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Attachment 3.

Alternative Experimental Flow Regimes in WY 2002-2003 for Consideration by the TWG

Preliminary Draft (March 14, 2002)

Recently, the GCMRC proposed three experimental flow scenarios for implementation in WY 

2002-2003. These scenarios are designed to address two issues: sediment conservation and nonnative fish control. An alternative flow regime should be considered for sediment conservation 

immediately following sediment input from tributaries — an alternative which uses either short 

duration maximum power plant capacity releases (31,500 cfs) or load following. The ad-hoc 

sediment group recently identified these as priority 1 experiments, i.e., experiments that should 

be considered immediately in a suite of experimental flows.

There are several reasons to first examine alternatives to employing low steady flows for 

sediment conservation: (1) the GCMRC experiment, as structured, does not include flows that 

could increase sediment conservation following significant sediment inputs should they occur in 

July or August; (2) low steady flows in December would require approval through the Annual 

Operating Plan (AOP) process; (3) low-steady flows would have significant adverse impacts on 

power revenues during peak demand months (July, August, and December); (4) potential 

benefits to non-native fish (and subsequent net disadvantages to native fish) could result from 

extended periods of steady flows; and (5) current understanding of sediment transport processes 

below Glen Canyon Dam leaves little doubt that low steady flows would reduce transport of fine 

sediment downstream and thus limits the amount of learning that would be gained by an

experiment of extended low steady flows. We believe that a better experiment would be to test 

whether sediment can be stored as it comes into the CRE while minimizing impacts to power and 

operating within the AOP and applicable governing regulations. The alternatives being proposed 

could be implemented without modification of monthly water volumes and with minimum 

impact to power revenues.

Rubin et al. (2000) suggested three approaches for restoring or retaining sand resources in the 

Colorado River Ecosystem below Glen Canyon Dam. Their first recommendation was to provide 

dam releases above power plant capacity immediately after substantial inputs of fine sediment 

from tributaries. In response to this recommendation, the ad hoc sediment group stated that 

“other dam release options exist that might promote potential eddy storage to conserve tributary 

inputs. Two of these options to be implemented immediately after substantial inputs of fine 

sediments are: (1) to implement Habitat Maintenance Flows (HMFs), or releases at power plant 

capacity; and (2) load following releases with fluctuations and magnitude greater than ROD 

restrictions.” We propose that these two sediment storage methods be considered for

implementation in WY 2002-2003 either as separate tests or as a combined strategy to maximize 

sediment conservation. The alternatives specifically focus on the period following sediment 

inputs in the late summer and fall. Either alternative could be combined with the GCMRC 

proposal of post-BHBF load following to disadvantage non-native fishes (and subsequently

advantage native fishes).
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Description of Alternative Experimental Flow Scenarios

1. Short-Duration Maximum Power Plant Capacity Spike Release

Habitat maintenance flows (HMFs) were included as an integral part of operations in the Record 

of Decision (ROD) for the Glen Canyon EIS. The purposes of HMFs, defined as high steady 

releases within power plant capacity for a period of 1 to 2 weeks, were to re-form backwaters 

and maintain sandbars. Since publication of the ROD, HMFs have been used on several

occasions (Hazel et al. 2001). These HMFs have had varying degrees of success in performing 

their intended function depending on timing of the HMF relative to sediment input, amount of 

sediment input prior to the HMF, and duration of the HMF.

Lessons learned from previous experiments suggest that to increase effectiveness, HMFs should 

be timed to coincide more closely with inflows from tributaries and be of shorter duration. Closer 

coordination with tributary inflows would accomplish two objectives. The combined flow of the 

maximum power plant release and the tributary inflow would increase the stage elevation at 

which sediment was deposited and make that sediment less vulnerable to erosion from later 

operational releases. In addition, shortening the time elapsed between sediment inputs and the 

spike flow would reduce the amount of sediment transported downstream and lost from the 

system. Reducing the duration of spike flows (one day or less) would allow deposition of

sediment at higher elevations while reducing transport downstream. In addition, small-bodied 

non-native fishes may be placed at a disadvantage by increased turbidity levels during and after 

these events and temporarily disrupting the relatively stable habitat conditions that favor their 

reproductive strategy.

