On the Colorado, a grand experiment meets Mother Nature

  • Water is released from Glen Canyon DAm in a 90-hour flood last November in an experiment that might have killed native fish

    US BUREAU OF RECLAMATION
  "It’s really hard to kill fish with water," says Joe Shannon, a professor of aquatic ecology with Northern Arizona University. But a recent experiment intended to help native fish in the Colorado River might have done just that.

In November, officials from the Grand Canyon Monitoring and Research Center released a 90-hour flood from Glen Canyon Dam to stir up sediment from downstream tributaries. The researchers wanted to redistribute thousands of tons of sand to enhance recreational beaches and create backwater habitats in which native fish thrive. But a recent report on post-flood fish counts showed a 63 percent decline in the juvenile population of the endangered humpback chub, compared to pre-flood counts. The chub is one of four species of native fish still found in the Grand Canyon.

During the experiment, the Little Colorado River — a tributary that’s the primary habitat of the humpback chub — surprised researchers by flooding naturally. "Mother Nature can throw you curveballs," says Lew Coggins, a fisheries biologist with the Research Center. The unanticipated weather may have washed fish downstream — dead or alive — and also created murky conditions that might have interfered with the post-flood count.

The rare chub might already be beyond salvation: Only 1,000 to 3,000 of the fish still swim in the Grand Canyon. Shannon notes that there is no scientific evidence to show that sand displacement would even help the species. Instead, he and other conservationists say that invasive fish that prey on chub, such as brown trout, should be eradicated.

He also suggests managing the river to mimic pre-dam flows. "It’s completely unnatural," says Shannon. "The whole thing is completely out of whack."