THE SPATIAL AND TEMPORAL CONTROLS ON IN-CHANNEL SEDIMENT AND MERCURY STORAGE OF A MID-ATLANTIC GRAVEL-BED RIVER

Here we determine spatial and temporal controls on in-channel sediment and mercury accumulation in South River, VA, a site of historic mercury contamination. We studied a 37-km reach of the South River, which starts in Waynesboro and flows north to Port Republic, VA. We hypothesize that frequency of sediment deposits results from geomorphology and changes in local climate and land use. Previous workers at the study site have documented loss of mill dams, increased storm frequency, and increased riparian reforestation has led to increases in bank erosion and wood recruitment rates. Additionally, sediment accumulations have a slope threshold (0.0025) and occur in four distinct physiographic geomorphic settings: 1) long pooled sections caused by bedrock exposures, former mill dams, and other causes; 2) downstream of LWD accumulations located along the margins of the pools in riffle-pool sequences, 3) bank obstructions (usually caused by trees leaning into the flow), 4) side channel backwaters where flow separates around islands. Most sediment is stored in long pooled sections (45% of the total) and associated with large wood (35% of total sediment, 37% of all deposits mapped). Most large wood associated with sediment occurs as jams and is oriented obliquely to the main flow (52%). The geometry and overall size of sediment deposits can be related to the obstruction: type, effective width, constriction ratio, volume, channel width, and diameter. Mercury concentrations are correlated with grain size (R² of 0.51 or higher) and organic content (R² of 0.42 and higher) and have also been related to sediment age. An integrated conceptual model emerges: larger deposits (>50m³), which occur in low slope areas, generally contain older sediment, longer turnover times, and are more likely to have higher mercury concentrations and higher clay content; conversely, smaller deposits contain younger sediment, shorter turnover times, lower mercury concentrations, and more sand.