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Beaver Dam Research Shows Many Impacts
by LuAnn Rolling, District Conservationist
An Iowa Nutrient Research Center-funded study being conducted at Iowa State University seeks to quantify beaver dam impacts on water quality, hydrology, and stream morphology (how dams shape the stream itself). Here are the researcher’s top three observations after one year of study.

Observation 1: Dams are solid, not porous filters
Beaver dams have minimal porosity and cross sections reveals an intricate, and not random, matrix of sediment, grass, logs, and other debris. Rocks as large as 6” across are shuttled to the dam top anchoring the sediment that is packed atop the dam each night. Dams maintain impressive quantities of water, even during drought conditions. As non-pool reaches dried out in summer 2021, the majority of study dams were able to maintain pools all summer. A dam that is 3-5’ tall may have a pool as deep as 7-9’. This storage represents critical habitat for aquatic life during drought. It also provides habitat for  wood ducks, herons, and other birds that were observed in pools during low-flow periods. In addition, pools allow for maintenance of riparian (streamside) groundwater levels – benefiting critical streambank vegetation.

Observation 2: Stream channel shape influences beaver activity and impact
Beavers in channelized reaches face many challenges. Beavers are cumbersome and vulnerable on land, thus less frequent floodplain inundation means less distance they are comfortable traveling for food. Because of this, beaver dams in channelized stream reaches exhibit feeding pressure confined to a concentrated area, quickly exhausting the woody vegetation.

Observation 3: Riparian tree felling patterns are somewhat predictable, somewhat random
Tree felling activity can impact intentional riparian conservation efforts. The researchers have observed that distance from the channel often outweighs species preference. As vulnerability to predation may increase with distance from water, they’ve observed lower-palatable 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 piece provides important clues as they begin to analyze water quality and hydrology data, and work to develop in-stream and riparian management guidance. The study’s researchers are: Andrew Rupiper (Graduate Research Assistant, Department of Natural Resource Ecology and Management, Iowa State University) and Billy Beck (Assistant Professor and Extension Forestry Specialist, Department of Natural Resource Ecology and Management, Iowa State University)