As icy rivers recede, nature's silent warnings of climate change grow louder, revealing its profound influence on our planet's delicate balance.
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River ice decline reflects the impact of climate change and human activities

As icy rivers recede, nature's silent warnings of climate change grow louder, revealing its profound influence on our planet's delicate balance.

Ice cover periodically forms on over half of the world’s rivers, estimated to occupy up to 120,000 square kilometers globally. Due to its nature, being relatively small in volume and occurring periodically, it is particularly vulnerable to climate change and human influence.

A detailed assessment of transformations in the occurrence of river ice cover allows us to understand how modern climate change and human activities are altering the natural environment of rivers and learn how to prevent the negative effects of these factors.

Studies of individual reservoirs indicate that their impact on ice cover at the local scale is signifcant, and their range of infuence can reach several hundred kilometers.

Maksymilian Fukś

The importance of river ice

River ice cover is a critical environmental element that affects nearly all river processes. It significantly shapes the physical and chemical properties of water, such as by reducing incoming solar radiation. As a result, ice cover affects river organisms, altering habitat conditions and food availability.

Scientific studies indicate that river ice impacts the rate of gas exchange between water and the atmosphere, playing an important role in releasing greenhouse gases (primarily CO2 and CH4) into the atmosphere. Ice phenomena in riverbeds also influence erosion and accumulation processes. For instance, researchers have demonstrated that the presence of ice reduces the ability of rivers to transport suspended material, resulting in decreased material transport during periods of ice.

Figure 1. Thermal breakup of ice cover on the Biała River, Central Europe
Credit. Author

The presence of ice cover is also crucial for human activities. In many areas,  especially in circumpolar climate zones like Alaska, frozen rivers serve local people as communication and transportation routes. This becomes particularly important in areas where the construction of traditional transportation routes such as (roads and railroads) is hampered.

Ice cover limits the possibility of navigation, necessitating icebreaking actions. Ice phenomena also affect humans by triggering jam floods and impacting hydrotechnical structures. Major jam floods, such as the 1952 Missouri River basin jam flood, resulted in damage amounting to hundreds of millions of dollars. The strong links between the occurrence of ice phenomena and air temperature make them excellent indicators of the impact of climate change on rivers.

How climate change impacts river ice phenomena

2023 review of scientific literature highlights the uncertainty surrounding the future of river ice in many parts of the world. The rise in air temperature during the second half of the 20th century has resulted in a worldwide reduction in the duration of river ice events.

Notably, there is a growing delay in the formation of ice cover, though the trends in the timing of ice cover formation vary spatially. In most areas of Europe, Asia and North America, ice cover is forming later and later, with delays ranging from 0.2 to 11.1 days per decade. However, some regions, like western North America and the Fennoscandian peninsula, have recorded earlier ice cover formation, although this is limited to specific locations.

Conversely, trends in ice cover breakup are much more uniform. In most areas studied, ice cover is breaking up earlier and earlier, with trends exceeding 10 days per decade. Eastern and central Europe, as well as large Asian rivers flowing into the Arctic Ocean, have recorded the most significant. Due to delayed ice formation and earlier breakup, many regions worldwide have observed a shortening of ice cover duration, reaching more than 10 days per decade.

In North America, trends are more spatially variable. While northeastern and northwestern parts of the continent, like New England and Alaska, have observed a shortening of ice cover duration by 10 days per decade, regions such as Atlantic Canada and the Rocky Mountains have recorded opposite trends.

In Eurasia, trends are relatively homogeneous, with most areas seeing a decrease in the duration of river ice cover exceeding 11 days per decade. The Yellow River in China records the most significant change, with a decrease of 11.5 days per decade between 1968 and 2001. Some areas also report the complete disappearance of ice cover in the second half of the 20th century, despite earlier annual occurrence, as seen in the lower Danube).  Moreover, many areas have reported a decline in the average thickness of river ice, accompanied by a decrease in its volume.

The impact of dam reservoirs on river ice cover

Dam reservoirs are structures that can significantly influence the occurrence of river ice cover both upstream and downstream. Firstly, thermal conditions in the reservoirs during winter cause an elevation in water temperature in the river below. This is due to the layered arrangement of water masses in the reservoir that differ in temperature, known as thermal stratification.

