Up until humans started to modify the landscape with machines using fossil fuels it could have been argued that the beavers had the greatest impact on the landscapes in North America. Two American geologists Ruedeman and Schoonmake in the 1930s were convinced that it was beavers working over many 100s of years that had changed a V-shaped valley into overlapping layers of sediments by damming streams to make small ponds which filled with sediment. This process is described in a book by Frances Backhouse, Once They Were Hats, In search of the Mighty Beaver.
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Some dams are over a kilometre long and many metres tall holding back small lakes. Not all dams are large as is the case in the Skagit River Delta 90 km north of Seattle. Many small narrow streams in steep channels have been dammed to form small pools that catch the rising ocean tides and retain the fresh and seawater mix when the tide recedes. These ponds retain a greater diversity of life than the unaltered streams.
Researcher Greg Hood has documented that the ponds also held 12 times as many juvenile Chinook salmon which are much safer from predators because of the dense overhanging vegetation. Most dams throughout the continent are made from wood (mostly small stems and branches) which are anchored in the mud and then built up with horizontal branches and packed with mud to seal the holes. Beavers are opportunists and will use any material in the area that they can move. For example in one Yukon stream that was logged out by the miners the resourceful beavers used rocks the size of soccer balls (weighing 4 to 5 kg) for a three-metre long section and a few metres high.
Author Backhouse gives a number of examples where abused areas are being reclaimed by using beavers dam building and channelling abilities. In southwestern Wyoming 100 years of overgrazing damaged many riparian zones resulting in serious erosion.
A few decades after beavers were reintroduced the vegetation has returned to the pond boundaries and expanding beaver populations continue to heal the land. Closer to home in the Aspen Parkland east of Edmonton, U of A scientists describe how the introduction of beavers in 1941 had an overwhelming influence on the wetland creation and maintenance. Not all beaver activities are positive especially close to human habitation which leads to constant removal of beaver dams. Sometimes a compromise can be reached when land owners were able to control the water level by the introduction of a large flexible plastic pipe under the dam with a triangular metal cage over the intake. As long as water was not breaching the dam beavers seemed to accept the lower levels and made no attempt to block the intake.
The most potential for success is in the more remote upper reaches of damaged watersheds which still exist in many former beaver habitats.
The author describes some work being done in peatlands where beavers dig channels in the flat wetlands to be able to move around safely during low water in dry years.
With the increasing threat of warming in many northern ecosystems the mighty beaver may be one of the best tools we have for controlling the water levels in these critical forests and wetlands.
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