Colorado River’s Low-Flow Sheds Light on Eventual New Normal for Grand Canyon

The Colorado River follows a 1,450-mile route generally southwest from north central Colorado to just east of Las Vegas. From there it turns south to form Arizona’s western border with Nevada and California, and then the border between Mexican states Sonora and Baja California before emptying into the Gulf of California.

Between the U.S. and Mexico, 40 million people depend on water from the Colorado. The snowmelt-fed river has seen its flows drop by 20% over the last 100 years as runoff efficiency – the percentage of precipitation that ends up in the river – has declined as summers have become hotter and drier, cooking the soil.

This year, for example, snowpack is 80% of average but sending just 30% of the average amount of water into the Colorado. Lake Mead, the reservoir behind Hoover Dam, is at an all-time low, and between them Lake Mead and Lake Powell, behind Glen Canyon Dam, are projected to be just 29% full within two years.

Completed in 1966, Glen Canyon Dam is 710 feet high and 1,560 feet long and named for the series of deep sandstone gorges flooded by Lake Powell. The lake draws its name from John Wesley Powell, the leader of the first boat expedition to traverse the Grand Canyon.

“Typically the Colorado River is coming out of Lake Powell fast and cold, which is a hostile environment for desert adapted organisms,” Lytle said.

For the recent experiment, low flow was maintained from March 15 through March 20, and immediately after that there was a big release of water, known as a high-pulse flow event, intended to scour out areas and possibly create new habitat for native fish and their food sources.

“During the first part of the low flow, we were in the far upper reaches of the canyon, and as soon as we finished sampling, we packed up the truck and raced across the desert 200 river miles away to Diamond Creek, where you can access the Grand Canyon from a road, just in time to capture the low-flow event moving its way down a long, sinuous canyon,” Lytle said. “And a USGS team was taking samples by boat throughout the entire canyon, complementary to what our group was doing. It was a real team effort, with people measuring riparian vegetation, taking drift samples of invertebrates in water, checking respiration of aquatic plants, and also noting the effect on fish and fisheries.”

As the climate continues to warm and the amount of water available for humans continues to drop, low flows such as the one during this year’s experiment may become the new normal, he added.

“That presents challenges but also opportunities for research,” Lytle said. “Prior to there being any dams on the river, low-flow events were part of the normal annual cycle of flows. In the spring, the river could flood quite spectacularly in some years, and by late summer or early fall into winter, flows could get to 4,000 CFS or even lower than that.”

Lytle says that kind of variation amounts to “exercise” for the river, which needs it for health just like a person needs both activity and rest.

“One question we’re asking is whether there could be ecological benefits, at least at certain important times of year, to low flows,” he said. “Low flows allow the water temperature to increase and let more light to reach the benthic zone, where the productivity of algae and invertebrates occurs. It also might favor greater production of those important native black flies, mayflies and midges.”

Documenting the work in March was photographer Jeremy Monroe of Freshwaters Illustrated, a nonprofit educational media organization headquartered in Corvallis.