The proposed experiment (Figure 1) is to produce one or more short-duration maximum power 

plant releases that coincide to the extent possible with inputs from tributaries (especially the 

Paria River, but also potentially the Little Colorado River). The duration of the peak that would 

be most effective in retaining sediment will vary according to the duration of the tributary

inflow, the volume of sediment that accompanies inflow, and the size of sediment particles. One 

of the intentions of the proposed experimental flow is to develop guidelines for determining 

effective spike flow durations.

We propose that each tributary inflow event be matched with a short duration maximum power 

plant release. These spike releases could vary from several hours to a day in duration, but in 

general, release duration would be a function of duration of peak tributary sediment input. 

Testing several durations in a single year in response to multiple tributary inflow events, would 

improve learning. Rapid ramping rates would conserve water, maximize sediment entrainment 

on the ascending limb of the spike, maximize fine sediment deposition on the descending limb, 

and minimize downstream sediment transport. Between spike releases, normal ROD flows (i.e., 

modified low fluctuating flows) would resume. These fluctuations could serve to stabilize new 

sediment deposits until a BHBF was released in January. The goal of this scenario would be to 

maintain full sediment storage in eddies and channel margins pending the January BHBF while 

delivering AOP-mandated water volumes.
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Null hypotheses to be tested

• Ho: short duration maximum power plant releases do not conserve sediment inputs from 

the Paria River in Marble Canyon.

• Ho: there is no relationship between the duration of maximum power plant releases and 

sediment storage.

• Ho: matching peak tributary input flows with short-duration maximum power plant

releases does not increase sediment storage.

• Ho: there is no difference in the sediment storage resulting from short duration 31,500 cfs 

spike releases and ROD fluctuations.

• Ho: the effectiveness of short duration maximum power plant releases in storing sediment 

is not dependent on the availability of storage space in eddies.

Note that the above null hypotheses specifically focus on sediment, but other hypotheses could 

be tested regarding the effect of experimental releases on other resources.

2. Maximum Load Following

Load following was proposed by the ad hoc sediment group as a possible means to conserve 

sediment and this experimental scenario would test the efficacy of that approach. The stage 

changes that occur daily when load following have two important components that reduce 

downstream sediment transport. First, the daily peak transports in-channel sediment and deposits 

it at higher elevations. Second, under maximum load following, the minimum daily flows are 

lower than under ROD flows and these lower flows have less capacity to transport sediment 

downstream. Added benefits would include destabilization of non-native fish habitats and

increased turbidity to enhance protective cover for native fishes.

With this alternative experimental flow scenario, a tributary inflow would be followed

immediately by load following without ROD restrictions (Figure 2). An appropriate length for 

the load-following period should be determined before hand, but is not proposed here (Figure 2 

shows a two-week period). The most effective length for conserving sediment is likely to be 

dependent on the amount and characteristics of sediment that is input by tributary events. As 

with duration of the spike flow of Alternative 1, the length of the load-following period could be 

tested as part of the experiment.

Null hypotheses to be tested

• Ho: load following does not conserve sediment inputs from the Paria River in Marble 

Canyon.

• Ho: there is no difference in the sediment storage capability of load following and ROD 

fluctuations. 

• Ho: the length of the load-following period does not affect sediment storage.

• Ho: the effectiveness of load following in storing sediment is not dependent on the 

availability of storage space in eddies.

Note that as with Alternative 1, the above null hypotheses specifically focus on sediment, but 

other hypotheses could be tested regarding the effect of experimental releases on other resources.
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Figure 1. Proposed Water Year 2002-03 Experimental Flow: Alternative 1

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ROD Flows

Short-Duration Maximum Power 

Plant Capacity Release Following 

Tributary Sediment Input

BHBF - 45,000 cfs.

ROD Operations - Unknown Hydrology for 2003
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Figure 2. Proposed Water Year 2002-03 Experimental Flow: Alternative 2

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Load Following Operations 

Initiated After Tributary Sediment 

Input

BHBF - 45,000 cfs.

ROD Operations - Unknown Hydrology for 2003

ROD Flows ROD Flows