THE BIG SPRING RUN, PA, PROJECT: EXPERIMENTING WITH RESTORATION OF AN ANTHROPOCENE STREAM

For centuries, milldams were constructed along valley bottoms throughout the eastern US to generate water power. Fine-grained sediment accumulated upstream of dams and buried the landscape beneath 1-5 m (average 1.2 m) of “legacy sediments”. As aging dams breached, single channel streams incised deeply into the historic sediment, accessing Pleistocene gravel to build bars and eroding high banks of millpond sediment. On Big Spring Run, a 1^st-order Piedmont tributary in the Conestoga River watershed, recent restoration 1.5 km upstream of at least one breached 2.5-m milldam involved removal of ~20,000 tons of historic sediment (mostly silt), with excavation down to the level of the pre-settlement wetland landscape.

A key goal of this state and federally funded project is to recover eco-hydrological benefits (e.g., denitrification) of the original wetland-floodplain ecosystem. Since removal of historic sediment and construction of small, sinuous channels with low banks in late 2011, a multi-channel system has evolved with lower water depth, flow velocity, and boundary shear stress than the former incised single channel with high banks. Post-restoration monitoring indicates significant reduction in mean particle size of bed load in comparison to pre-restoration high-flow conditions. Analysis of sediment load data acquired over the past 4 years from 3 USGS gauge stations equipped with turbidity sensors both upstream and downstream of the study reach, along with repeat RTK-GPS cross section surveying, reveals a marked reduction erosion and increased retention of fine sediment within the restoration reach, decreasing the sediment load transported downstream.