CONTROLS ON LEGACY SEDIMENT MOBILITY IN A LOW-GRADIENT POST-GLACIAL RIVER

Dams significantly impact the morphology of rivers by altering flow regime, grain size and sedimentation, and channel morphology. Recent studies hypothesize that impounded, or legacy, sediment is widespread in many streams with historical dam sites, creating fill terraces that may or may not act as floodplains after the dams are gone (Walter and Merritts, 2008). We analyze channel morphology and sedimentation patterns upstream of two breached dams to determine the mechanisms controlling the mobility of impounded sediment. In the past several decades, observable channel morphologic changes occurred at the two study sites in the Sheepscot River watershed in mid-coastal Maine: Maxcy's Mills Dam (built in 1809, it was 4-m high and breached in the late 1950s), and at Head Tide Dam (built in the 1760s, it is 6-m high and was partially breached in 1952). The Sheepscot River is one of the state's eight rivers with native anadromous Atlantic salmon populations. Because Atlantic salmon are a federally listed endangered species, understanding the existence and transport of legacy sediment has become an important component of habitat restoration efforts. The goal of this investigation is to determine the extent of legacy sediment in order to better understand how historical dam sites affect morphology and sediment transport in a post-glacial, low-gradient river system. Analyzing the mechanisms and rates of legacy sediment erosion will improve understanding of how rivers return to a natural state after dam removal and will facilitate current and future restoration projects. Field and remote sensing analyses (using lidar digital elevation models and historical topographic maps and aerial photographs) indicate that fill terraces (up to 2-m high) composed of mud and sand function as floodplains for 1.5-2 km upstream of both former dam sites. Preliminary analysis of four radiocarbon dates from pieces of tree bark sampled from the stratigraphy (58-187 cm below the surface) of the two study sites suggest the fill terraces were deposited within the past ~300 years and are therefore composed of legacy sediment.