Understanding how water and sediment move through different parts of a deltaic system is of fundamental importance to resource managers who are charged with sustainable futures of critical marsh and wetland habitat. Channel bifurcations (locations in a primary channel where the flow divides into two separate downstream channels) are common natural features of fluvial deltas. Despite their abundance, river bifurcations have not been studied extensively because it is conventionally assumed that suspended sediment "goes with the flow" -- that suspended sediment is partitioned between the downstream branches in proportion to water discharge into each downstream branch. This assumption is contrary to the well-known fact that suspended sediment is typically distributed unevenly across channels because of secondary flows and because of varying source-sink areas within a channel. The goal of this Doctoral Dissertation Research Improvement project is to address these contrary notions by identifying the hydrologic and morphologic parameters that influence the division of suspended sediments through a bifurcation. High-resolution field data will be collected in a tidally influenced river bifurcation on the Sacramento-San Joaquin Delta located in central California. Instrumentation will include a boat-mounted, three-dimensional Acoustic Doppler Profiler, several Optical Backscatterance Sensors, a Global Positioning System, and a range of other electronic sensors and manual devices to measure water flow and the distribution of suspended sediment throughout the bifurcation over a variety of hydrologic conditions. Shear stress, turbulence intensity, momentum flux, and the suspended sediment transport capacity of each branch of the bifurcation will be calculated and related to the observed pattern of sediment concentrations. The field results will be complemented by the use of a two-dimensional hydrodynamic model, which allows extrapolation to longer time frames (months to years) and to alternative sites in order to produce generalizations about cyclic and long-term evolution of bifurcation morphology.
A bifurcation in a river channel represents a "decision point" in a channel network where suspended sediment is preferentially directed towards one downstream branch over another. Scientists have yet to decipher how this complex "decision" is made despite its critical role in the development of river deltas and estuaries. Suspended sediment provides the base on which tidal marsh habitat grows, and knowing why one downstream branch receives more suspended sediment over another will guide managers in selecting successful marsh restoration strategies. This is particularly important when flow control structures such as floodgates, barriers, and dams modify the natural dynamics of the system. The Sacramento-San Joaquin Delta-Estuary system is under immense environmental stress and this study contributes to a larger understanding of the processes that shape this vital ecosystem. While the processes at a channel bifurcation are complex, recent developments in field techniques and computer modeling allow researchers to address this lapse in basic scientific understanding for the first time. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
