WSP12615 ‎(2)‎

• File Number: 8220.101.10
• Description: Colorado River Water Projects - Glen Canyon Dam-Lake Powell - Adaptive Management
• State: CO
• Basin: Colorado Mainstem
• Date: 10/27/1997
• Author: Glen Canyon Dam TWG - Spike Flow Subgroup
• Title: Report of the Spike Flow Subgroup - Glen Canyon Dam Technical Work Group - 10-27-97
• Water Supply Pro - Doc Type: Report/Study

002551

Back~round - Evolution of Spiil Expectations

GCES conclusions

The majority of the GCES phase i research work lOok place in the mid-1980's, when the releases
from Glen Canyon Dam were at an all time high since the construction of the dam. Flows were
generally high and constant. providing almost no opportUnity for the researchers 10 observe the
impacts of fluctuating !lows. The !lood flows were so different than historic releases and caused
such large effects downstream that they had a great influence on GCES recommendations.

On page 83 of the final GCES Phase I report. the first and foremost conclusion was that
"Adverse downstream consequences are caused primarily by sustained flood releases
significantly greater than powerplant capacity and by fluctuating releases", noting the erosive
effect of floods on sand deposits and vegetation. Generally. these conclusions suggested the
elimination or reduction of flood flows, Significant effort was taken to understand the
operational causes of spills. and in 1987 modifications to monthly release patterns were made by
Reclamation to reduce the frequency of spills from about I year in 4 to about I year in 20. Due
to the limited number of years of actual dam operation and forecasts. there was uncertainty
associated with these estimates, but a reduction in this frequency was cenainIy desired by the
researchers involved in GCES.

Despite the enonnous beaches created particularly by the 1983 spill event, the general thinking at
that time was that there was a very limited supply of sediment below Glen Canyon Dam and that
spills destructively moved much of this sediment out of the Grand Canyon.

1992 Grand Canyon Protection Act

In the committee report accompanying this legislation. the Congress continued this thinking of
adverse impacts by stating that "Flood releases from the dam erode beaches used by recreational
rafters and campers, The river's now reduced sediment loads are inadequate to replenish
beaches, even if flood releases occur once every twenty years. Flood releases destroy riparian
vegetation and birds." The Act did not specify remedial measures. but seemed to imply that even
the aggressive spill avoidance strategy that had been implemented to reduce spill frequency
might be insufficient.

GCDEIS Transition Work Group and Sediment Balance

After the passage of the GCP A, the thinking of some sediment experts began to change,
primarily as the result of the hypothesis that the sediment rating curves below the dam were not
static with time, Additional thought was also being given to the location of stored sediment in
the canyon and the mechanisms for moving sediment from the channel bottom to eddy areas.
Significant mQdeling by the sediment researchers changed to a great degree the wav in which
transport mechanisms "';ere viewed. The long tenn balance of sediment in the G~d Canyon
continued to be an important issue in these discussions.

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