Winter floods are important natural 'disturbances' for maintaining species-rich riparian zones along northern watercourses. Movies like The Day After Tomorrow and An Inconvenient Truth exaggerated some of the big effects of a climate gone crazy but less attention is given to smaller, realistic scenarios, like a disturbance in existing winter floods.

Riparian forests are important as they supply habitat, store carbon, provide shading, and filter water. Ice formation and winter floods are significant factors for vegetation and wildlife in northern regions.  According to Ph.D. candidate Lovisa Lind at Umeå universitet, during cold winter days, frazil ice (tiny ice particles) forms in the super-cooled water of open turbulent stream reaches.

Researcher Lovisa Lind. Photo: Gina Lind

Frazil ice has adhesive features and may attach to submerged boulders and other objects in the water, forming anchor ice. As a consequence, water can be dammed by the anchor ice causing flooding and ice growth in the riparian zone. When the air temperature rises the anchor ice detaches, floats away and might jam up and lead to new floods and even more ice – the stream has become an ice machine.

In her thesis work, Lovisa Lind investigated which factors control ice formation in northern watercourses, and how and to what degree ice and winter floods affect the vegetation along streams. She has inventoried ice and winter conditions along tributaries to the Vindel River in northern Sweden and related them to differences in vegetation. Her results show that turbulent, steep streams far away from a lake outlet have a high production of frazil and anchor ice, and consequently a high number of winter floods.

Field picture of winter stream. Photo: Lovisa Lind

“This type of stream has a more species rich riparian vegetation than streams without frazil and anchor ice production. This is because competitive species of evergreen dwarf shrubs are eliminated by ice and winter floods, and that opens up patches for less competitive species of forbs and grasses,” says Lind.

The ice formation in the streams also affects the vegetation in the water through scouring and pulling away bryophytes and algae. Ice break-ups during late winter can also have implications for the riparian forest by bending, breaking and pulling away vegetation, thus opening up patches for new species to colonize.

Meteorologists predict that winter temperatures will vary more, which will lead to more shifts between freezing and thawing. If the mean temperature rises with only a few degrees, this type of ice formation can even be lost. Today, the ice formation in northern systems is already affected by for example dams, channelzation, agriculture and pollution.

Since northern Sweden has a large number of small watercourses, extensive areas could potentially be affected by climate change induced shifts in ice dynamics.

“If the disturbance from ice and winter floods would be lost, the riparian vegetation in northern watercourses could potentially be more dominated by dwarf shrubs. This could have further implications for the fauna and the stream productivity,”says Lind.