If global warming ends up decreasing precipitation and historical precedents repeat themselves, the region could be in far worse shape than policy-makers currently anticipate, warn the authors of the study, which appears in the current issue of the Journal of the American Water Resources Association.

"Past droughts and corresponding declines in river flow have been worse than anything we've seen for the past 100 years, including the recent drought, and this was before man began modifying the climate," said lead author Roslyn A. Case, who conducted the research as a UCLA graduate student but who now works for the Venice, Calif.-based environmental consulting firm of McDaniel Lambert Inc. "Human-induced climate change could make the situation even worse."

The UCLA findings, which represent the first large-scale reconstruction of the river system's flow rates, call into question the wisdom of current approaches to river water management in the basin, which encompasses Alberta, Saskatchewan and Manitoba and covers 168,000 square miles, including some of Canada's longest rivers.

"If future water policy and infrastructure development in the Canada prairies continue to take only 20th-century water resources into account, then the region is in for real trouble," said Glen M. MacDonald, co-author and chair of UCLA's geography department. "One reason the current drought seems so severe is the 20th century was one of the wettest periods the region has experienced."

With core samples from old-growth trees in Alberta and Saskatchewan, the researchers pieced together annual water flow over varying periods for the region's three main rivers: 1,113 years for the North Saskatchewan River, 522 years for the South Saskatchewan River and 325 years for the Saskatchewan River.

Among their findings:

• Between 900 and 1300, the North Saskatchewan River experienced 10 decades of the lowest flow in its history; over those 400 years, the average flow of the river was 15 percent lower than the 20th-century average.
• Between 1702 and 1725, river flows on the South Saskatchewan River were almost 20 percent below the 20th-century average.
• Between 1841 and 1859, river flows on the Saskatchewan River were at least 22 percent below the 20th-century average.
• Along the South Saskatchewan River, the early 20th century saw the highest river flows of the segment's 522-year reconstruction.

Tree rings from sites adjacent to Canada's prairies are reliable records of past precipitation and river flows in the area because trees form larger rings during years of high precipitation and form thin rings during years of little precipitation. The patterning persists for the life of the wood.

Case and MacDonald examined samples from 178 trees, the majority of which were living and none of which were damaged by their work.

To ensure the accuracy of their findings, the researchers initially checked their river flow predictions against historical records for river flows in the area.

In addition to recording the amount of precipitation in any given year, tree rings reflect the volume of river water flowing in any given period since rivers serve as the principle destinations for the region's watershed.

About 75 percent of water from the Saskatchewan River system is used for domestic and agricultural purposes, including wheat production. The Saskatchewan River Basin is one of the world's leading producers of wheat.

Debate surrounding future allocations of these waters has been hampered by the fact that government officials did not begin keeping consistent, systematic records of the river system's flow rates until 1912.

"In order to understand what nature can throw our way in terms of drought, we have to look deeper into the past than historical records currently go," MacDonald said. "That view suggests the possibility of some unpleasant surprises."

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