Morphological Response to Discharge Reduction in a Partially Abandoned Channel of the Catawba River, North Carolina, USA.

Examples of partial avulsion and river bifurcation around large stable islands make up a small but locally important portion of the total length of rivers in the Piedmont physiographic province of North Carolina (U.S.). Despite remaining ecologically significant, the partially abandoned (or "abandoning") branches along such reaches have received less attention than the often dominant newly created branches. The existence of perennial flow through abandoning channels may be contingent upon their ability to rapidly adjust their morphologies to declining flows. In this paper we study the response of a partially abandoned channel of the upper Catawba River in North Carolina to major loss of flow following flood-induced partial avulsion in 1977. Human activities have strongly modified the floodplain in this area, assisting the avulsion and possibly influencing the natural erosion and sedimentation processes responsible for subsequent adjustments of the abandoning channel. Using a combination of downstream gaging data, flow estimation functions, channel mapping, and hydraulic geometry analyses, we assess the degree to which channel capacity reduction matches the amount of post-avulsion diminished discharge. Construction of fluvial sediment benches inset within the old single-thread channel has occurred along 83% of the channel's length, and most of these have crest elevations in the 1–2 yr flood stage range. Hydraulic geometry reconstructions indicate that length-weighted mean channel width, depth, and capacity deviate from predicted values by 36%, 18%, and 61%. Thus, channel cross-section dimensions for the abandoning channel appear substantially larger than predicted, a condition that is potentially discordant with a hypothetical necessity of rapid width adjustment for maintaining an open channel, depending on what constitutes a rapid rate of adjustment. Variables such as the morphology of the bifurcation node and load properties also influence the longevity of abandoning channels, and rates of adjustment or infilling can vary widely. Stratigraphic, pedogenic, and vegetative evidence together suggest that channel narrowing via inset bench formation has occurred gradually over at least two or more decades following initial flow loss. Comparisons with channel dimensions predicted from existing Piedmont regional curves show greater agreement, supporting the contention that Piedmont valley characteristics exercise at least as strong an influence on channel morphology as does the higher relief watershed topography and associated flow regime. The presence of perennial flow and active geomorphological processes in the abandoning channel indicates that it remains part of a dynamic and ecologically significant river reach, despite being currently designated as land in county land parcel records. • Partial avulsion of the upper Catawba River (U.S.) occurred in 1977. • The new channel course was influenced by floodplain gravel mining. • Contraction of the abandoning channel has occurred via bench formation. • Hydraulic reconstruction indicates contraction not commensurate with discharge reduction. • Comparison to regional curve predictions indicate the importance of valley setting. [ABSTRACT FROM AUTHOR]