INCLUDING BEAVERS IN THE RIVER RESTORATION BASELINE

Goals for river restoration, particularly dam removals, are often based on the model of a free-flowing, single-thread, rocky substrate channel. When considering precolonial conditions of headwaters in the northeastern U.S., additional complexity associated with beavers (Castor canadensis) should be included. By historical accounts, beavers were pervasive at the time of European colonization. Examination of modern watercourses re-colonized by beavers reveals features that counter the free-flowing baseline model.

Observations of forested headwater streams in northeastern Connecticut show a beaver dam density of up to 3 dams / 100m, with check-dams located downstream of larger dams. This creates a patchy diversity of habitats including large impounded ponds, “check-dam” impoundments, and beaver meadows interspersed with free-flowing reaches. In turn, this provides potential to alter the natural flow regime due to increased storage and saturated areas. It also dramatically affects biogeochemical cycles through alternating high- and low-oxygen reaches.

Sediment distribution is patchy in beaver-created habitats. The major impoundments are sites of deposition, with organic and fine-grained mineral sediments. Downstream of beaver dams, the river channel has multiple threads. These channels are actively eroding downstream of new dams and in beaver meadows. Of the ten beaver dam complexes examined in this study, 80% are drained by multiple channels. A new dam, less one year old, effectively diverts a portion of the watercourse into the riparian forest downstream of the dam. The diverted water shallowly floods 722 m² of the forest, eroding and altering that soil. By comparison, only 214 m² is newly flooded upstream of the new dam. The flooded forest downstream of the dam is drained by 5 separate side-channels newly scoured in the forest soil, 117m in combined length adjacent to 60m of the primary channel.

Given the recognition of beavers as ecosystem engineers, their landscape alteration should be included in the river restoration baseline. In headwater systems this includes patchy distribution of habitats and sediments, alternating high- and low-oxygen conditions, alternating sediment mobilization and deposition, and multi- and single-thread channels.