WHEN DO PLANTS MATTER FOR FLUVIAL PROCESS AND FORM? PLANT-MORPHODYNAMIC INTERACTIONS UNDER VARIABLE FLOW AND SEDIMENT SUPPLY RATES

The balance between abiotic and biotic factors dictates the strength and direction of plant-morphodynamic interactions. These interactions determine the size and shape of vegetated river channels. We present the results from an experiment designed to test the impact of the abiotic/biotic balance on flow hydraulics and channel topography. Live seedlings of two species, tamarisk (Tamarix spp.) and cottonwood (Populus fremontii), with distinct morphologies were placed in different configurations in a mobile sand-bed flume. The hydraulic and topographic response of the channel was measured for different flow rates and two sediment supply conditions, sediment equilibrium (supply = transport rate) and sediment deficit (supply < transport rate). The morphodynamic response during sediment equilibrium conditions represented the balance between abiotic and biotic conditions, responding to increasing flow rates and the species and configuration of plants present. Species, more so than other biotic characteristics, controlled the hydraulic and topographic response. Tamarisk had larger, more numerous, stems for its height that decreased the degree of pronation, and thus reduced near-bed velocities, increased near-bed turbulence, and increased topographic variability. Frontal area density (i.e., frontal area of plants per bed area), a secondary control on the morphodynamics during sediment equilibrium conditions, was the only plant characteristic that explained some of the variability in the morphodynamic response during sediment deficit conditions. Abiotic conditions, however, dominated the topographic response during sediment deficit conditions. We identified thresholds in the abiotic/biotic balance, such for some runs, plant characteristics prevailed over differences in flow or sediment transport rates, and vice-versa. These results provide insight into how and when plants influence channel form in natural systems where rapid rates of change in discharge and sediment concentrations are typical.