In persistent negative air temperatures, the water with the highest density (4°C) sinks to the bottom, pushing cooler water with lower density towards the surface. The release of warm bottom water into the river below the dam raises its temperature relative to its natural state, thereby reducing the formation of ice phenomena.

Reservoirs also restrict the migration of mobile ice forms from upstream, causing them to accumulate above the reservoir and preventing the development of a stable ice cover below. Additionally, reservoirs alter the volume of river flow. Increasing the flow during the winter season can accelerate the velocity of water in the riverbed, which translates into limiting the formation of a stable ice cover.

A review of scientific studies by Fukś demonstrates that the mechanisms by which reservoirs alter the river ice regime are still not fully understood. There is a shortage of studies examining a broader range of reservoirs with varying parameters, functions, and natural conditions, which would allow for a statistical assessment of the nature of their impact on the ice regime. In addition, the lack of studies based on a long series of satellite observations poses a challenge.

Previous studies indicate that dam reservoirs can transform the ice regime at local (single river) and regional (river basin) scales. The largest reservoirs, particularly those with dam heights greater than 60 meters, limit the formation of ice cover for up to several hundred kilometers downstream of their location, leading to the complete disappearance of ice cover in significant sections below.

Figure 2. Williston Lake reservoir (western part of the imagery) and the Peace River in Canada. On the river below the reservoir, ice cover is not present, even though it is present in all its tributaries. As a result of the reservoir’s operation, ice cover does not form between 100 and 300 kilometers below the dam. Sentinel-2 satellite imaging using near-infrared.
Credit. Copernicus Sentinel data 2023

Smaller reservoirs restrict the occurrence of ice cover over stretches of several to tens of kilometers. The reduction in the frequency of ice cover occurrence can vary from a few to tens of percent. Studies using mathematical models indicate that the degree to which reservoirs reduce the occurrence of ice cover will increase due to climate change. In many areas with a high concentration of reservoirs, their operation is the driving factor for the decline in ice cover occurrence, outweighing the influence of climatic conditions.

Interaction of climate change and operation of dam reservoirs

The rise in air temperature, the construction of dam reservoirs, and other factors like the thermal pollution of rivers will increasingly reduce the occurrence of ice phenomena in rivers in the future. This may lead to significant changes in the functioning of river ecosystems and the course of hydrological processes and force human adaptation to new conditions.

As research suggests, it is necessary to consider the operation of dam reservoirs for the accurate assessment of progressive transformations in the ice regime of rivers. This is particularly crucial given the significant number of large reservoirs, estimated to exceed 8,000, with dam heights exceeding 15 meters in areas where ice cover occurs.

Figure 3. Location of dam reservoirs (red dots) in areas of ice cover (blue area, Fukś, 2023).
Credit. Aquat Sci

Such a large number suggests that they may play an important role in shaping river ice conditions, particularly in Europe, central North America, and northeast Asia, areas where ice phenomena are shortest-lived.

Conclusions

As a result of climate change, later formation and earlier ice cover breakup can be expected in the future, as well as changes in other parameters such as its thickness. In areas where frozen rivers are used as roads, it is necessary to develop appropriate engineering solutions, such as advanced ice thickness monitoring networks, that allow their safe use in the future.

Since the operation of reservoirs exacerbates changes due to climatic conditions, considering their impact on the river ice cover is important when planning new developments. The construction of reservoirs in areas where the occurrence of ice cover is important in the functioning of the natural environment or human activities can exacerbate the negative effects resulting from the disappearance of ice.

On the other hand, dam reservoirs can reduce ice cover where its occurrence negatively affects humans, such as the formation of ice jam floods. Through appropriate engineering solutions such as reducing or increasing the reservoir’s impact on river water temperature, it is possible to thoughtfully control the occurrence of ice cover to maximize the positive aspects of reservoir operation for humans and minimize the negative ones for the environment.

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Journal reference

Fukś, M. (2023). Changes in river ice cover in the context of climate change and dam impacts: a review. Aquatic Sciences85(4), 113. https://doi.org/10.1007/s00027-023-01011-4

Maksymilian Fuks is a doctoral student at the Institute of Geography
and Spatial Organization of the Polish Academy of Sciences. His scientific interests focus on hydrology and the application of advanced tools in hydrological research. He is currently working on anthropogenic transformations of ice and thermal regimes of mountain rivers.