Invertebrate Drift Downstream of Colorado River Basin Dams

Science Center Objects

Aquatic invertebrates are critical food for fish and other species that inhabit large rivers. In the Colorado River Basin, invertebrates that get transported down the river (“in the drift”) are particularly important to rainbow trout and other species of interest to recreational users. This research seeks to compare rivers downstream of large dams throughout the Colorado River Basin in order to understand how dam operations and the local environment may be affecting differences in drift concentrations, and thus higher levels of the food chain as well.

USGS researcher on a cobble beach adjacent to the Green River.

Sampling invertebrates downstream of Flaming Gorge Dam, on the Green River near Dutch John, Utah (May 8, 2015). (Credit: Adam Cobb, USGS. Public domain.)

Background & Importance

Food webs in streams and rivers rely heavily on aquatic invertebrates as food for fish, birds, bats, and numerous other groups. However, effectively sampling these invertebrates in large rivers is notoriously difficult. Our research group has developed methods for sampling invertebrates when they enter the drift; that is, when they are in the water column being swept downstream. Nearly all invertebrates enter the drift at some point in their life cycle, so measuring drift provides a good representation of the condition of the entire aquatic invertebrate community.

In this project, our group is studying drift downstream of large dams throughout the Colorado River Basin, including Fontenelle Dam (Green River, Wyoming), Flaming Gorge Dam (Green River, Utah), Crystal Dam (Gunnison River, Colorado), Navajo Dam (San Juan River, New Mexico), and Glen Canyon Dam (Colorado River, Arizona), and Hoover, Davis, and Parker Dams (Colorado River, Arizona/Nevada/California border). Our goal is to relate invertebrate drift conditions across these dams to dam operations such as flow fluctuations and environmental variables such as latitude or the presence of tributaries, in order to better understand the controls on invertebrate drift in large, regulated river settings.

General Methods

We use a motorboat equipped with a large boom to sample drift in large rivers. A winch is mounted to this boom, with a net and 75-pound weight attached. The net is raised and lowered throughout the water column of the river, at approximately mid-channel, for approximately five minutes. The resulting sample is taken back to the lab, where any collected invertebrates are counted and identified. We also sample benthic invertebrates as a part of this project for comparison with the drift data. These benthic invertebrates are collected using a Hess or Surber net sampler, or by manually scrubbing rocks from several locations throughout a river reach.

USGS researcher standing in a river holding a metal Hess sampler to sample aquatic invertebrates

Sampling invertebrates on “Casino Row” at Laughlin, Nevada, just downstream of Davis Dam on the Colorado River (September 7, 2015). (Credit: Adam Copp, USGS. Public domain.)

A larval stonefly on the hand of a USGS researcher

A stonefly (family Perlidae), in its larval stage, from the Green River just downstream of Fontenelle Dam, near Kemmerer, Wyoming (May, 10, 2015). (Credit: Adam Copp, USGS. Public domain.)