General comments

The authors present the results of laboratory experiments that reproduce the development of floodplains within estuaries. They compare a control case with only sand to one with mud and one with live vegetation, which enables them to show subtle differences in the way mud and vegetation affect floodplain formation. Their conclusion that mud affects tidal propagation in the estuary by filling accomodation space while vegetation imposes additional hydraulic resistance certainly makes sense and is well supported by their data. The vegetated estuary starts off more like the sandy estuary and becomes more like the muddy estuary as vegetation establishes. The main limitation of their experiments seems to be that the experiments were not run long enough to determine if the vegetated and muddy estuaries will completely converge. These experiments provide an important complement to field and numerical studies of estuarine floodplain formation, and this manuscript is an excellent description of the experiments.

Specific comments

I was somewhat skeptical of using a numerical model to replace measurements of hydrodynamics, especially when it comes to the effects of vegetation on the flow, but the citations to Weisscher et al. (2020) and Lokhorst et al. (2019) suggest that this is not unreasonable. In any case the authors' interpretations do not rely too heavily on the exact flow velocities achieved in the different experiments, and the tidal prism estimates are probably more robust to model inaccuracies.

Are the sand and mud experiments identical to some of the runs in Braat et al. (2019) or are they new runs with similar sediment and forcing? The wording was a little unclear, and I am having some trouble matching up the experimental conditions between those described here and those in Braat et al. (2019).

Technical corrections

Minor typographic corrections:

- Lines 297 & 299: The parentheses are misplaced around the citations on these lines.

- Line 306: "Mudflat" should be capitalized.

General Comments

In the manuscript “Estuarine morphodynamics and development modified by floodplain formation”, the authors present a combination of physical and numerical modeling results to demonstrate the role of vegetation and mud in estuarine evolution. The authors suggest two separate mechanisms that affect morphology: mud changes estuarine dynamics by filling in accommodation space, while plants increase roughness. This is interesting because both mud and vegetation are often thought to stabilize systems and prevent change, but the authors demonstrate that they do this in different ways. Overall, I think the paper contributes novel results that directly relate to field observations of estuarine sediment transport and dynamics, and by using a physical model you are able to separate confounding variables that are impossible to separate in field work.

Specific Comments

Plants:

One component of the physical experiments that seems perhaps too simplified is the vegetation and the role of the roots. First, root morphology is important for stabilization, which is neglected in this paper. For these experiments, it seems that you chose the two species of plants with the shortest roots and smallest diameters (based on Lokhorst 2019). Do you think your results would be different if you had used plants with more extensive (or interlocking) root structures? How do your plant choice relate to plant root proprieties in the field?

Second, stabilization by plants can be affected by the sediment type. Previous studies have demonstrated that in sandy sediments, vegetation may not provide stabilization (e.g., Feagin et al. 2009). I think your experiments may underestimate the potential role of plants in bank stabilization given that you use sandy sediments. Have you considered doing experiments that have both mud and vegetation? That is likely beyond the scope of this paper, but I would be interested to see if plants have a more profound effect with finer grained sediments.

I was also curious about why you used two different plant species and then did not discuss the difference between the two other than in the methods. For roughness, you used stem density and diameter, which would have taken into account some of the differences between the plants. You also state that the plants have different zonation. Was the use of the two species just to get maximum vegetation cover (plants that would inhabit all elevations of bars and floodplains)? Was there any difference in the flexibility or surface area of the species of plants that may alter the roughness?

Other comments:

Did you measure the grain size distribution of the sand at the end of the mud experiments? I am curious if mud infiltrated the porespace of the sand. This would have morphodynamic importance – if the mud simply deposits on the top, it would behave like mud-capped sands, but if the mud infiltrated, the sand may behave cohesively.

Small question, but could you clarify the lights used in the lab – are they full spectrum? Were the lights on all the time, or was there a dark cycle (“night”)? From the text, it seems that the lights were always on, but experiments often use a light/dark cycle to simulate days. I suppose this would be difficult in scaling your experiments, so how do you think having the light on all the time affected plant growth? (See Smith and Sitt 2007 for some discussion of this).

In response to previous comments on the manuscript from Will, I agree with the authors about the difficulties of working with physical experiments and pest control.

Technical Corrections

Line 178: Figure 8 is referenced before Figure 7.

Line 218: It seems like the end of this paragraph was cut off.

Citations:

Feagin, Rusty A., et al. "Does vegetation prevent wave erosion of salt marsh edges?." Proceedings of the National Academy of Sciences 106.25 (2009): 10109-10113.

Smith, Alison M., and Mark Stitt. "Coordination of carbon supply and plant growth." Plant, cell & environment 30.9 (2007): 1126-1149.