8274

• UCREFRP Catalog Number: 8274
• Author: Pitlick, J. and R. Cress.
• Title: Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
• USFW Year: 2000.
• USFW - Doc Type: Recovery Implementation Program Project 48-C,
• Copyright Material: NO

RESULTS
The results of our analyses are split into four sections. The first two sections focus on historical
changes in channel morphology in the uppermost study reach (Rifle-DeBeque), and long-term
trends in suspended sediment loads at the four mainstem gauging stations. The last two sections
focus on existing channel characteristics and bed load sediment transport relations. All of these
results have implications for habitats used by native fishes and other organisms, but the last two sets
of results are perhaps most important for providing information and data on contemporary
processes of habitat formation and maintenance.
Historical Changes in the Geomorphology of the Rifle-DeBeque Reach
Comparison of aerial photographs of the alluvial reach between Rifle and DeBeque indicates that
many portions of this reach have undergone significant geomorphic change since 1937. Figure 12
shows relative differences in main channel area, island area and side channel-backwater area for
individual 1.6-km segments of the river. Figure 12a shows that he main channel has experienced
relatively small changes in planform area overall, with individual segments of the river equally
divided between those where channel area increased and those where channel area decreased. In
contrast, Figures 12b and 12c show that there have been significant changes in the area of islands
and side channels/backwaters. The change in island area is due mostly to the loss of several large
islands between RKM 352 and RKM 376 which appear to have been used as gravel pits during the
construction of the interstate highway. The change in side-channel/backwater area (Fig. 12c) is
very systematic, with 25 out of 39 subreaches showing a decrease in area.
Table 3 provides a summary of these data. When the difference in area is proportioned over the
total reach length of 58 km, the decrease in main channel area of -14 ha amounts to a reduction in
average channel width of about 3 m; such a change (-3%) is probably too small to be considered
significant. The changes in island area (-75 ha) and side channel area (-33 ha), on the other hand,
are clearly significant, and would probably be greater if we could remove the bias introduced by the
difference in flow levels in 1937 and 1995. Overall, the total area of islands in the study reach has
decreased by 20%, while the total area of side channels and backwaters has decreased by 31%
(Table 3). When the difference in area is proportioned over the total reach length, the total change
amounts to a 6 m decrease in the average width of side channels. Given that these features are
typically 10 to 20 m wide, a decrease in average width of 6 m represents a significant loss in
potential fish habitat.
Table 3. Summary of channel changes within the Rifle-DeBeque reach of the Colorado River.
--------------------------------------------------------------------------------------------------------------
1937 1995
Total Area Total Area
(ha) (ha)
--------------------------------------
Main Channel 469 ----------------
455
Islands 386 311
Side Channels 105 72
Change in Change per Change
Total Area Unit Lengths in Area
(ha) (m)
-------------------
- (%)
------------
----------------
-14 -
--
-3 -3
-75 -13 -20
-33 -6 -31
1. The change in area per unit length is computed on the basis of a total reach length of 58 km. Islands and side
channels are not continuous over this length.
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