Abstract Vegetation is a major control on delta evolution, but its impact over intermediate (10 2 –10 3 yrs) and long (>10 3 yrs) timescales is relatively unconstrained. We particularly lack these constraints in the context of rising sea level. To address this, we conducted laboratory flume experiments to isolate vegetation effects on delta evolution over intermediate‐to‐long timescales with sea level rise, using alfalfa ( Medicago sativa ) as a vegetation proxy. We found that vegetation effects promoted delta shorelines that migrated more slowly and across a smaller area, preventing rapid area fluctuations and slowing area loss with sea level rise. Vegetation effects also promoted confined channels within branching channel networks that maintained relatively fixed planforms as sea level rose. This localized shoreline deposition at fixed channel termini and enhanced shoreline roughness across spatial scales. However, non‐vegetated shorelines were transiently rougher during periods of rapid area fluctuation as lobes migrated. The relatively fixed vegetated channel networks also spatially restricted floodplain reworking, resulting in longer floodplain residence times, particularly in proximal delta regions. Differences between non‐vegetated and vegetated delta evolution reflect vegetation's role in increasing sediment cohesion and sediment trapping. Importantly, we found the effects of onshore vegetation extend offshore, modifying deposition and accommodation and creating feedbacks that reinforce vegetation's control on delta evolution. These results help predict how vegetation will modulate delta response to sea‐level rise, climate change‐induced vegetation turnover, and anthropogenic activities. More broadly, these results will help to quantify delta floodplain storage and interpret how modern and ancient deltas evolved.
Lyster et al. (Wed,) studied this question.