Sediment Supply as a Control on Plant-Morphodynamic Interactions

Friday, 19 December 2014: 10:35 AM
Rebecca Manners1, Andrew C Wilcox1, LI Kui2, John C Stella3, Anne Lightbody4 and Leonard S Sklar5, (1)University of Montana, Geosciences, Missoula, MT, United States, (2)SUNY-ESF, Syracuse, NY, United States, (3)SUNY College of Environmental Science and Forestry, Forest and Natural Resources Management, Syracuse, NY, United States, (4)University of New Hampshire, Earth Sciences, Durham, NH, United States, (5)San Francisco State University, San Francisco, CA, United States
The caliber and quantity of sediment delivered to a channel influences its size and shape, yet we know little about how the sediment supply affects rivers whose geomorphic form is influenced by riparian vegetation. We present results from flume experiments that test the impact of sediment supply on plant-morphodynamic interactions. We introduced two sediment supply conditions to a 28-meter long, sand bedded flume (60 cm wide and 71 cm deep) at the UC-Berkeley Richmond Field Station: equilibrium (balance between sediment transport and supply) and deficit (transport exceeds sediment supply). We conducted ten runs with different riparian seedling configurations (individual plants or patches) and species (tamarisk or cottonwood), and stem and leaf density (0.003-0.47 cm2/cm2), under both sediment supply conditions. Plant species, size, and configuration were important in predicting the topographic adjustments that occurred during our experiments. These influences may be attributed to differences in plant morphology; tamarisk is shrubby while cottonwood is more tree-like, with a single stem and leaves concentrated higher on the plant. The plant-morphodynamic relationship, however, differed for the two sediment supply conditions. During sediment equilibrium, only patches of cottonwood served as sediment sinks compared to an unvegetated bed, but tamarisk patches had no impact on the sediment mass balance. During sediment deficit, in contrast, tamarisk patches accumulated more sediment than unvegetated beds. Stem and leaf density also controlled changes in bed elevation. During equilibrium conditions, increasing the density of cottonwood stems and leaves resulted in greater bed degradation. Conversely, aggradation occurred with increases in the density of tamarisk. For sediment deficit conditions, the relationship between stem and leaf density and the rate of bed change was negative for both species (i.e., higher density resulted in faster rate of scour). The shifting influence in how tamarisk and cottonwood mediate topographic changes under low versus high relative sediment supply rates highlights the need to account for this variability in future research on vegetated rivers.