TRANSPORT AND RETENTION OF PARTICULATE ORGANIC-MATTER IN 2 LOW-GRADIENT HEADWATER STREAMS

Transport and retention of particulate organic matter (POM) were examined in the channels and on the floodplains of two low-gradient headwater streams on the Coastal Plain of southeastern Virginia. During base discharge, POM was primarily retained as it settled onto the sediment surface, but during high discharge, debris dams became primary retainers. During overbank flooding much of the coarse particulate organic matter (CPOM) moved from the channels onto the floodplains. The mean distance that wood moved over a year at Colliers Creek, which had low current velocity and a broad, frequently inundated floodplain, was 23 m in the channel and 46 m on the floodplain; at Buzzards Branch, with higher current velocity and a smaller, less-frequently inundated floodplain, wood moved a mean distance of 136 m in the channel and only 2 m on the floodplain. Mean leaf transport distances in the channels ranged from 1.6 m in Colliers Creek during summer base discharge to 156 m at Buzzards Branch during a winter spate; mean leaf transport distances on the floodplains were 0.5-1.7 m. Fine particulate organic matter (FPOM) transport, studied only during base discharge, was farther than that of wood or leaves and varied from 1.8 m during summer base discharge in the Colliers Creek channel to 84.0 m during winter base discharge in the Buzzards Branch channel. Over one year, 97% and 27% of the marked wood placed in the channels was transported into the Colliers Creek and Buzzards Branch floodplains, respectively; only 4-9% of the marked wood placed in the floodplains moved into the channels. Base flow flux of POM was 24,000 kg/yr and 1700 kg/yr on the floodplains, and 7000 kg/yr and 13,000 kg/yr in the channels, at Colliers Creek and Buzzards Branch, respectively. Retention, transport distance, and magnitude of POM exchange between the channels and floodplains were dependent on the timing, frequency and extent of spates, and floodplain inundation, which thus were critical determinants of POM dynamics and hence system structure and function.