Grand Canyon floods and native fish
by Cally Carswell
The last time the Colorado River plunged unhindered through the Grand Canyon, swollen by snowmelt to 126,000 cubic-feet per second, was in 1957. Glen Canyon Dam rose soon after, delivering cheap hydropower and reliable water to cities, farms and industry.
For native fish, the transformation was debilitating. Most of the river's sediment -- which built sandbars that shelter backwater habitat favored by young fish -- settled in Lake Powell, the reservoir behind the dam. And the water downstream became much colder, since dam discharge comes from deep in the reservoir. This limited the ability of native fish to spawn in the mainstem Colorado, and stifled young fishes' growth there. "Growth is a proxy for survival," says Ted Kennedy, a USGS biologist with the Grand Canyon Monitoring and Research Center. "They have to get to a size where they're large enough to not be eaten by other fish." Four species have disappeared from the Grand Canyon, and another, the humpback chub, is endangered.
The scarcity of sandbars, which make primo camping spots, also disappointed river runners and hikers. So in the late 1990s, a plan was hatched to allow controlled floods to sweep the canyon. Scientists theorized that high water releases from the dam would mobilize tributary-deposited sediment and build up shrunken sandbars. "Flooding is a natural part of river dynamics," says Kennedy. "There was a lot of hope that it would be beneficial to the system as a whole," much as it was hoped that spills over Fort Peck would boost the Missouri River ecosystem.
The Grand Canyon releases -- made from low in the reservoir -- weren't thwarted as the Fort Peck spills were. Since 1996, three experimental floods have been unleashed, and scientists have gained important insight into their effect. Sixty-hour floods of around 40,000 cubic-feet per second in 2004 and 2008, timed to follow natural flooding in tributaries, successfully enlarged sandbars, though they eventually diminished. The floods also created more backwater habitat. But fish didn't benefit as hoped. The water didn't warm to optimal temperatures, says Kennedy, and the habitat quickly vanished once normal operations resumed. Humpback chub still largely ignored the main stem for spawning, crowding instead into the Little Colorado River, a warm, silty tributary.
The takeaway, says Kennedy: Floods alone aren't likely to boost native fish, since they can't remedy other alterations to the natural system -- especially water temperature.
It's even possible that the 2008 flood had a slight negative impact on chub, by boosting mainstem invertebrates that rainbow trout love. The non-native sport fish thrives in cold water, competes with chub for forage, and preys on them. Newly hatched trout feasted on the invertebrate bounty, according to a new study of the post-flood food web, and their numbers skyrocketed: Near the Little Colorado's mouth, trout catch rates grew by 800 percent. It's not yet clear how that's impacted chub.
Nevertheless, Interior Secretary Ken Salazar announced this spring that the controlled floods will begin taking place whenever conditions allow. If they occur annually or more often, their impacts could change. Regular experimental floods on a Swiss river took three years to shift the composition of organisms at the bottom of the food web, which influences which other species thrive. Native trout redds, for example, increased sixfold.
"That probably wouldn't have happened if they hadn't (flooded that river) consistently," says Wyatt Cross, a Montana State University ecologist who researched the 2008 rainbow trout bump. "We don't know how multiple floods will affect the Grand Canyon. But we want to."© High Country News