Tiny Insects Tell Us a Lot About River Health

By Erin Madison

Although they’re small, macroinvertebrates, or aquatic insects, are important indicators of how a river is doing.

“They can tell us about the overall health of a river system,” said Jordan Tollefson, Water Quality Specialist with NorthWestern Energy.

Every summer, NorthWestern Energy’s Environmental Group covers some 300 miles collecting macroinvertebrate samples throughout the Missouri-Madison river system.

Anglers enjoy an autumn afternoon on the Madison River.

Macroinvertebrate sampling has been occurring since 2000, with some sites going back as far as 1996. It’s one of the longest continuously running data sets in the state.

The macroinvertebrate data allows us to see trends over time. For example, NorthWestern Energy scientists have found through this data collection and monitoring process that macroinvertebrate populations respond to changing environmental conditions. Macroinvertebrate communities in the Madison and Missouri rivers in 2018 were extremely healthy following the high spring river flows. However, drought conditions last year led to low river flows and a more tolerant community of macroinvertebrates.

Gathering macroinvertebrates is done by scrubbing rocks and raking the river bed, with a net used to capture material that flows downstream.

This large data set allows us to compare different factors to get a big picture of what’s going on in the river system.

“Everything ties together,” Jordan said.

The group monitors 11 sites starting inside Yellowstone National Park and going all the way down river to below Morony Dam. The macroinvertebrate sampling is done at the same time every year, in late summer when water is lower and easier to work in.

Each summer, NorthWestern’s Hydro Compliance Team samples macroinvertebrates to monitor the conditions of Montana’s rivers.

Macroinvertebrates are large enough to see with the naked eye, but that doesn’t mean they’re easy to spot in the river. To collect samples, Jordan and his team select a 2-foot square area and scrub the rocks to knock off the insects. They also rake the river bed to kick up bugs hanging out on the river bottom. Another sampler stands downstream holding a net to collect everything that flows downstream during the raking and scrubbing.

It takes three to four days to monitor all the sites. Various partner organizations, including Montana Fish, Wildlife & Parks and the Montana Department of Environmental Quality, pitch in to help with the field work.

A team of NorthWestern employees gathers macroinvertebrates in the Missouri River.

“It’s a good chance to get some face time with our partner organizations,” Jordan said.

The Hydro Compliance Team also measures water temperature, pH and looks at chlorophyll levels, which offer insight into the overall nutrient levels in a river. They look at sediment and fish data as well.

NorthWestern Energy Biologist Jon Hanson gathers samples during our annual macroinvertebrate testing.

The data collected is used to create bioassessment scores. Bioassessment scores are a way to provide a measure of environmental stress on macroinvertebrates, and those scores are developed based on a reference to expected normal or natural conditions. In 2021, 10 of the 11 sites monitored had lower bioassessment scores when compared with 2018, indicating a decline in the health of macroinvertebrates, likely due to ongoing drought conditions.

This monitoring and data collection is one of the many requirements outlined in the licenses that allow us to operate dams on public waterways.