DYNAMICS OF EPHEMERAL CHANNELS IN HUMID, FORESTED WATERSHEDS

In the moderate relief Piedmont of the southeastern United States, headwater watersheds are drained by dendritic ephemeral channel networks that contribute to perennial streams. Such headwater watersheds (<1km²) account for approximately 60% of the total land area in the North Carolina Piedmont, where rapid population growth is converting forest and farmland to urban land uses, but little is known about the magnitude and frequency of flow and erosion in the ephemeral channel network and their controlling factors. Using map grade GPS units, >100 forested channel heads are being mapped, to evaluate the contributing area-slope relationships and their variation with soil type and land-use history. Six ephemeral streams have been instrumented with 41 crest stage gauges to monitor the magnitude and frequency of peak flows, relative to antecedent and event moisture conditions and position within the channel network. Source areas for ephemeral channels are on the order of 0.95 ha (range= 0.8-1.5 ha) with an average length 161.1 m (range= 88.4-245.9m). Preliminary data suggest there are no significant relationships between slope and area at channel heads within or across soil types (r²< 0.53). In summer 2010, two ~2.5 cm rain events generated measurable flow in all portions of the creeks with the exception of the channel heads. These results suggest that channelized overland flow occurs during typical precipitation events over a significant portion of the flowpath from ridgeline to perennial channel. Disturbances to the ephemeral channel network, such as occur during urbanization, may have an under-appreciated impact on larger streams.