The circumpolar tundra and taiga are drained by great numbers of great rivers (and by many more lesser ones, too) that have played important roles in the history, exploration, and exploitation of the North. Most of the great rivers of North America and Eurasia have served as avenues for prospectors and explorers, as routes of commerce, and as highways for anadromous fish (which spend most of their life cycle in the sea but spawn in fresh water, e.g., salmon, Arctic char). Most importantly, the rivers of the North serve as vital links between terrestrial and marine ecosystems. Smaller streams (and in Siberia, even some sizable rivers) typically freeze solid in winter. In other cases, a shallow portion of a stream freezes solid, while deeper channels freeze over but retain an unfrozen layer along the bottom where fish and invertebrates may remain active. When water from upstream encounters an ice dam downstream, the water usually and repeatedly overflows the existing surface ice and then freezes, forming aufeis (from the German for "ice on top," also known as naleds). The dimensions of accumulated ice just before spring thaw may greatly exceed the depth and width of the stream flow in summer. In most instances, however, winter essentially shuts down tributary inputs to lakes and streams so that water movement is minimal. Such is not the case for the many great rivers of the North—in these, water continues to move downstream under the covering ice and continues to support aquatic life during the winter. Some lakes tend to lose significant volume during winter even while being completely ice covered: water escapes through outflow rivers (also ice covered), but no significant replenishment can occur until spring thaw. Spring thaw is often accompanied by flooding of river valleys either because ice jams temporarily dam the channel, causing water to overspread the valley, or because the channel is too small to accommodate the sudden influx of great volumes of water from snow and glacier melting.
Measurements of 67 Alaskan glaciers taken between 1955 and 1995 indicate an average annual loss of thickness of about 50 cm. Repeat measurements of 26 glaciers from 1995 to 2001 indicate an average annual thinning of 1.8 m, suggesting an increasing rate of thinning in this more recent time frame. This meltwater contributes to a sea level rise of 27 cm per year. One of the practical observations that Paul Okalik, premier of Nunavut, Canada, offered in his argument that Canada should ratify the Kyoto Protocol (to reduce carbon dioxide emissions to the atmosphere) is that he has seen a river running at unusually high levels due to unprecedentedly rapid melting of glaciers near the town of Pangnirtung.
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