Wild Side: Rivers of ice are powerful
The North American Laurentide ice sheet was an enormous river of ice that covered most of Canada, the Upper Midwest, New England and parts of Montana and Washington. It bulldozed our region several times, conveying sediment and rocks from the north. The most recent maximum advance was about 21,000 years ago.
The glacier finally retreated about 10,000 years ago leaving behind bare land with lumpy topography, lake basins, and moraine hills. Today we can look at the local rivers of ice and see some of the same processes at work.
Water is a strange compound. Its properties are life giving and unlike other materials. In its liquid form water readily absorbs and loses heat. Water gives up a lot of heat as it freezes. This latent heat of fusion plus the heat stored in the water column is what keeps lakes and rivers open well into the winter.
Water becomes less dense as it cools from 39 degrees Fahrenheit to freezing. That's why the water temperature profile of lakes is reversed in winter, with the warmer water at the bottom and the coldest water on top. We're very fortunate that this quirky substance expands as it freezes. Otherwise, our lakes and oceans would freeze from the bottom up and ice cubes wouldn't float in our beverage glasses.
Black (congelation) ice forms as the latent heat of fusion is conducted upward through the ice and snow to the atmosphere. Black ice is clear, allowing light to shine through to the water below.
White (snow) ice forms when the weight of snow overcomes the buoyancy of the ice. Water flows upward through cracks in the ice. The snow becomes saturated and then freezes.
Ice formation in rivers is more complex than in lakes because of water movement and turbulence. Unlike in lakes, the turbulent mixing causes the entire water column to cool uniformly even after the temperature has fallen below 39 degrees, the maximum density of water.
There are a number of kinds of ice that form in rivers. Border ice forms along the shore. Anchor ice is attached to the bed of the river. Frazil ice is small needles or circular plates of ice suspended in water. Sheet ice is a continuous cover of ice formed by expanding border ice, accumulations of frazil ice, or formed by pieces of ice jamming together and freezing.
Our local rivers are often covered with sheet ice during longer periods of below zero weather. They open back up as the temperature moderates because of the relatively warm ground water base flow.
Canoeing rivers in winter is fun but it is dangerous if there is sheet ice across the river. It's best to wait for nice sunny days to enjoy a winter float when it warms above freezing and the river opens up.
Moving ice is powerful. Wind-blown ice pushed west onto the shore of Lake Mille Lacs in the spring has mowed down cabins, trees, and covered Highway 169. Rock piles and shoreline disturbance caused by ice push is evident on many lakes and rivers in this region.
Ice-out on rivers can be dramatic with loud cracks of breaking sheet ice that sound like cannon fire. Broken ice can form jams acting as dams and flooding areas upstream. Torrents of ice and water scour the river bed, banks and floodplain when ice dams break. We look forward to watching the natural drama when the rivers of ice make way for spring.
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