Riparian Areas as Filters for Agricultural Sediment1

Abstract

Riparian buffer areas are known to trap sediment leaving agricultural fields, but little quantitative data on this process are available. This study was initiated to determine the amount of sediment deposited in riparian areas of two watersheds during the last 20 yr. These watersheds contained approximately 800 and 1400 ha, and about half of each watershed was cultivated land. The areal extent and thickness of the sediment were mapped and described using 137Cs data and sediment-soil morphology. At the field-forest edge 15 to 50 cm of 137Cs sediment accumulated while <5 cm of 137Cs sediment was deposited in the flood plain swamp downstream. Although only a thin (≤5 cm) layer of sediment accumulated in the flood plain swamp, the large area available made this an important depositional area. The full potential of the flood plain swamp for sediment deposition has not been utilized. About 80% of the 137Cs sediment measured in the watershed was deposited above the flood plain swamp with >50% of this sediment deposited within 100 m of the exit location from the cultivated fields. Sand dominated the deposition at the forest edge while high silt and clay contents occurred in the flood plain swamp. Sediment delivery estimates for Coastal Plain watershed indicated that 84 to 90% of the sediment removed from the cultivated fields remained in the watershed. It is concluded that riparian areas serve a valuable function by trapping sediment from agricultural fields.