River ice in a shrinking cryosphere
Warmer wetter winters will reduce ice jams and change river ecology
This seems like a perfect time to consider the effect that warmer winters might have on watersheds in the Northeast. Changes in the hydrology of the Hudson-Mohawk watershed are expected as we transition to warmer and wetter winters. As explored below, we are already seeing differences in the frequency and intensity of precipitation, and in warmer winter temperatures (despite the ongoing chill). It is a worthwhile exercise to consider how these changes may affect flooding, ice jams, hydropower, river ecology, and water quality. Certainly the river ecosystem in 30 years will be different, and changes are probably going to be driven by warmer winters. While we don’t have a crystal ball, there are a number of changes that are ongoing and the rate of change that we see today may help us understand future conditions.
The cryosphere is the part of the Earth system with frozen water, and it enlarges and contracts seasonally. Warming temperatures in the Northeast US mean that the seasonality of ice on rivers and lakes is changing, and this change has important implications for river ecology, water quality, and ice hazards.
A recent and accessible summary of projections of temperature and precipitation for NY can be found in the “Northeast” chapter of “Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment” by Lesley-Ann L. Dupigny-Giroux and colleagues [01]. They point out that under several emission scenarios, total rainfall and rainfall intensity are expected to increase [see also 02]. They also note that as winters continue to warm in the Northeast, there will be a decrease in the freezing season, an increase in the number of winter rain events, fewer days with snow on the ground, and an earlier snowmelt. Much of this change is in response to the shift northward of the snow–rain transition zone [01].
Ice-jamming is a cold-winter process, and given the finding that climate change scenarios tend to project warmer winters in the Northeast, it’s logical to conclude that in the future ice jams will be less of a problem. Warming winters and more precipitation will certainly affect our approach to future floodplain management and how we build resiliency into river-lining communities.
Canadian observations. Naturally we can look to Canada for expertise with ice. An influential paper by Spryos Beltaos and Terry Prowse, both with Environment Canada, published in 2008 is entitled “River-ice hydrology in a shrinking cryosphere” [04]. The reduction in the length of the freezing season and ice cover may have important effects. They note: “This [change] may have wide-ranging consequences because ice is a critical component of cold-regions hydrologic systems and strongly affects, for example, extreme floods, low winter flows, river transport, hydroelectric production, and numerous ecological and water quality characteristics.” [04]. They also point out that we can expect more mid-winter breakup and associated jamming events. One could argue that on the Mohawk we have already transitioned to these mid-winter events, the first step in this progressive change [09,10,11]. The next step would be reduction and elimination.
In an analysis of the Canadian River Ice Database, de Rham and colleagues show that from 1985-2015 there was an overall reduction of the duration of complete ice cover (73% of the sites), and 20 sites show a reduction at an average rate of ~11 days per decade [05]. A recent study by Benoit Turcotte and colleagues involves understanding change in rivers in southern Quebéc, just north of the Mohawk [06]. Their modeling shows that warming will reduce ice formation and ice-jam occurrence in studied rivers in southern Quebec (but not in northern rivers). The duration of the winter period when ice-jam breakup events can occur gets shorter, mid-winter events become more likely, and the overall probability of spring breakup events (end of season) is reduced by about half [06]. They note:
Observations from the Northeastern US also propose that the short duration and low intensity of cold periods combined with dominant above-freezing temperatures and more frequent rain events are preventing the formation of a complete ice cover at an increasing number of locations... In addition...mid-winter and spring breakup ice jams in Midwest US were becoming less frequent, but potentially more intense [06].
Warming Winters on the Mohawk. Let’s look at winter temperatures in Upstate New York and see how things have changed over the last few decades. The best long-term climate records for the Mohawk are from the National Weather Service in Albany, and they are collected in the very lowest part of the watershed (near the confluence with the Hudson). Bear in mind that this means the temperatures elsewhere in the watershed, which are at higher elevations, would be slightly colder, but the Albany record provides important insight of overall trends [03].
To do this we can use data from NOAA’s National Centers for Environmental Information, and the useful “Climate at a Glance” that allows user-controlled plots of precipitation, temperature, etc. You can access this database for anywhere in the US, but here is the link for Albany NY [03]. While sifting through these data sets I am interested in how winter temperatures have changed in Albany since the records began in 1895. How has the mean temperature of the main winter months – December-January-February (DJF), or just the coldest months (JF) changed over time?
Mean temperatures in these months matter for the total accumulation (or growth) of ice on lakes and rivers. Ice accumulation (and therefore thickness) is primarily a function of freezing degree days (FDD), warming degree days (WDD), and snow cover. Mean temperature in the winter months provide an intuitive metric for the two primary controls on ice (FDD and WDD). The temperatures for December (27.1°F or -2.7°C), January (22.3°F or -5.4°C) and February (23.4°F or -4.8°C) are the only months of the year in Upstate NY that have mean temperatures below freezing, and hence these are the months to focus on for the formation of ice.
