the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2026
Ice jam formation at river confluences: comprehensive field investigation and comparison to laboratory-derived predictive equations
Abstract. Ice jams can cause significant damages to riverine communities during breakup season. Tributaries often play an important role, leading to complex spatial ice processes and occasional flooding. The behaviour of tributaries during breakup as well as their impact on the initiation of ice jams at confluences is very site-specific and depends on ice, morphological and hydrometeorological conditions. The objective of this study is to characterize the interaction between tributary and main river ice during breakup, to quantify hydro-environmental factors controlling breakup sequences and jam formation at confluences, and to evaluate the applicability of laboratory-derived ice jam formation mechanisms to these real-world systems. These objectives were achieved through a comprehensive field monitoring program of four confluences in Québec, Canada (three on the Chaudière River and one on the Sainte-Anne River) over four consecutive winters (2021–2024). Field monitoring included continuous water temperature and depth, time-lapse photography, drone surveys and meteorological data. The results showed that a flow impact mechanism consistently produced ice jams when the flow from ice-free tributary restricted ice passage in the main river. Also, the presence of a downstream ice control structure maintained a stationary ice cover at the confluence, forcing a jam to form on the tributary. Finally, morphological controls including islands and bridge piers at the tributary mouth were found to be preventing ice evacuation at the confluence. These observations were then compared to those developed in the laboratory by Ettema & Muste (2001). It was shown that laboratory derived equations could be used to predict jam formation at confluences when the governing breakup mechanism is well defined and related parameters are adequately measured.
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Status: open (until 11 Apr 2026)
- RC1: 'Comment on egusphere-2025-6452', Robert Ettema, 11 Mar 2026 reply
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RC2: 'Comment on egusphere-2025-6452', Anonymous Referee #2, 06 Apr 2026
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Summary
This very interesting manuscript reports field observations of ice jam formation at 4 confluences over 4 years in Quebec, Canada. Field monitoring including water level, water temperature, and ice thickness measurements were conducted and combined with existing weather datasets. The study goal to compare to laboratory studies of Ettema et al. and validate equations describing formation of ice jams. The description of jam formation at the study sites is clear, but in places the manuscript reads more like a case study. To move beyond this, it would be helpful if the structure of the manuscript were clarified and in places the analysis were futher developed to clearly identify insights from the data for model validation and put forward specific directions for future work.
Because the core goal appears to be to validate the Ettema lab studies, I suggest to make comparison to Ettema an organising factor of the manuscript. For example, were the confluences chosen to find specific processes? Did the authors observe the processes they expected to, or not observed some processes they anticipated? The site description in the Results is clear, but it is difficult to follow the model validation between sites. It may make sense to order the site descriptions according to complexity, allowing clear development of model validation, or to report the site-specific results in the Results and develop a targeted Discussion focused on model validation. Finally, it would be helpful if the uncertainty in the measurements and calibrated parameters was developed in more detail to clearly assess the model validation, and if the Discussion were focused on a clear assessment of points to take forward based on this work, as at present this is a very broad range of possibilities.
Line Comments
L1- this is a key reason to study ice jams, it would be helpful if additional details were included in Introduction. For example, are the major historical incidents of flooding due to ice jams or any annual figures of damages in Quebec?
L16- This portion of the Abstract seems to generalise observations across sites, but the Results are very site-specific with different mechanisms for ice jam formation occurring at different sites. It could be helpful to set out in the Abstract that the mechanisms observed occurred at different confluences.
L19-20: This summary seems overly general as for much of the Results, comparison is not possible due to complexity of the field setting or the spatial scale of the jam formation mechanism.
Line 25: As above, it would be helpful if a bit more context could be provided motivating the study of ice jams due to the impact of them on society and the environment.
Line 45: This paragraph provides key information describing the theoretical basis of the study, the lab-derived observations and models by Ettema et al. At present the information is somewhat difficult to follow. It would be helpful if the different jam formation mechanisms were organised using a table or list so the reader can easily follow along.
L70/L80: It is not clear to me why these specific sites were chosen. Were the authors expecting to see a range of jam formation processes among them?
Table 2: ‘Ice’ is a vague descriptor—I assume this refers to ice thickness measurements. Could this be described in more detail, perhaps by defining a variable to use in the table header.
Section 4: The structure of this section would be easier to interpret if a table or list were used in the Introduction to clearly set out the previously observed processes and perhaps references for existing model equations for them. I realise this section comes before the Results, but it is a bit jarring to only see equations for some processes. Perhaps it would make sense to state that only processes 3,5,6 were observed and so Section 4 describes only these.
Results—the rationale for the structure of the results is unclear. I can see that the Results is organised by site, but why were the sites ordered in the current manner? To clarify the overall comparison, I would suggest either ordering the respective sites by increasing complexity, so that the clearest comparisons to existing models appear first, or to report the site results first, and then develop a comparison to existing models in the Discussion.
In each site description, I appreciate the narrative of breakup procedures but I am not sure why a given year was chosen for the narrative. I realise that the details of the other years is provided in the supplemental material to conserve space, but a bit more developed analysis in the description would be helpful. Could a bit more detail be provided so that the reader can understand if this is a typical year, with a short summary what variations were observed on the other 3 years.
As in the Summary above, inclusion of error in the Results (ex. estimated uncertainty for parameters in Table 4, Table 5, Table 6) would be helpful in understanding the model validation.
Table 4- Here, it would be helpful if the authors could comment using their expertise from a field perspective as to why these parameters are difficult to obtain in the field, requiring calibration. As a 3-parameter calibration necessarily limits the usefulness of the model, perhaps a discussion of future experiments or field measurements could be helpful in terms of looking to future research directions in order to understand these terms better.
Citation: https://doi.org/10.5194/egusphere-2025-6452-RC2
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- 1
The version of the manuscript (paper) that I have viewed is very good and worthy of publication. I do have a few comments, intended to enhance the paper. My comments are for the numbered paragraphs (e.g., In 10, I mean in Paragraph 10).
In 10: Replace "objective" with "objectives" and "is" to "are", as multiple objectives are stated.
In 40: Data are plural (datum is singular). So, replace "has " with "have".
Figure 1: Ensure the readability of this figure (actually, all figures). Some fonts are very small.
In 225: Use "All data were..."
Ln 290: Ensure the readability of this lengthy paragraph (here and there, elsewhere too). I suggest breaking the paragraph in two parts.
Table 6: I use "variables" rather than "parameters" to describe dimensional quantities. "Parameters" (to my mind) are dimensionless ratios determined from dimensional analysis.
In 650. Use "Conclusions" rather than "Conclusion", as you have several conclusions to state, and besides the you do not want to say: "The End".
Table A1: Be consistent with use of italics (e.g., in the Description you use text whereas under Symbol it [say F] is italicized).
Also, the authors should cite the 2008 book "River Confluences, Tributaries and the Fluvial Network", authored by Rice, Roy, and Rhoads, published by Wiley. Chapter 6 of this book concerns ice jams, giving several U.S. examples.