Effects of vegetation distribution on experimental river channel dynamics

Abstract

Strong feedbacks exist between channel dynamics, floodplain development, and riparian vegetation. Earlier experimental studies showed how uniformly distributed riparian vegetation causes a shift from a braided to a single‐thread river because riparian vegetation stabilizes the banks and focuses discharge off the floodplains into channels. These experiments tested anemochorously distributed vegetation, i.e., by wind, whereas many riparian species in nature are also distributed hydrochorously, i.e., by flowing water. The objective of this study is to test experimentally what the different effects are of hydrochorously and anemochorously distributed vegetation on channel pattern and dynamics. The experiments were carried out in a flume of 3 m wide and 10 m long. We compared experiments with the two forms of vegetation distribution methods to control experiments without vegetation. To independently quantify bank retreat rate as a function of seed density and vegetation age, we used a small bank erosion test. In agreement with other work, the uniformly distributed vegetation decreased bank retreat, often stabilized banks and tightened meander bends. Vegetation seeds distributed by the flow during floods settled at lower elevations compared to the uniformly distributed vegetation. Inner bend vegetation stabilized a part of the point bar and hydraulic resistance of the vegetated bar forced water into the channel and over the floodplain. As a result, sediment was deposited upstream of vegetation patches. We conclude that seeds distributed by the flow during floods lead to island braiding: a patchy multithread river with stable vegetated bars, whereas vegetation uniformly distributed on the floodplain of a single‐thread meandering river increases sinuosity and decreases bend wavelength. This implies that the combination of discharge variations and vegetation settling behavior has a large effect on the morphology and dynamics of rivers. The experimental approach opens up a wide range of possibilities to explore hydro‐bio‐geomorphological interactions with a high degree of control.