SEDIMENT RESPONSE TO DAM REMOVAL IN A RIVER WITH LOW STREAM POWER

This talk presents observations of sediment transport following the removal of the Millie Turner Dam on the Nissitissit River in Pepperell, MA. During this removal, the impounded sediments were allowed to mobilize without human intervention. Due to the combination of drought at the time of removal and the low gradient of the river at this site, stream power was exceptionally low. Therefore, this removal provided an unusual opportunity to study an end member of the relationship between stream power and sediment response post dam removal. In this case, sediments from littoral areas of the previous impoundment were mobilized laterally during the rapid lowering of water level during the initial dam breach; however, the stream that soon developed in the previous impoundment lacked the power to continue to move many of these sediments. Instead, the mobilized material formed highly sorted sand deposits in the new channel within the previous impoundment. These sands slowly saltated downstream, with bulk movement in a slow wave. Months passed before these sand deposits within the previous impoundment moved far enough downstream to reach the site of the former dam. The sediment response to this dam removal can be contrasted with removals at sites wtih larger stream power to suggest a model of sediment response based on stream power at dam removal sites.