Understanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in 3 forested catchments (<75 ha) in each of 4 physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length 7 times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.
We use unique fl uvial gravel deposits preserved atop a regional drainage divide to confi rm the role of stream capture in driving ~250 m of incision in the transient Roanoke River basin of the Appalachian Mountains (United States). Gravel provenance constrains the pre-capture position of the divide, indicating that ~225 km 2 of basin area were abruptly connected to the base level of the capturing stream. The resulting wave of incision is currently manifest as major knickzones separating adjusting reaches from relict headwaters resembling streams of the New River basin, from which the Roanoke River was captured. The unusual preservation of the unconsolidated gravels on small relict surfaces adjacent to bedrock gorges indicates extreme spatial variability in erosion rates within the Roanoke basin, which is the fi rst documented example of a transient passive margin basin connected to a capture event by stranded fl uvial debris. Our results show the potential for stream capture across an asymmetric drainage divide to drive major transient incision independent of external forcings, such as climate change or tectonic uplift. A continuation of this process will lead to eventual capture of ~7000 km 2 of the New River basin in the relatively near geologic future.
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