Natures's Water Towers: Many river systems start in the ice
By Sid Perkins as part of his article "On Thinning Ice: Are the world's glaciers in mortal danger?"
In the October 4, 2003 issue of Science News
Glaciers store precipitation that falls in winter months and discharge it gradually during the summer, a season when rainfall may be lacking and demand for water can be high. For example, about 15 percent of the Himalayas is swathed by glaciers. An additional 35 percent of the region is covered in snow at the height of winter. Each year, as many as 800 cubic kilometers of meltwater from these sources nourish the Himalayan streams that eventually feed into major Asian rivers such as the Ganges, Indus, Hawang Ho, and Yangtze.
Most of the main rivers that cross Canada's western plains originate at glaciers high in the Rockies, says David W. Schindler, an ecologist at the University of Alberta in Edmonton. The river's flow volumes typically surge in the spring as the seasonal snowpack melts, but for the rest of the year, the rivers are fed only by glaciers and groundwater. These sources are particularly important because the rivers drain watersheds that don't receive much summer rainfall and where rates of evaporation are relatively high.
Despite increased glacier melting in recent decades, the Canadian river's flow volumes now measure only about 60 percent of those gauged a century ago, says Schindler. That reduction is affecting water quality in Lake Winnipeg because the river water entering the lake now holds higher concentrations of algae-nourishing chemicals than it did in the early 1900s. The problem is compounded by increased fertilizer use by farmers in the watersheds, he notes.
The freezing waters that tumble from the feet of melting Canadian ice masses bear a burden of eroded and dissolved minerals. They also carry pollutants-including pesticide residues and fallout from atomic bomb tests-that were lofted to the glaciers on and in snowflakes that fell decades ago, says Schindler.
Field studies suggest that DDT and some other pesticides found in modern meltwater were deposited on the ice masses in snowfall as many as 50 years ago. Analyses of ice layers from intact portions of the glacier indicate that the concentrations of DDT and other pesticides actually peaked about a decade after use of these substances had been banned. That finding provides scientists with insight about how such chemicals cycle through the environment.