SMALL MILL DAMS AS DRIVERS FOR AVULSIONS IN SEDIMENT-LIMITED LANDSCAPES: ANALYSIS OF THE MAY 2006 SUNCOOK RIVER AVULSION IN EPSOM, NEW HAMPSHIRE AND NEARBY RIVERS

Avulsions, or rapid changes in the location of a river, usually occur in environments where net-deposition can raise the bed above the surrounding floodplain. They are less common in rivers with a more limited influx of sediment, like many in New England. One of these rare avulsions occurred during a 100-year flood on the Suncook River in Epsom, NH, between May 14 and 15, 2006.

We studied the Suncook River event to develop a model for the drivers of avulsions in sediment-limited landscapes. We suggest that a strong substrate in the parent channel, such as bedrock or immobile boulders, can facilitate an avulsion by preventing incision and driving water overbank during heavy flooding. Easily erodible substrates in the path of the new channel are necessary to allow a knickpoint to migrate quickly upstream and create a channel with a more favorable slope during a single flood.

We propose that small mill dams, common throughout New England, increase the risk of avulsions by facilitating the distribution of their impounded water on the floodplain during floods. Based on Slingerland and Smith's (2004) model, these small dams could also directly drive avulsions by reducing the water surface slope of the parent channel and thus making a new path gravitationally favorable. Huckins Mill Dam, located downstream of the site of the Suncook River avulsion, could have driven the 2006 event. We used digital elevation models of several tributaries to the Merrimack River in New Hampshire, including the pre-avulsion Suncook River, to delineate zones at risk for avulsion based on Slingerland and Smith's (2004) water surface slope ratios. These zones were found to concentrate behind these small mill dams, where the water surface slope of the main channel was lowest.