Unnaturally high beaver densities found today negatively impact coldwater fisheries and thwart efforts to restore riparian forests and water quality.
Beavers are a part of the natural ecology of Northern Minnesota. However, they were scarce on the streams of the North Shore of Lake Superior prior to settlement. Prior to the Logging Era the northern forest was largely coniferous, including in the riparian areas and offered little suitable habitat for beaver. Logging and other human activities which occurred over the past 150 years have substantially altered forest stand composition and created habitats which are far more suitable for beaver. The artificially high beaver populations which now exist, and the periodic inundation of riparian forests caused by their dam building, continue to thwart efforts to restore riparian forests in the Lake Superior basin (and elsewhere in Northern Minnesota) to conditions essential to improving and maintaining water quality and coldwater fisheries.
Numerous aspects of stream function are disrupted or negatively impacted by beaver impoundments, including increasing water temperature, decreasing base flow, disrupting sediment transport, reducing stream channel stability and altering invertebrate communities, which in turn influences food availability for fish. Beaver dams severely impede or block fish movement, denying trout access to colder or deeper areas critical to survival at certain times of the year, and blocking access to spawning areas. While there are some stream conditions under which beaver impoundments may provide some fishery benefits, these conditions do not exist in northeastern Minnesota. The majority of flow in trout streams in northeastern Minnesota originates not from cold groundwater, but from surface water from wetlands, shallow seepage, and storm water runoff which do not provide buffering capacity for stream temperature increases. Trout streams in northeastern Minnesota are particularly susceptible to temperature impacts caused by beaver impoundments and loss of riparian forest canopy. MNDNR research has established that beaver impoundments often raise water temperatures 10 to 15 degrees in the summer months, and into the lethal range for trout and other coldwater species. There is direct correlation between the number of dams present on a stream and the magnitude of water temperature increases. Beaver dams located on headwater reaches have negative thermal impacts which extend far downstream.
The Knife River system in St. Louis and Lake Counties illustrates the negative impacts which human caused changes in forest composition, coupled with unnaturally high beaver densities, continue to have on trout streams in northeastern Minnesota. Since the 1990s Minnesota Trout Unlimited, MNDNR fisheries personnel, soil conservation agencies and other conservation groups have been working collaboratively with forest managers and landowners to gradually restore the riparian area forests in the Knife River watershed to conifers and other long lived trees species. In 1998 the Knife River was formally identified as impaired water and recently a plan was developed to address turbidly problems. A watershed assessment identified the unnaturally high beaver densities in the watershed as both a cause of erosion and sedimentation problems, as well as an impediment to restoration of riparian forest stands. In short, the artificially high number of beavers and beaver impoundments must be reduced throughout the watershed in order to restore the forest and reduce erosion, turbidity and sedimentation for the benefit of all aquatic life.
The Knife River watershed has supported a coldwater fishery for many hundreds, or thousands, of years. The artificially high beaver numbers created by relatively recent logging activity is now threatening the survival of coldwater fisheries, as well as the health of the watershed and Lake Superior. Minnesota Trout Unlimited, the Lake Superior Steelhead Association, the MNDNR and others continue to make very substantial investments of time and money to sustain and improve the fishery, and restore the watershed to long term health. Experience has shown that the artificially high beaver numbers caused by human activity in the watershed can only be effectively reduced by active management in the near and intermediate term.
In recent years one or more special interest groups have pressured the MNDNR to restrict beaver management measures to but a portion of the Knife River system. Unfortunately, this political decision allows already artificially high beaver populations to quickly grow and re-colonize extensive areas of the Knife River watershed. This may prevent restoration of the forests in the Knife River watershed to conifers and other long lived trees species and undermine measures to reduce erosion, turbidity and sedimentation.
Minnesota Trout Unlimited believes that restoration of North Shore watersheds (as well as the coldwater fisheries they support) should be the top priority of all divisions of the MNDNR, as well as the Counties, and intensive beaver management measures employed throughout the watersheds until such time as the riparian forests are restored. Once riparian forests are restored beaver populations will return to more natural levels and be sustainable with little future management.