Please see the attached pdf file for my detailed comments.

This paper presents two-dimensional morphodynamic modeling simulations investigating how the so-called hydrograph boundary layer influences morphodynamics in straight channels with migrating alternate bars. This topic is a natural progression from prior work on hydrograph boundary layers, which have focused on the one-dimensional (bar-free) condition up until now. I therefore think this paper is relevant and of potential interest to the readers of Earth-Surface Dynamics.

The research presented in the paper is nicely conceived and generally well presented. There are some areas of the presentation and analysis that I find somewhat confusing, and I encourage the authors to consider the comments below in revision.

• More details on the specific set up of the model are needed. What are the initial conditions of the simulations? Was an initial equilibrium bed developed before imposing the hydrograph and constant sediment supply? What was the time step and cell size?
• The description of the calculation of sediment supply rates is also unclear (lines 285-290).
• For Fig 5 (and others) – is it possible to provide movie files of the simulation results as supplemental information? It would be informative to be able to see the evolution of these simulations as they occur.
• Can you provide more description of how the bars emerged? No mention is made of any initial topographic instability, so what led to bar emergence and why at 3km?
• Figure 8: what is “upper low” and “lower low”?
• Figure 13 is somewhat challenging to understand. What does the hysteresis look further upstream (within the HBL)? I wonder if also showing the hysteresis of bed elevation deviation, and/or average surface geometric mean grain size, at the locations chosen for Figure 13 would more clearly show the impact of bedload sheets.