Monitoring the exchanges of water, solids, and solutes between channels and islands of Wax Lake Delta, Louisiana: Key to defining the resiliency of this coastal environment

Abstract:

Deltas are typically treated as binary systems composed of a channel network and the land separating adjacent channels. Field studies of these systems have tended to focus on collecting data either from the channels or from the land, and by doing so have missed a central characteristic of deltas, the connectedness between the land and channels. We propose that the resiliency of any delta can only be accurately assessed if the naturally occurring exchanges of fluid, solids and solutes between the channels and islands (neighboring land) are understood. These exchanges control the growth of land via the deposition of sediment and accumulation of plant biomass, and also affect delta ecology by mediating water temperature and solute concentrations. The deposition of sediment and organic material in turn influences future growth and pattern development for the deltaic channel network. Exchanges between channelized flow in the delta network and the more distributed flow over submerged island tops is currently being monitored and studied at an NSF-funded observatory under development at Wax Lake Delta, Louisiana. Characterization of flow in distributary channels and on island tops reveals that a considerable fraction of water originally travelling in the large channels is transferred onto island tops either through focused entry points (tie channels) or via distributed flow through island-bounding levees. These volume transfer fractions range between 10 and 60 percent, and are sensitive to location within the delta, as well as river discharge, tides, and winds. Island tops develop tributary-like networks through which the fluid, solids, and solutes drain back into adjacent channels or drain out of the system at the front of the delta, in between the mouths of primary distributary channels. Characteristic fluid velocities vary over roughly two orders of magnitude (centimeter- to meter-per-second) depending on whether a fluid parcel is located in shallow laterally unconfined flow on an island top or positioned in a primary channel. We will discuss how these monitored exchanges affect the growth of new land and delta ecology, as well as how overall delta growth influences future exchange of water, solids and solutes between the channels and islands composing this prograding coastal environment.