THE LEGACY OF NATURAL DAMS LOST

The impact of human-constructed dams on channel morphology and sediment transport has been relatively well characterized. It includes the understanding that sediments impounded behind these dams leave a legacy on the river valley and corresponding channel. Although river restoration efforts typically focus on the comparison of these impacts with a free-flowing baseline condition, the baseline condition prior to dam construction was not necessarily free-flowing. Instead, prior to European colonization of the northeast, log jams and beaver dams generated frequent obstructions to river flow. This talk will draw on examples from Connecticut and New Hampshire to describe the impacts of beaver dams and log jams on channel shape, sediment distribution, and hydrologic function. The impacts described in this talk include generation of multi-thread channels, creation of new channels, and retention of sediments. We will also use estimates of hydrologic storage in beaver ponds and human-created impoundments to describe the landscape-scale impacts of natural dams. We will then compare the impacts of these natural dams with the free-flowing condition to infer the missing legacy of these dams from the river-scape following their removal by European settlers and loggers. We will compare this missing legacy with the known legacies of human dams to infer baseline conditions for river restoration.