Grand Canyon flood exercise ends as Colorado River flows return to normal at Lake Powell

Glen Canyon Dam operators began slowing high flows from Lake Powell into the Colorado River Thursday, ending a 72-hour exercise aimed at improving environmental conditions through the Grand Canyon.

Bureau of Reclamation crews reduced flows through the bypass tubes on the side of the dam Thursday morning, then slowed releases through the power turbines. Regular operations were scheduled to resume late in the day.

Water levels on the river as it flows through the Canyon will drop over the next few days, according to the National Park Service, which is advising river rafters, hikers and campers to expect high-flow conditions until early Sunday in areas farther downstream, near Pearce Ferry.

The bureau ramped up water flows from the dam early Monday for the three-day high-flow experiment. At peak times, the turbines and bypass tubes were gushing water at a rate of almost 40,000 cubic feet per second, about four times the average for this time of year.

The goal of the experiment was to use the higher flows in the river to move sediment downstream, rebuilding beaches and backwater fish habitat that had eroded in recent years. Before Glen Canyon Dam was completed in 1963, the river did that work itself, shifting sand and sediment with the higher flows of spring runoff.

This was the first high-flow experiment since 2018 and the first during the spring runoff season. Although Lake Powell is still less than one-quarter full after years of sustained drought, bountiful snowpack in the Rocky Mountains is expected to raise water levels significantly over the next few months.

Forecasters say runoff from snowmelt could total 177% of the average from April through July. New projections estimate Oct. 1 water levels at Lake Powell will be almost 50 feet higher than Oct. 1 of last year, while Lake Mead levels are projected to start October about 22 feet higher than last fall. That's still about 160 feet below what's considered capacity.

The water released from Lake Powell this week won't be lost for users on the river. It will remain stored downstream in Lake Mead, which is less than 30% full. The two reservoirs are the largest on the river and help manage water and hydropower for more than 40 million people in seven states.

High-flow experiments were authorized through the 1992 Grand Canyon Protection Act and are based in part on the availability of sand and sediment along the river's length. One of the primary sources of that sediment is the Paria River near Lees Ferry, where an active 2022 monsoon washed tons of sand toward the Colorado.

River rafters and environmental groups were especially keen on another high-flow experiment this year.

"The last two summers we have witnessed the unraveling of sandbars in Grand Canyon as an effect of violent monsoon patterns," said Ben Reeder, a professional river guide and a representative on a technical work group for the Glen Canyon Dam Adaptive Management Program. "The result has been the worst beach conditions downriver in 20 years."

He said those conditions detract from river trips and signify a declining ecosystem.

The success of the experiment won't be immediately evident. Scientists and river managers will survey the beaches and habitat areas in the coming weeks to measure how much sediment shifted downstream and where the greatest changes occurred.

Biologists will also watch carefully for any effects on fish and other wildlife. The high flows could help reduce populations of the invasive brown trout around Lees Ferry, but questions remain about smallmouth bass, a more recent invader that has taken advantage of low water levels at Lake Powell to escape downriver.

Bypassing the power turbines meant the loss of hydroelectricity at the dam. Declining water levels in Lake Powell have reduced the dam's capacity for generating power in recent years, so power providers raised concerns about the high-flow exercise.

“I understand the (environmental) concerns,” said Leslie James, executive director of the Colorado River Energy Distributors Association. But power consumers are affected when the water is released without generating electricity, which raises the cost of power that has to be secured from more expensive sources. “The water and hydropower resources have also been significantly impacted for more than five years. Any bypass is a direct impact to the hydropower resource.”

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This article originally appeared on Arizona Republic: Grand Canyon flood exercise ends as normal Colorado River flows resume