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Beaver dam observations surprise researchers

Billy Beck Dam cross sections revealing intricate matrices of diverse materials
SUPRISING SOLIDITY: Dam cross sections reveal intricate matrices of diverse materials, with minimal porosity.
Iowa Learning Farm: Beaver dams influence water quality, hydrology and the shape of streams on which they are built.

As a part of an ongoing three-year study funded by the Iowa Nutrient Research Center, researchers from Iowa State University have been digging into the details of beaver dams in Iowa. The study is designed to help quantify beaver dam impacts on water quality, hydrology and stream morphology (i.e., how dams shape the stream itself).

In looking back over the first year of fieldwork, observations and data collection, they’ve made some interesting and surprising discoveries about beavers and the dams they construct.

“Although the primary goals of the study focus on how dams influence sediment, nitrate and phosphorus levels in waterways, our time in the field has generated numerous ancillary observations about these unique creatures,” says William (Billy) Beck, ISU assistant professor and Extension forestry specialist.

Andrew RupiperA central Iowa dam during extreme drought, July 2021

DROUGHT EFFECTS: This is a central Iowa dam during extreme drought in July. Note the elevation difference between upstream and downstream segments; the flow is moving toward the camera.

Beck and ISU graduate research assistant Andrew Rupiper compiled the following top three surprising observations; they don’t necessarily address the core study goals but are nonetheless interesting and may merit further investigation and research.

Observation 1. Beaver dams are solid, not porous. “Of course, you would expect a dam to be relatively solid in order to stop moving water and create a pond, but it is commonly thought that dams are somewhat porous and act to filter the water that flows through them,” Beck says “By taking a closer look at and into the dams, we see that they interact with streams much like human-constructed dams.”

Beaver dams have minimal porosity and cross sections reveal an intricate and non-random matrix of sediment, grass, logs and other debris (even traffic cones). Sediment is dredged from the pool nightly and is anchored in place atop the dam with building materials — and sometimes rocks as large as 6 inches in diameter. These solid structures help to maintain impressive quantities of water in pools, even during drought conditions.

Andrew RupiperCentral Iowa dam and multi-acre pool, March 2022

LARGE STRUCTURE: This central Iowa dam and multi-acre pool shown here in March is more than 200 feet long and inundates more than 2.5 acres.

As non-pool reaches (a reach is a length of stream or valley used as a unit of study) dried out in summer 2021, the majority of study dams were able to maintain pools all summer. A dam that is 3 to 5 feet in height may have a pool that runs as deep as 7 to 9 feet, thanks to the persistent efforts of the beavers. This storage provides critical habitat and refuge for a diverse aquatic community during drought.

“The ponds also support other wildlife, including the wood ducks, herons and other birds that we observed in pools during the worst of the drought,” Beck said. “In addition, pools contribute to maintaining riparian [streamside] groundwater levels — benefiting streambank vegetation.”

Observation 2. Stream channel shape influences beaver activity and impact. Channelized reaches (stream sections), particularly those with steep, incised banks, appear to influence the behavior of beavers. The stark transitions from the floodplain to the stream and the retention of streamflow energy in the channel can cause frequent dam breaches that necessitate rebuilds. In addition, the deeper channels prevent floodplain inundation, which allows the beavers to safely travel farther from the channel to find food and construction materials.

“We observed feeding pressure among beavers inhabiting channelized streams, as well as the exhaustion of desirable woody vegetation nearby,” Beck says. “Beavers are cumbersome and vulnerable on land, and tend to stay near the water.”

Andrew RupiperA large fallen tree and creek

LEAVE BIGGEST TREES FOR LAST: Large trees are generally felled last and allow access to more palatable young branches.

In a scenario with a wider, readily accessible floodplain, with less incision, beavers may expand their territory. To do this, they construct lateral canals and satellite dams to extend safe access to preferred woody species, thereby diminishing impact on the riparian vegetation near the primary dam.

Observation 3. Riparian tree felling patterns are somewhat predictable, somewhat random. “Beavers eat the living exterior layers of trees and will often fell trees to access the tender upper branches for food as well as construction material,” Beck says. “Their lumberjack activities can impact intentional riparian conservation efforts aimed at nutrient loss reduction, so understanding these behaviors can be important to conservation and forestry planners.”

While beaver preferences for certain tree species are well-documented, Beck and Rupiper observed that there appears to be a balancing act between distance from the stream and trees that are more or less desirable. Beck says, “As vulnerability to predation may increase with distance from water, we’ve observed lower-palatability species close to banks [even cedar] sampled before they hit more desirable species at greater distances.”

At this point, these observations are simply that — observations. Although fascinating sidebars to the overall project goals, each one provides important clues to help understand water quality and hydrology data and work to develop in-stream and riparian management guidance.

Ripley is Iowa Learning Farms manager and a Water Rocks! conservation outreach specialist.

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