Rtver Tce Modification

There are a variety of means of modifying ice in rivers. Icebreaking vessels are used to clear paths for other vessels and occasionally to assist in relieving jams on large rivers. Icebreakers are used extensively in northern Europe and to some extent on the Great Lakes and St. Lawrence River of North America. Dusting the ice cover with a dark material such as coal dust or sand can increase the absorption of solar radiation and thus create areas of weakness that aid in an orderly breakup. Dusting has limited effectiveness, however, if a later snowfall covers the dust layer. Trenching of the ice cover with a ditching or similar machine has been practiced to create a weak zone in areas that are historically prone to jamming. Once ice jams have formed, they are sometimes blasted with explosives. However, if there is no current to transport the ice away after blasting, such measures are usually of little effect.

Ice-retention structures such as floating ice booms are used to hold ice in place and prevent it from moving downstream, where it might cause problems. There have been some attempts to control water releases from dammed reservoirs so as to induce breakup in an orderly manner, but these measures are limited to a narrow range of conditions. Air bubbler systems and flow developers (submerged motor-driven propellers) are used to melt small portions of the ice cover by taking advantage of any thermal reserve, relative to the freezing point, that may exist in the water. These are usually more successful in lakes or enclosed areas than in rivers, since the water temperature in rivers is rarely much above the freezing point.

Wastewater from the cooling of power plants, both fossil-fueled and nuclear, has sometimes been suggested as a source of energy for melting ice downstream of the release points. This method may be advantageous in small areas, but the power requirements for melting extended reaches of ice are immense. Discharges from smaller sources, such as sewage treatment plants, are generally too small to have more than a very localized effect. On the other hand, the water held in reservoirs is often somewhat warmer than freezing, and it can be released in quantities sufficient to result in extended open water downstream— the precise distance depending on how much surface area is required to cool the water back to the freezing point by heat loss to the cold air above.

Priit Vesilind
An icebreaker clears a path for smaller boats near Finland. Travel by river in cold climates may be possible only when ice jams are broken up or removed in some manner. Priit Vesilind/National Geographic/Getty Images.
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