Reply to Hall et al. Comment on “Beltaos, S. Ice Jam Flooding of the Drying Peace–Athabasca Delta: Hindsight on the Accuracy of the Traditional Knowledge and Historical Flood Record. Environments 2025, 12, 376”

I thank the commenters (Hall et al. 2025 [1]) for their interest, but it appears that they did not discern what is “new knowledge” in my “latest” paper. For their benefit, the new knowledge generated by Beltaos (2025 [2]) consists primarily of the following:

• Validation of the H-TK record of large ice jam floods (IJFs) using three hydroclimatic criteria, discharge magnitude, freeze-up elevation, and winter snowfall as a proxy for years in which discharge is unknown;
• Detection and quantification of substantial uncertainty about the actual occurrence of the H-TK record’s lesser (moderate/small) events using water surface elevation differences between L. Athabasca and the junction of the Riviere des Rochers with the Peace River.

The claim of inadequate reference to prior research is unfounded. The mentioned prior research involves work by the commenters and addresses two related but distinct topics: (a) the question of whether regulation has contributed to the scarcity of ice jam flooding of the Peace–Athabasca Delta (PAD), which is the topic of the 2019 discussion–reply sequence mentioned by the commenters, is beyond the scope of Beltaos (2025 [2]) and (b) the accuracy of the H-TK spring flood record, which is also the subject of Beltaos (2025 [2]). After noting in my Introduction an overall favourable assessment of the accuracy of the H-TK record by others, I stated that “The accuracy of the H-TK record has been assessed much less favourably by the authors of paleolimnological studies within the PAD, who used proxy evidence to infer a very different flood history during the 20th century” (citing three references pertaining to paleo studies, including Wolfe et al. (2020 [3])). Therefore, the commenters’ “prior research” on the accuracy of the H-TK record was duly acknowledged, while the difference in prior opinions about the accuracy of the H-TK record was one of the motives for Beltaos’ (2025 [2]) work.

Figure 5 of Beltaos (2025 [2]) was a by-product, not a “key product”, of the H-TK record accuracy assessment. It was presented in the Discussion Section as an update from Beltaos (2018 [4]) that now carries increased confidence and includes 8 more years of data (2018–2025). The increase in confidence stems from the aforementioned finding regarding the validation of the H-TK record of large IJFs. There was no discussion in Beltaos (2025 [2]) of statistical significance or why the large IJF frequency has been reduced after regulation, two key points in the 2019 discussion–reply sequence mentioned by the commenters. This sequence addresses the question of possible regulation effects on the scarcity of ice jam flooding and is beyond the scope of Beltaos (2025 [2]). Though irrelevant to the content of Beltaos (2025 [2]), I note for the record that my reply to both Hall et al. (2019 [5]) and Timoney et al. (2019 [6]) thoroughly debunked their “important criticisms”, as indicated in that reply’s abstract: “The writer systematically refutes all of the discussers’ criticisms and supplies more details on the rigour of his “slope-based” frequency analysis. He also identifies key flaws of alternative and conflicting statistical interpretations proposed by the two discussion teams and provides detailed documentation of the large uncertainty associated with paleolimnological assessments of ice-jam flood frequency” (Beltaos 2019 [7]).

Incredulously, the commenters went on to claim that “…the author’s analytical approach and the conclusions are no different from those previously communicated in the 2018 article…”. {Here, the “2018 article” refers to Beltaos (2018 [4])}. Table 1 aptly demonstrates the falsity of this claim.

Those readers whose interest goes beyond the accuracy of the H-TK record can find a summary of the regulation-effect debate in Beltaos (2023 [8]). This summary cites many articles and reports published during the period 1993 to 2023 and includes those mentioned by the commenters.

Table 1. Comparing summaries, Beltaos (2018 [4]) versus Beltaos (2025 [2]).

Summary of Beltaos (2018 [4])

Summary of Beltaos (2025 [2])

A robust approach for quantifying the frequency of ice jam floods, based on cumulative numbers, has been developed and applied to the historical record associated with the Peace–Athabasca Delta. The results of this study support previous physically based findings indicating that regulation has played a role in the drying trend that has been experienced after 1967.

