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USGS Says High Colorado River Flows Benefit the Grand Canyon
Before the first of nearly 5 million cubic yards of concrete was poured to create the Glen Canyon Dam, and behind it Lake Powell, the Colorado River ebbed and flowed with the seasons through the Grand Canyon. Heavy snowpacks high up in the Rocky Mountains of Colorado and Wyoming, and in the Uintas of Utah, in Spring would turn to heavy, sediment-laden surges that churned and ripped at the river corridor, creating sandbars here and there and with them habitat for native fishes and vegetation.
Those pulses were tempered beginning in 1960 with that first load of concrete, and by 1963, when the 710-foot-high dam was finished, the river largely was tamed and the seasonal flushes that had been so important to the Grand Canyon were smothered.
In recent years there has been a tug-of-war of sorts between the National Park Service, which wants large, repeated releases from the dam to mimic the seasonal flushes, and the Bureau of Reclamation, which would rather the releases be timed to generate the most electricity possible to meet demand in the Southwest.
The last staged high flow, a 60-hour experiment conducted in March 2008, saw torrents of Colorado River water released through the dam’s powerplant and bypass tubes to a peak of about 41,500 cubic feet per second, about twice the normal peak. Two previous experiments were conducted in 1996 and 2004. At the time of that March 2008 experiment, BuRec officials proposed that it would be conducted just once, while September-October "steady" flows would be repeated for five years.
At the time, strongly worded opposition from both park officials and the Grand Canyon Trust questioned how carefully BuRec officials had developed their flooding strategy and whether it really was intended to benefit the canyon's natural resources. Indeed, Trust officials contended the staged flood was aimed to benefit downstream water users and hydropower interests, not natural resources. Park Superintendent Steve Martin said he didn't believe BuRec should be locking itself into one scenario for the next five years and that its plan should be more science-based. He said he'd prefer a situation where flooding is allowed in the spring on a more regular basis, perhaps every one or two years, depending on sediment conditions.
This past November, Interior Secretary Ken Salazar voiced his preference to see more high flows from Glen Canyon, and added that he thought they could be arranged to benefit both the natural resources of the canyon and energy demands.
“We must find a way to protect one of the world’s most treasured landscapes – the Grand Canyon – while meeting water and clean energy needs in the face of climate change,” Secretary Salazar said at the time. “Today, I am directing the development of a protocol for conducting additional High Flow Experiments at the (Glen Canyon) Dam. These experimental high flows [like the one in 2008] send sediment downstream to rebuild sandbars, beaches and backwaters. The rebuilt areas provide key wildlife habitat, enhance the aquatic food base, protect archeological sites, and create additional camping opportunities in the canyon.”
On Tuesday the U.S. Geological Survey agreed with the Interior secretary on the value of the flushes to the canyon. In a report built on research conducted around last March's flush, the agency said high flows through the Grand Canyon that mimic natural ebbs and flows are beneficial for the river corridor. However, the report also notes that many of the benefits are erased within six months due to energy demands that dictate releases through the Glen Canyon Dam.
Last March's release was designed to test the premise that such high flows would help rebuild eroded Grand Canyon sandbars, create habitat for the endangered humpback chub, and benefit other resources such as archaeological sites, rainbow trout, aquatic food for fish, and riverside vegetation.
"Before the dam’s completion in 1963, spring snowmelt produced floods that carried large quantities of sand that created and maintained Grand Canyon sandbars," the USGS said in a release. "Today, because Glen Canyon Dam, which provides hydropower to customers in six states, traps approximately 90 percent of the sand once available to maintain Grand Canyon sandbars, high flows are the only way to rebuild these important resources.
Here's a look at the highlights of the report's findings:
The 2008 experiment resulted in widespread increases in the area and volume of sandbars, expansions of camping areas, and increases in the number and size of backwater habitats (areas of low-velocity flow thought to be used as rearing habitat by native fish).
Six months after the experiment, the new sandbars had been largely eroded by typical fluctuating flow dam operations driven by electrical energy demand; however, median sandbar elevation was still slightly higher and backwater habitats still slightly more abundant than before the experiment. Although stable and relatively lower monthly volume releases are the most effective at limiting sandbar erosion, the volume of water that must be released from Glen Canyon Dam annually is determined by basin hydrology and legal requirements to deliver water from the upper to lower Colorado River Basin.
Timing the 2008 experiment in March likely reduced successful nonnative seedling germination and created new sandbars during the spring windy season, which allowed for the greatest transport of windblown sand to archeological sites where it protects sites from weathering and erosion.
In the Lees Ferry rainbow trout fishery, high flows reduced the New Zealand mud snail population by about 80 percent. This nonnative species is considered a nuisance species because the snails cannot be digested when eaten by trout. In contrast, midges and black flies, high-quality food items for fish, increased.
Young rainbow trout in the Lees Ferry river reach had better survival and growth rates following the experiment, which scientist think may have resulted from improved habitat conditions and better food quality. Additionally, data show that rainbow trout did not move downstream in significant numbers as the result of the high flows.
“Insights gained about the effects of the 2008 experiment will be invaluable in helping decision-makers determine the best frequency, timing, duration, and magnitude for future high flows to benefit resources in Glen Canyon National Recreational Area and Grand Canyon National Park,” noted John Hamill, Chief of the USGS Grand Canyon Monitoring and Research Center.
The USGS's research is expected to play a role in that new protocol that Interior Secretary Salazar called for back in November.
Research completed by the U.S. Geological Survey and cooperating scientists about the effects of the 2008 high-flow experiment will be discussed at the Glen Canyon Dam Adaptive Management Program meeting February 3–4, 2010, in Phoenix, Ariz. The findings will also be taken into consideration in development of a new protocol for conducting additional high-flow experiments, announced by Secretary Salazar in December 2009.
You can find the report stemming from this research at this site.