Understanding the feedbacks between water, sediment, and vegetation in deltas is an important part of understanding deltas as ecomorphodynamic systems. We conducted a set of laboratory experiments using alfalfa (Medicago sativa) as a proxy for delta vegetation to investigate: (1) the effects of plants on delta growth and channel network formation; and (2) the timescales controlling delta evolution in the presence of plants. Experiments were conducted with fluctuating discharge (i.e. flood and base flow periods) and variable seeding densities. We found that when deltas were small, channels had no memory across flood cycles, as floods could completely fill the incised channel network. When deltas were large, the larger channel volume could remain underfilled to keep channel memory. Plant patches also helped to increase the number of channels and make a more distributive network. Patchiness increased over time to continually aid in bifurcation, but as vegetation cover and patch sizes increased, patches began to merge. Larger patches blocked the flow to enhance topset deposition and channel filling, even for the case of large deltas with a high channel volume. We conclude that both plant patchiness and delta size affect the development of the channel network, and we hypothesize that their influences are manifested through two competing timescales. The first timescale, T v , defines the time when the delta is large enough for channels to have memory (i.e. remain underfilled), and the second, T p , defines the time when vegetation patches merge, amplifying deposition and blocking channels. When run time is between these two timescales, the delta can develop a persistent distributary network of channels aided by bifurcation around plant patches, but once T p is reached, the channel network can again be destroyed by vegetation.
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Acknowledgments—Data for this paper will be made available through the Sediment Experimentalists Network and the Sustainable Environment, Actionable Data Repository. This work was supported by NSF EAR grant to W. Kim 1324335. We thank Jim Buttles for insightful discussion regarding these experiments and Paola Passalacqua, Doug Jerolmack, and David Mohrig for early reviews of this manuscript. We appreciate comments from Lucy Clarke, Michal Tal, and the editor and associate editor that greatly improved the manuscript.
All Science Journal Classification (ASJC) codes
- Geography, Planning and Development
- Earth-Surface Processes
- Earth and Planetary Sciences (miscellaneous)