The H-TK record of Peace River ice jam floods near the PAD, which was published in 1995, can inform research and decision making pertaining to actions that can help recharge the drying Peace–Athabasca Delta. Its accuracy over the years 1900–1992 has been assessed in the light of current understanding of relevant physical processes and of local hydrometric datasets for Peace River at Peace Point, Riviere des Rochers, above Slave River, and Lake Athabasca at Fort Chipewyan. Reported large ice jam floods typically involve witnessed overland flooding at one or more locations along the lower Peace River, while reported lesser events were typically inferred from observed reversals of Peace River tributaries towards Lake Athabasca. Based on earlier numerical modelling results, it was first noted that single-location overland inundation implies more extensive flooding because ice jams need to be tens of km long before they can cause overtopping of the riverbanks. Hydroclimatic indicators of a large IJF, such as breakup flow, winter snowfall at Grande Prairie, and freeze-up level, have been favourable for large IJF years, with the possible exception of the 1942 event. Overall, the H-TK record of large IJFs is shown to be reliable, in the sense that all or nearly all the reported events did occur. However, the possibility that some large events were missed, e.g., the 1972 large IJF, cannot be precluded. The available evidence suggests that 1967 may have also been a large IJF year. Frequency estimates indicate that large IJFs were about twice as frequent under natural conditions (1900–1967) as under the regulation regime (1972–2025). Unlike for the large IJFs, the record of lesser events (small, moderate), which is largely based on inference pertaining to tributary flow reversals, may not be reliable: hydrometric records and field observations have shown that the Riviere des Rochers and thence the Chenal des Quatre Fourches often reverse in non-event years. Consequently, the primary value of the H-TK record is in the set of large IJFs, which are the predominant agents of PAD basin recharge. The present validation of the large events adds confidence in the H-TK record, which is partly based on Indigenous knowledge. In turn, this reinforces climate-related projections of severe reductions in large-flood frequency during this century and underscores the need for implementation of remedial measures like Strategic Flow Releases. The success of such measures will partially depend on comprehensive monitoring activities, including Indigenous community-based monitoring.

Table 1. Comparing summaries, Beltaos (2018 [4]) versus Beltaos (2025 [2]).

Summary of Beltaos (2018 [4])

Summary of Beltaos (2025 [2])

A robust approach for quantifying the frequency of ice jam floods, based on cumulative numbers, has been developed and applied to the historical record associated with the Peace–Athabasca Delta. The results of this study support previous physically based findings indicating that regulation has played a role in the drying trend that has been experienced after 1967.

The H-TK record of Peace River ice jam floods near the PAD, which was published in 1995, can inform research and decision making pertaining to actions that can help recharge the drying Peace–Athabasca Delta. Its accuracy over the years 1900–1992 has been assessed in the light of current understanding of relevant physical processes and of local hydrometric datasets for Peace River at Peace Point, Riviere des Rochers, above Slave River, and Lake Athabasca at Fort Chipewyan. Reported large ice jam floods typically involve witnessed overland flooding at one or more locations along the lower Peace River, while reported lesser events were typically inferred from observed reversals of Peace River tributaries towards Lake Athabasca. Based on earlier numerical modelling results, it was first noted that single-location overland inundation implies more extensive flooding because ice jams need to be tens of km long before they can cause overtopping of the riverbanks. Hydroclimatic indicators of a large IJF, such as breakup flow, winter snowfall at Grande Prairie, and freeze-up level, have been favourable for large IJF years, with the possible exception of the 1942 event. Overall, the H-TK record of large IJFs is shown to be reliable, in the sense that all or nearly all the reported events did occur. However, the possibility that some large events were missed, e.g., the 1972 large IJF, cannot be precluded. The available evidence suggests that 1967 may have also been a large IJF year. Frequency estimates indicate that large IJFs were about twice as frequent under natural conditions (1900–1967) as under the regulation regime (1972–2025). Unlike for the large IJFs, the record of lesser events (small, moderate), which is largely based on inference pertaining to tributary flow reversals, may not be reliable: hydrometric records and field observations have shown that the Riviere des Rochers and thence the Chenal des Quatre Fourches often reverse in non-event years. Consequently, the primary value of the H-TK record is in the set of large IJFs, which are the predominant agents of PAD basin recharge. The present validation of the large events adds confidence in the H-TK record, which is partly based on Indigenous knowledge. In turn, this reinforces climate-related projections of severe reductions in large-flood frequency during this century and underscores the need for implementation of remedial measures like Strategic Flow Releases. The success of such measures will partially depend on comprehensive monitoring activities, including Indigenous community-based monitoring.

As for Timoney (2024 [9]), I was aware of that paper but had found it to be a source of misinformation and confusion; therefore, it was not to be cited unless the omission was questioned. Appendix A details key reasons for my reticence regarding the citation of Timoney (2024 [9]).

Though I disagree with the commenters’ various points, I appreciate their mentioning of my replies to their prior criticisms. Though not always followed, this practice is consistent with scientific integrity principles and ensures that readers will not be misled into thinking that no replies exist.