Plot showing mean winter temperatures (December, January, February – DJF) at Albany NY since 1900. Blue line is the trend of all data. Black dotted line is the mean for all years since 1895. The green line is a LOESS filter, which is similar to a moving average but it puts a lower significance on outlying points. Data source: NOAA [03].
Winter temperatures (December, January, February - DJF). Since 1895 (or 1900), the mean DJF temperature is 24.9°F (-3.9°C), with a long-term increase (warming) of +0.4°F (0.22°C) per decade. The six warmest winters have been since 1997, and of these six winters, the mean DJF temperature was above freezing in three years: 2011, 2001, and 2015. Warming is most distinctive since 1980 and has increased in each decade since. From 1980 to 2020 the mean DJF temperature was 26.0°F (-3.3°C) and the trend is an increase of +1.0°F (0.56°C) per decade. From 1990 to 2020 the mean was 26.8°F (-2.9°C) with a +0.9°F (0.50°C) increase per decade. Finally, from 2000-2020 – the last 20 years - the mean is 27.5°F (-2.5°C) with a +1.4°F (0.78°C) increase per decade. At this rate of change, the mean winter temperature will be above freezing in just over three decades (between 2050 and 2060). (The 2020-21 DJF winter is obviously not over, but the current mean temperature for Dec through mid Feb is ~25°F - close to the long-term average, but lower than the last few years).
“Deep Winter” Temperatures (January and February - JF). For the coldest two months of the year (Jan-Feb or here JF) are when we typically have significant ice accumulation. In recent years, however, temperature excursions in the deep winter have resulted in mid-winter ice jams, some very large [09,10]. Since 1895, the mean JF temp was 23.4°F, with an increase (warming) over the entire period of +0.4°F (0.22°C) per decade. In this interval, the six warmest winters have all been since 1990, and two winters (2002 and 2017) had mean temperatures above freezing. Since 2000, the mean JF temperature is 26.0°F (-3.3°C) and this 20-year period shows a warming trend of 1.4°F (0.78°C) per decade. If this rate continues, mean winter temperatures will be above freezing after 2060.
So no matter how you slice it, our winters are getting warmer, a condition not favorable to ice formation.
Mohawk Projections. At the 2014 Mohawk Watershed Symposium, Steve Shaw and Ashley Ryan, both then at SUNY ESF in Syracuse, presented the first model that attempted to predict the future ice-jam potential on the Mohawk in a warming world [07]. Their model predicts ice-jam probability based on the number of freezing degree days and warming degree days. For historical ice jams – here the last 70 years on the Mohawk – the model correctly predicted appropriate ice-jam conditions 81% of the time. They then used an emission scenario (see below) to look at potential ice accumulation in future. I recently contacted Steve and asked him about the status of their research and he was kind enough to send a copy of Ashley’s MSc thesis that has a full analysis of their approach [08].
An important issue that they had to wrestle with was the lack of historical data of ice thickness. They looked at a few rivers, and only the Piscataquis in Maine has a robust data set for historical observations of measured ice thickness [see 08]. The Mohawk River does not, and this needs to change. Nonetheless Ashley used her thermal modal and observed ice thickness on the Piscataquis to test the model for ice-thickness prediction, which is based on FDD (and WDD). With verification of that model, she then applied the approach to the Mohawk River. She shows that ice thickness is the most important variable for the prediction of ice jams.
Plot of modeled historic ice thickness (blue) and possible future ice thickness on the Mohawk River based on a warming scenario (explained below). These data are from Ashley Ryan’s MSc thesis, and here I combined her graphs that show: 1) past; and 2) future modeled ice thickness.
Ice thickness can be modeled based on the number of freezing degree days. In this analysis, known temperatures for the historical record were used to estimate ice thicknesses and these were then correlated to years with ice jams. Ashley Ryan’s model correctly predicted years with ice jams about 80% of the time – most occur when ice thickness is greater than one foot. She then modeled future conditions using local data from a Global Climate Model that provides daily temperature and precipitation for 2050 to 2080 using the GFDL CM3 model with the A2 emission scenario. NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) has developed ocean-atmosphere coupled models that are then run using different emission scenarios. You can read about CM3 here, and here, and the A2 emission scenario, which is on the higher end, is explained here (in A2, CO2 is projected to be 538 ppm by mid-century). In this warming scenario, the occurrence of ice jams is substantially reduced – largely because warmer winter temperatures result in thinner ice (with some years with little or no ice).
