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 (USA). During base discharge, POM was primarily retained as it settled onto the sediment surface, but during high discharge, debris dams became primary retainers. During over-bank 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. Meanleaf transport distances in the channels ranged from 16. 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 basedischarge in the colliers Creek channel to 84.0 m during winter base dischargein the Buzzards Branch channel. Over one year, 97% and 27% of the marked woodplaced in the channels was transported in the Colliers Creek and Buzzards Branchfloodplains, respectively; only 4-9% of the marked wood placed in thefloodplains moved into the channels. Base flow flux of POM was 2,000 kg/yr and1700 kg/yr on the floodplains, and 7000 kg/yr nd 13,000 kg/yr in the channels,at Colliers Creek and Buzzards Branch, respectively. Retention, transportdistance, and magnitude of POM exchange between the channels and floodplainswere dependent on the timing, frequency and extent of spates, and floodplaininundation, which thus were critical determinants of POM dynamics and hencesystem structure and function.
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