SEDIMENT BUDGETS AND STREAM TERRACES: RECONSTRUCTING THE EFFECTS OF HISTORIC LAND-USE ON SEDIMENT TRANSPORT IN THE BIRCH BROOK CATCHMENT, MASSACHUSETTS

Sediment budgets for the South Branch of Birch Brook, NW Massachusetts, indicate that annual bedload discharge (1.6 tons/sq. km/year) is a function of the number of peak events above bankfull (0.34 cu. meters/sec.) and sediment supply. Sediment availability in the South Branch, a small, forested mountain stream draining 1.25 sq. km, is limited due to low erosion rates on hillslopes. Significant sediment storage occurs behind organic debris dams, in pools associated with bedrock reaches, and along the channel. When annual storms events are < 80 to 90 percent of bankfull, relatively little sediment is available for transport. Because sediment storage along the channel is substantial, the relationship between discharge and sediment delivery shifts after high discharge events, such as the 15-year flood, flush out significant debris dams and transport sediment downstream.

Deforestation, grazing and cultivation within the Birch Brook catchment throughout the 19th century resulted in increased erosion on hillslopes and stream aggradation. We studied stream terraces <1.2 meters above the stream channel along 3.4 km of channel length in the Birch Brook catchment. Stream terraces record increased sediment load and account for aggradation and redistribution of sediment within stream channels of the Birch Brook catchment during this period of high sediment yield. Based on our analysis of modern sediment storage and annual bedload discharge, we estimate that sediment load was potentially three to ten times greater than modern load during this period of increased erosion and fewer organic debris dams. The widespread distribution, volume, and composition of these terraces suggest deposition from continuous aggradation rather than a single storm event such as the 500 or 1000-year flood. Tree core dates frame the stabilization of these terrace surfaces between 60-80 years before present, correlating with major reforestation in the catchment.