Conclusion. River ice is part of the natural ecosystem in northern temperate rivers. Warmer winters are occurring in the Northeast, including in the Mohawk Watershed. If the decadal trend of warming continues, we will soon routinely have winters with mean temperatures above freezing, which will have far-reaching effects on the hydrology and ecology of the river. This trend will tend to reduce ice thickness, and that may well mean that the ice jam hazard lessens over time. It may also mean that higher flows occur in the winter and that the spring floods are reduced in size.
However, these change do not mean floods are going away - in fact we may see the opposite. In the last 100 years, precipitation has increased in the Northeast, as have extreme precipitation events, and this change to wetter conditions may have the effect of causing more frequent and more severe floods [01]. A recent paper by M.J. Collins suggests that floods that occur later in the season (September to November or SON) may become more common [12]. Frei and colleagues show that warm-season extreme events have increased throughout much of the 20th century [13].
Thus the overall conclusion of a number of studies is that extreme precipitation has increased in the Northeast in the past few decades, with a high frequency of those events in the warm season, but not during the cold season [13]. Was the Halloween Flood of 2019, which was so devastating on the East and West Canada Creeks in the upper part of the Mohawk Watershed, an indication of things to come?
This and other Notes from a Watershed are available at: https://mohawk.substack.com/
Further Reading
[01] Dupigny-Giroux, L.A., E.L. Mecray, M.D. Lemcke-Stampone, G.A. Hodgkins, E.E. Lentz, K.E. Mills, E.D. Lane, R. Miller, D.Y. Hollinger, W.D. Solecki, G.A. Wellenius, P.E. Sheffield, A.B. MacDonald, and C. Caldwell, 2018: Northeast. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 669–742. Available here.
[02] Howarth, M.E., Thorncroft, C.D. and Bosart, L.F., 2019. Changes in extreme precipitation in the northeast United States: 1979–2014. Journal of Hydrometeorology, 20(4), pp.673-689. Available here.
[03] NOAA National Centers for Environmental information, Climate at a Glance: City Time Series, published January 2021. Website here.
[04] Beltaos, S., and Prowse, T. (2009). River‐ice hydrology in a shrinking cryosphere. Hydrological Processes, 23(1), 122-144.
[05] de Rham, L., Dibike, Y., Prowse, T.D. and Beltaos, S., 2019. Overview of a Canadian River Ice Database Derived from Water Survey of Canada Hydrometric Archives. In Proceedings of the 20th Workshop on the Hydraulics of Ice Covered Rivers, Ottawa, Canada (available here).
[06] Turcotte, B., Morse, B. and Pelchat, G., 2020. Impact of Climate Change on the Frequency of Dynamic Breakup Events and on the Risk of Ice-Jam Floods in Quebec, Canada. Water, 12(10), p.2891. (Available here).
[07] Shaw, S.B., Ryan, A.M., Predicting occurrence of Ice jam flooding on the Mohawk River at the end of the 21st century, In Cockburn, J.M.H. and Garver, J.I., Proceedings of the 2014 Mohawk Watershed Symposium, Union College, Schenectady, NY, March 21, 2014, p. 39-40.
[08] Ryan, A., 2014. An investigation of flood hydrology on the Mohawk River. MSc Thesis, State University of NY College of Environmental Science and Forestry, Syracuse NY. 93 p.
[09] Garver, J.I., 2019. The 2019 mid-winter ice jam event on the lower Mohawk River, New York. In: Garver, J.I., Smith, J.A., and Rodak, C. 2019. Proceedings of the 2019 Mohawk Watershed Symposium, Union College, Schenectady, NY, March 22, 2019, Volume 11, p. 12-17. (Download paper here).
[10] Garver, J.I., 2018. Ice Jam flooding on the lower Mohawk River and the 2018 mid-winter ice jam event. In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2018 Mohawk Watershed Symposium, Union College, Schenectady NY, 23 March 2018, v. 10, p. 13-18. (Download paper here).
[11] Garver, J.I., 2014. Insight from Ice Jams on the Lower Mohawk River, NY. In Mohawk Watershed Symposium 2014. (Download paper here)
[12] Collins, M.J., 2019. River flood seasonality in the Northeast United States: Characterization and trends. Hydrological Processes, 33(5), pp.687-698. (Download paper here).
[13] Frei, A., Kunkel, K. E., and Matonse, A. (2015). The seasonal nature of extreme hydrological events in the Northeastern United States. Journal of Hydrometeorology, 16, 2065–2085. https://doi.org/10.1175/JHMD‐14‐0237.1
[14] Huang, H., Winter, J. M., & Osterberg, E. C. (2018). Mechanisms of abrupt extreme precipitation change over the northeastern United States. Journal of Geophysical Research: Atmospheres, 123, 7179–7192. https://doi.org/10.1029/ 2017JD028136