9700

• UCREFRP Catalog Number: 9700
• Author: Hayes, J.W., K.E. LaGory and G.L. Burton.
• Title: Consideration of site-specific floodplain inundation thresholds in implementing peak flow magnitude and duration recommendations in the middle Green River, Utah.
• USFW Year: 2005.
• USFW - Doc Type: Argonne National Labortory, Argonne, Illinois.
• Copyright Material: NO

Green River Floodplain Inundation

3

June 2005

2 EXISTING PEAK FLOW RECOMMENDATIONS

Reach 2 of the Green River contains the largest amount of floodplain habitat in the Upper
Colorado River Basin and is considered the most important reach for endangered fishes in the
Basin. Muth et al. (2000) recommended peak flows in Reach 2 to achieve the following
objectives:

· Rework and rebuild in-channel sediment deposits;
· Increase habitat complexity;
· Form in-channel sand bars;
· Prevent or reverse channel narrowing;
· Inundate floodplain and other off-channel habitats;
· Establish river-floodplain connections; and
· Provide warm, food-rich environments for growth and conditioning of razorback suckers
(especially young), Colorado pikeminnow (Ptychocheilus lucius), and humpback chub
(Gila cypha).

To achieve these objectives, Muth et al. (2000) made separate recommendations for
instantaneous peak flow magnitudes and peak flow durations. When the recommendations were
developed, the recommended instantaneous peak flows were intended to accomplish the channel
maintenance functions identified above and the recommended extended durations were intended
to maintain connections between floodplain nursery habitats and the main channel long enough
for larval endangered fish (especially razorback sucker) to have adequate access to those areas.
These recommendations and their anticipated benefits are identified in Table 1. Recommended
instantaneous peak flow magnitudes were 235 m3/s (8,300 cfs) in dry and moderately dry years;
?.527 m3/s (18,600 cfs) in one of two average years; ?.575 m3/s (20,300 cfs) in moderately wet
years; and ?.748 m3/s (26,400 cfs) in wet years. Connecting flows of 527 m3/s (18,600 cfs) for
two weeks or more were recommended in one of four average years and in all moderately wet
years. Connecting flows of 643 m3/s (22,700 cfs) for two weeks or more and 527 m3/s
(18,600 cfs) for four weeksor more were recommended in wet years. The recommended peak
flow magnitudes were largely based on the relationships shown in Figure 1 for inundation of
floodplain areas with levees in place. The recommended duration of two weeks or more for
connecting flows was chosen to match the length of time over which most razorback sucker
larvae would be available for entrainment into floodplain habitats (Muth et al. 2000).

The Muth et al. (2000). flow recommendations state that the duration of peak flows in
Reach 2 of less than 527 m3/s (18,600 cfs) should be limited, because useable floodplain habitats
are not available at such flows (Table 1). However, on the basis of the information presented in
Valdez and Nelson (2004) and summarized below, we believe this recommendation is not valid
and, if implemented, could result in lost opportunities in some average years to provide flows
needed by native fish.

Razorback sucker nursery habitats are less prevalent in Reach 3 than in Reach 2 of the
Green River (Figure 1). Peak flow recommendations for Reach 3 focus on the magnitude,
duration, and timing of flows that would provide floodplain inundatfon in the upper portion of