BEAVER-MODIFIED FLOODPLAINS CONTRIBUTING TO ARABLE FARMLANDS ALONG THE COLD RIVER IN SOUTH ACWORTH, NEW HAMPSHIRE

Along the Cold River floodplain in South Acworth New Hampshire is a 13-acre field composed of arable fine-grain sediment that does not reflect the typical bouldery, bedload channel or gravelly flood deposits currently associated with the river. Although LIDAR data reveals several relict channels across the field, there are no stone walls or other signs that boulders were removed from the field, which has been farmed since the late 1700’s. This study investigates the probability that broad arable fields of fine-grain sediment along the Cold River may be the result of long-term ecosystem modification by beaver.

To reveal the underlying floodplain stratigraphy Ground Penetrating Radar (GPR) was run along a 10x10m grid covering all 13 acres with a 200 mHz antenna. Several trenches, which aided in the interpretation of the GPR data, were dug within and between relict channels. These data were compare to the recently acquire NH GRANIT LIDAR, and the observed stratigraphy in actively forming beaver meadows along the river.

The stratigraphy exposed in trenches reveal over a meter of sand, silt and organic material in varying amounts with rare thin gravelly lenses, comparable to deposits currently accumulating behind modern beaver dams. The thickest accumulations of organic material is found in the downstream reaches of the abandoned channels where beaver have been historically active. Along most survey lines, the GPR data did not identify bouldery beds similar to those observed in the present river channel, but revealed a largely homogeneous substrate at depths below the plow zone. However, along one line GPR data did show a possible buried beaver den, and at least two channels with a gravel and sand layer at depths measured in the trenches.

Modern beaver activity is locally prevalent along the river, and is a major cause of channel avulsion. Beavers continue to dam partially abandoned channels adding to the long-term accumulation of fine-grain sediment. The floodplain stratigraphy and distribution of organic-rich sediment strongly suggest that the abundant fine-grain arable sediment composing the floodplain is attributed to channel shifting and subsequent infilling caused by beaver activity. If so, then this study documents the beneficial effects of beaver repopulation along New England streams.