Ice processes in streams
Why is ice and winter disturbance important for river ecosystems?
As much as 60 % of the major river basins north of the equator experience ice processes during winter. Although the volume of ice in rivers constitutes a small part of the cryosphere it plays an important role in freshwater ecosystems, especially since small streams constitute a large part of the boreal landscape. In northern river systems, dynamic ice formation is common during winter when air temperature is low and water is super-cooled. Different types of ice characterize the rivers and the formation of ice is dependent upon different conditions such as local stream flow, meteorological factors and topography.
How are winter processes important for plant ecology?
Traditionally, plant ecologists have focused their studies on the summer period, largely ignoring the fact that processes during winter also impact vegetation dynamics. For example, the freeze-up period in early winter may result in extensive formation of underwater ice that can restructure the channel, obstruct flow, and cause flooding and thus formation of more ice. Anchor ice dams can almost double the wetted area and reduce flow velocities of streams. During damming, the water may freeze solid in the riparian zone, thereby also freezing riparian vegetation. When an anchor ice dam drains and the stream water recedes, the weight of the ice rests mainly on the riparian vegetation and can rip away the plants and erode the banks. In midwinter, slow-flowing reaches develop a surface-ice cover that accumulates snow, protecting habitats under the ice from formation of underwater ice but also reducing underwater light, thus suppressing photosynthesis. Towards the end of winter, ice breaks up and moves downstream. During this transport, ice floes can jam up and cause floods and major erosion. The magnitudes of the floods and their erosive power mainly depend on the size of the watercourse, also resulting in different degrees of disturbance to the vegetation.
What influences the ice formation?
Significant changes in the ice conditions could develop due to changes in almost any of the major meteorological fluxes associated with climate change (e.g. temperature, precipitation) which could have important geomorphological, ecological and socio-economic impacts. Human impacts, such as flow regulation, channelization, agriculturalization and water pollution have modified ice dynamics. Understanding and predicting ice formation are important because it can have large impacts and implications on, for example, fish and vegetation communities, hydropower production and infrastructure in close vicinity to the river channel.
Frazil ice: tiny ice particles that forms on the water surface during sub-zero weather and can be mixed into the water column. Frazil ice has adhesive features in super-cooled water and may attach to submerged objects forming anchor ice.
Super-cooling: or undercooling, when the temperature for water is lower than its freezing point without it freezing to ice.
River basin: the entire geographical area drained by a river and its tributaries.
Cryosphere: the frozen water on Earth´s surface. This includes sea ice, river ice, lake ice, glaciers, ice caps, ice sheets and frozen ground.
Lind, L., Nilsson, C., Polvi, L. E. & Weber, C. (2014). The role of ice dynamics in shaping vegetation in flowing waters. Biological Reviews, 89: 791-804.
Lind, L., Nilsson, C & Weber, C. (2014). Effects of ice and floods on vegetation in streams in cold regions: implications for climate change. Ecology and Evolution, 4: 4173-4184.
Weber, C., Nilsson, C., Lind, L., Alfredsen, K. T. & Polvi. L. E. (2013). Winter disturbances and riverine fish in temperate and cold regions. BioScience, 63: 199-210.
Prowse, T. (2001). River-ice ecology. I: Hydrology, geomorphic, and water-quality aspects. Journal of Cold Region Engineering, 15: 1–16.