WSP09464

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 5/24/2000
• Author: Hazel et al.
• Title: Sand Deposition in the Colorado River Ecosystem from Flooding of the Paria River and the Effects of the Nov 1997 Glen Canyon Dam Test Flow
• Water Supply Pro - Doc Type: Report/Study

NAU Sand Bar Studies

Final Report

MANAGEMENT IMPLICATIONS

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As controlled flooding becomes a tool for regulated river resource management, predicting the

outcome of floods under a range of antecedent conditions is important. Greater understanding is

needed of the basic physical processes that control sediment transport following tributary sand inputs

because limitations in sediment supply during flooding control main-channel concentrations (Rubin et

al" 1998; Topping et al" 1999) and eddy bar deposition rates (Wiele et al" 1999), In addition to

sediment availability and discharge, the volume of sand occupying the depositional site prior to

flooding (antecedent storage) is an important factor in determining the magnitude and persistence of

flood-related deposition (Hazel et al., 1999; Wiele et al" 1999), Prediction of change is further

complicated by variability in channel and debris fan geometry (Webb et al" 1989; Schmidt and Graf,

1990; Melis, 1997), the cumulative downstream nature of the limited sand supply (Topping et ai"

2000a, 2000b), and in the spatial and temporal variability of study sites utilized for monitoring

(Schmidt et al" 1999b; Grams and Schmidt, 1999),

The 1997 Test Flow was the first attempt by the Glen Canyon Dam adaptive management group to

implement a release closely timed with tributary floods on the Paria River. The potential benefit of

floods timed closely with tributary inputs is that in the presence of finer sediment, sand concentrations

will be higher resulting in higher rates of deposition in eddies (Topping et al" 1999), As a result,

planned floods can be shorter in duration and lessen the economic cost associated with loss of

hydropower generation and altered water-release patterns (Harpman, 1999), Because of these

associated costs it was important for ri ver managers to know if a peak power plant discharge, the 1997

Test Flow, could prolong the residence time of sediment supplied by the Paria River, Our data

suggests that this management goal was not achieved, and we conclude that the discharge of future

planned floods designed to redistribute sediment to high elevation will need to be at least similar to the

magnitude of the 1996 Controlled Flood, if not greater. Controlled floods designed to prolong the

residence time of newly input sand in Marble Canyon need to take into account where storage

environments are available, especially at higher elevations, as well as the antecedent conditions of sand

storage on the bed with respect to volume, grain size, and spatial distribution,

Timing future floods to coincide with years or months of above average sediment delivery by the

Paria River is a critical resource issue (Schmidt, 1999). Large floods on the Paria River typically occur

during the late summer or early fall (Topping, 1997). Coordination of high releases at this time of year

is difficult because flows in excess of power plant capacity can only be released to avert hydrologic

emergencies, a situation that is most likely to occur in late spring or early summer when inflow to Lake

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