Role of Debris Dams in the Structure and Functioning of Low‐Gradient Headwater Streams

The importance of debris dams to organic matter dynamics and the macroinvertebrate community of two low—gradient, headwater streams on the Coastal Plain of Virginia was examined through sampling of natural dams and by experimental manipulation of dam abundance. Buzzards Branch had permanent flow and a sand substrate; Colliers Creek had intermittent flow and an organic substrate. Dam abundance varied from 8 to 13 dams/100m of stream length and covered only 1—3% of channel surface. Annual mean storage of organic matter (ash—free dry mass; particles >0.15 mm) in debris dams was 922 g/m² and 3356 g/m² in Colliers Creek and Buzzards Branch, respectively. Wood >16 mm constituted 73—80% and fine particulate organic matter (FPOM; particles 0.15—1 mm) <1% of total organic matter in the streams. Dams stored 21% and 85 of the coarse particulate organic matter (CPOM) on the channel surface in Colliers Creek and Buzzards Branch, respectively. Storage was highly seasonal, with highest concentrations following autumnal leaf fall and only 5—17% of December storage present in February. Dams were net exporters of large amounts of small particles of CPOM and FPOM. Macroinvertebrate density and biomass in dams was correlated with changes in organic matter storage both over seasons and between streams. Annual mean densities were 8915 individuals/m² and 22 302 individuals/m², and biomass was 0.3 g/m² and 3.2 g/m², in Colliers Creek and Buzzards Branch, respectively. Densities were at least 10 times, and biomass 5 times, as great in dams as on sediment. Increasing the abundance of dams increased organic matter storage, macroinvertebrate abundance, and the relative contribution of shredders to biomass, both in dams and on sediment. Increasing dam abundance also increased retention of leaves during base and especially storm flow. Widespread removal of riparian vegetation throughout the southeastern Coastal Plain, resulting in a decrease in the number of dams in streams, no doubt has altered the ecological characteristics of these streams relative to historical, pristine conditions. In particular, the transport of both energy and nutrients to downstream riverine areas likely is greater, and production of higher consumer levels probably is lower under present conditions.