Geomorphological Response of Alluvial Streams to Flood Events during Base-Level Lowering: Insights from Drone-Based Photogrammetric Surveys in Dead Sea Tributaries

Round 1

Reviewer 1 Report

This manuscript presents the results of a case study examining geomorphic change to two ephemeral stream channels draining to the Dead Sea by using drone-based photogrammetry. This is a well-written manuscript and it clearly presents an improved understanding of the morphodynamics of ephemeral channels where field-data collection and direct measurement are difficult and potentially unsafe during channel-altering flood events. While other studies have adopted similar methodologies, the application to this geomorphic setting provides insight about how channels respond to a sequence of floods as their base level is reduced. This application of photogrammetry is an example of how remote techniques are increasingly used to gain knowledge of previously inaccessible geomorphic events. The questions and comments that I share below are very minor; this is a strong paper.

Line 54: “impacts” instead of “impact”

Line 57: Do not use contractions in a formal paper – “it is” instead of “it’s”

Line 64: Data are plural – “. . . data for discrete flood events are routinely . . .”

Line 108: Do not use contractions in a formal paper – “is not” instead of “isn’t”

Lines 214-216: What are the average channel gradients of the Darga and Arugot streams downstream of the ramps?

Line 324: Capitalize HEC-RAS

Line 326: Do not use contractions in a formal paper – “It is” instead of “It’s”

Figures 12 and 13: It looks like it, but are the y-axis values plotted equally between cross-section plots?

Conclusions: Can you identify an idea or two for future work stemming from this research?

Line 712: Delete extra period

This is a well-written and constructed manuscript. I indicate a few grammatical corrections in my comments. Otherwise, the paper would benefit from copy editing to catch any other minor errors.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

This work presented a relatively new application of drone based remote sensing in fluvial geomorphology in semi-arid rivers. Overall, the methodology is reasonable, the data analysis is valid, and the writing is clear. I suggest a minor review and hope the authors can make the manuscript stronger.

The study ranked the causes of channel erosion (e.g., fig 15) and these are supported by the data analysis. But I think a more interesting point is to link the erosion to a bigger picture as the authors indicated earlier. The water level drop in the Dead Sea seems to be a bigger factor than peak discharge (my guess), because the local gradient increased as water recedes, increasing the stream power given the same discharge (fig 1). Meanwhile, changes in precipitation intensity, duration, and frequency also play an important role. The current manuscript treated channel erosion and lake water level drop in a loose way, and I am wondering if the authors can make their connection stronger so that the entire manuscript will be more coherent and interesting. This is because fig 1 seems to serve as the governing conceptual model while the discussion lacks enough emphasis on fig 1.

The manuscript mentioned equilibrium multiple times. I am curious if any of the studied channel reach has reached a new equilibrium after the lake water level drops as indicated in fig 1d, on lines 676 and 708. My guess is, if this ever existed, it would be more likely to be away from the estuary. This could be a good point for the discussion.

I would also like to see more discussion on the sediment budget. In general, why does the current flow regime lose sediments in the study reaches? The upstream is detachment limited and the sediment supply is less sensitive to discharge, so, does the negative sediment budget indicate a change in precipitation or base water level drop? How did other researchers explain this phenomenon? The work by Hooke (line 587) does not explain the negative sediment budget as the entrained sediments may be deposited again in the channel or in the floodplain.

The statistical method section requires more clarification. It is not clear how exactly the statistical methods were applied, and equations can be added if necessary. Further, I suggest a more physically based approach by using stream power other than discharge. The stream power considers the local gradient that is expected to be variable due to the knickpoints.

Minor comments

L83: this seems to be the beginning of section 2 background information

Fig 2b: fix the longitude error

Fig 7: is A related to B and C? Where are B and C?

Fig 9b: add date of photo

L474: what is tolerance value?

L475: define VIF

L491: GLM or GLMM?

Fig 14: Some of the figure caption is redundant. Add the legends for the dashed lines to the plots.

L539: this is another reason to compare stream power to channel erosion as written here.

What is the difference between geomorphological and geomorphic? I suggest choosing one term and keeping it consistent.

Author Response

Please see the attachment

Author Response File: Author Response.pdf