Role of Organic Debris Dams in Regulating the Export of Dissolved and Particulate Matter from a Forested Watershed

Abstract

An organic debris dam is an accumulation of organic matter in a stream which obstructs water flow. Debris dams trap sediments in the pool formed upstream from them and the dam structure itself collects particulate organic matter. This study was done at the Hubbard Brook Experimental Forest, in New Hampshire, to examine the relative importance of these structures in retention of sediment and organic matter in a small stream ecosystem. An experimental approach was used in which all organic debris dams were removed from a 175—m section of second—order stream, just above a gauging weir. The material being exported from the watershed was separated into three size categories: dissolved matter (<0.5 μm), fine particulate matter (0.5 μm to 1 mm), and coarse particualte matter (>1 mm). Export of each size fraction was monitored for at least 1 yr prior to dam removal, and for 1 yr following removal. Following dam removal, export of dissolved matter increased slightly due to an increase in the concentration of dissolved organic carbon in the stream water during periods of high discharge. Fine particulate matter export increased dramatically at high discharges following dam removal; concentrations in some instances achieved values five times higher than any observed before dam removal. Coarse particulate matter export also was greatly increase. Calculating dissolved matter and particulate matter export from the watershed, with an without organic debris dams, showed that dam removal brought about a 6% increase in the export of dissolved matter and a 500% increase in the export of both fine particulate and coarse particulate matter.