Channel bifurcation and adjustment on the upper Yadkin River, North Carolina (USA)

The bifurcation of river flow around large stable islands, also known as anabranching, represents a distinctive form of river adjustment that is uncommon in the Appalachian Piedmont province of the eastern U.S. Within this province, highly localized river branching similar to classical anabranching forms exists in one area near the foot of the Blue Ridge Escarpment. This paper examines channel form and processes along a reach of the upper Yadkin River in North Carolina (USA) before and after a flood-induced division of its flow into two subparallel branches, both of which remain active 35 years later. The research draws on aerial photograph analyses, channel surveying, and observations of inset channel benches, flood frequency analysis, and discharge monitoring and modeling to analyze planform history and to track and explain changes in newly excised and losing channels along the bifurcated reach. The characterization of this branched reach resembles some descriptions of gravel-dominated laterally active anabranching, although the match is imperfect. Reductions in valley slope and confinement and the presence of local valley constrictions near the base of the escarpment steeplands have been conducive to long-term sedimentation and a greater likelihood of branching. A period of relatively rapid cross section adjustment occurred on both branches soon after bifurcation, and this may have permitted the losing branch to remain open. Well-stratified benches currently observed along the losing branch are interpreted to be largely the late product of a waning early major adjustment phase and moderate, recent, and perhaps ongoing variations in flow division and appear to be not fully adjusted to prevailing discharges. Coarse benches along the newly excised branch are better-adjusted inset floodplain fragments created primarily by lateral migration and point accretion. Patterns of benchfull flow frequency variation are interpreted to indicate that the flow bifurcation ratio has been neither static nor monotonically changing in recent decades. Further exploration of branching in this environment, otherwise dominated by single-thread meandering patterns, is of importance for a variety of management and ecological concerns.