

GUNNISON RIVER
RIVERBED SUBSTRATE SIZE

19
LL
?

0.01 91 1 10 100 1000
BED SUBSTRATE SUE (mm)
-•? SPRING 84-«- FALL 9f - FALL 99

The trend in the river-wide decrease of
substrate sizes in the Colorado River
was also evident in the calculated D 16
and D50 values (Figure 4). This data
indicates that in the spring of 1994, the
river-wide average D50 substrate size
sampled was 91 mm. This size
decreased to 75 mm in the fall of 1994
and reached a minimum of 68 mm in
spring 1995. Following the runoff of
1995, the average D50 increased to 82
mm, which was less than the spring of
1994 but greater than the fall of that
year. The D16 size, which is the
substrate size where 16 percent of the
sample is smaller in size, followed the
same pattern as the D50 except that
the response between spring 1995 and
fall 1995 was more dramatic,
increasing to a value equal to the
spring of 1994.
Figure 3. The cumulative distribution (% less than) of bed In the Gunnison River, the D50 values
material substrate size in the Colorado (above) and
Gunnison (below) rivers. Data are river-wide averages. for the substrate size slightly increased
from a mean of 65 mm in the spring of
1994 to 72 mm in the fall of 1994. In
the fall of 1995 the D50 increased to 83 mm; however, there was not a significant difference
between the mean D50 values for all three dates in the Gunnison River.
The impact of the change in substrate sizes noted above was also observed in the depth to
embeddedness (DTE) and the percent of surface area embedded (PAE).
The river-wide mean DTE values were significantly lower in spring 1995 and significantly
higher the following fall (1995) compared to the spring and fall of 1994 (Figure 5). Although
having slightly lower DTE values, the Gunnison River demonstrated the same temporal
variation as the Colorado River. For a given date, there were no significant differences
between the two systems.
10