It’s that time of year again, when the sun has crossed over the equator, and spring is bringing everything in the Northern Hemisphere to life. The storms abate, the temperatures rise, and if you live where it’s cold enough in winter, you see the snow and ice begin to melt away. If you live where it’s really cold, that ice includes big, stationary bodies of water, and rivers. Rivers that may have ice thick enough to drive a car on, eventually clear themselves in what is called the spring break-up. It happens when the strength of the ice becomes less than the pressure of upstream water pushing down the river channel. Because there are a multitude of factors affecting this process, the moment it may occur is not knowable, except within a range of time that it has happened historically. This period of possibility is typically days or weeks long. People living along iced-over rivers have betting pools on exactly when the ice will break up. Watching for river ice break-up is especially popular in Alaska, where there are so many frozen rivers.
At Nenana, Alaska, on the Tanana River, a tripod is placed out on the ice, with a tether attached to a clock on shore, and when the ice breaks, the clock is stopped. Bets come in from around the world from anyone who thinks they can predict the date and time. With climate change, the possible date range for the break-up has shifted earlier.
River ice breaks up in a couple of different ways. The melting ice may thin and weaken in place, eventually crumbling in the flow that has increased as runoff has grown upstream. This is known as a thermal break-up, but Alaskans call it a “mush-out.” This slow-motion process results from gradually warming days, and freezing nights. If warming is rapid and snowfall at higher elevation has been heavy all winter, runoff can quickly grow, sometimes aided by spring rains. The massive water pressure from upstream is then able to lift much thicker ice and break it by force. Curiously, heavy snowfall on the river itself can make for a thinner layer of ice because the snow’s power of insulation keeps temperatures from falling so low. The timing of cold spells in autumn and the kinds of precipitation add to the complexity of river ice buildup. Things other than the temperature and water volume also play into the break-up. The curvature of the channel, its depth, and width affect the timing and behavior of the event.
Source: Wikipedia: Cantwell, J. C.
At some point in the spring, the ice gives way. Besides witnessing a spectacle of nature in transition (and the fun of guessing the moment and winning the pot), why should anyone care? The unsticking of a stretch of solid water brings with it a number of environmental effects. Of course the underwater inhabitants of the river are once again able to access the surface, and perhaps of more importance to them, land creatures can once again go fishin’ and threaten their existence. Water temperatures will begin to rise, sometimes suddenly and markedly, signaling changes in natural processes, and increasing biologic activity. In what is called a dynamic break-up, ice will tear away from the banks, pulling in sediment and detritus. Large ice chunks may pile up in the stream and tumble, scraping the river bed, a process known as ice scour. Sometimes the sheer volume of broken-up ice may clog the stream and cause the flow to back up—an ice dam, or ice jam. The surge of water following the release of any ice blockage further roils the water. Ice shoving, where masses of ice are piled along the banks, can reach several meters above water level and take out shrubs and even trees. All this gouging and scraping and turbulence raisies the water’s sediment content, which affects its chemistry and nutrient supply. This is key to the growth of insect and fish life in both positive and negative ways. Added nutrients are beneficial, but muddy, cloudy water can interrupt a filter-feeder’s ability to eat, or impact visibility for fish trying to hunt down their prey.
Source: Wikipedia: Unknown
Ice dams may impede flow to a great extent, if enough ice is involved. The massive river back-up behind them can push damaging floodwaters into human settlement. A thirty-mile-long ice jam on the Yukon River in Alaska inundated the entire town of Galena in spring of 2013, but minor flooding is a routine spring occurrence along many frozen rivers. Alaskan rivers are often wide and shallow, and of course the winter temperatures are extremely low, so freeze-up is the norm. Flooding from ice dams creates dangerous situations, because waters can rise fairly quickly, and may catch inhabitants by surprise. Interestingly, the danger of ice jams is less severe if the river flows in a direction opposite to the movement of regional warming. In the Northern Hemisphere, that means that southward-flowing rivers should be less likely to suffer the effects of ice dam flooding. Lower reaches should become clear of ice before colder, more northerly waters melt and flow southward. But the Mississippi can experience ice jams too, so even a big, deep river flowing south is not immune. Further, the whole mechanism of ice break-up, ice dams, and water surge can be repeated sequentially down the river, starting and stopping as the flow encounters varying conditions. It’s not an all-at-once event, or even a smoothly progressing one, and until all the ice has melted or broken into tiny chunks, riverside settlements are wise to keep abreast of the situation upstream when spring warming turns the seasons.
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