In 1996, a helicopter dropped Alejandra Duk-Rodkin and a field assistant on a terrace above the confluence of the Yukon and Indian Rivers. The two geologists planned to eat their lunches before setting to work, but their sandwiches were soon forgotten.

Soon after sitting down, Duk-Rodkin glanced down at the anthills at her feet and saw the evidence for which she had come looking; small pieces of argilite, a form of shale, were scattered over the surface of the churned-up soil.

She had just found proof that three million years ago the Yukon River used to flow south, instead of north and west as it does today. "We forgot about lunch. I was so happy!" says Duk-Rodkin, a research scientist with the Geological Survey of Canada.

The source of the argilite was in the sedimentary mountains to the north, on the other side of the Tintina Trench. A paleo-Yukon River had once carried this rock south and had also formed series of high terraces, such as the one where the two geologists were working.

The Fifteen Mile River northwest of Dawson City formed the headwaters of this ancient river until sometime between 2.9 and 2.6 million years ago when the Cordilleran ice sheet blocked its flow to the south and east, and the river changed its course.

The Yukon eventually cut a new channel, flowing northwest to the Bering Sea. Its headwaters are now far to the south among the glaciers of the coastal mountains in northwestern British Columbia.

Earlier geologists had speculated that the river had changed direction, but they never had positive proof. Duk-Rodkin, a research scientist with the Geological Survey of Canada (GSC), describes this discovery as the high point of her work in the Yukon.

She has spent 14 years mapping the glacial history of northwestern Canada, and has nearly finished a GSC map that will show glacial limits for all of the Yukon. Her research has rewritten the glacial history of this corner of the world, where ice has played a major role in shaping the land.

"95 percent of northwestern Canada has been changed by glaciation," says Duk-Rodkin.

In bluffs along the Tintina Trench, she found unusual soils that helped to determine the age of the ice sheet that once blocked the flow of the ancient Yukon River.

After studying the soils, Scott Smith, a soil scientist, confirmed that they were paleosols, or ancient soils. The oldest ones were dated at about 2 1/2 million years old.

The soil profile also showed that the Tintina Trench had been filled with ice many times. Duk-Rodkin concluded that ice flowing north and west from the Cordilleran ice sheets joined with local valley glaciers flowing south out of the Ogilvie Mountains. When the ice intermingled, it filled the Tintina Trench.

This finding explained why the Yukon River did not follow the apparent line of least resistance, the Tintina Trench, when it began flowing north. Instead the river cut its way through much harder rock south of the Trench.

"The most incredible thing is that the Yukon River is incised in the northern part of the Dawson Range. The river didn't follow the Trench, which was lower and softer, because it was filled with ice," says Duk-Rodkin.

The Yukon is not the only northern river that changed course when it was blocked by ice. A few years earlier Duk-Rodkin found that the Mackenzie River, which empties into the Beaufort Sea, once flowed east to Hudson's Bay and the Atlantic Ocean until ice got in the way.

While working in the Mackenzie Mountains in the Northwest Territories, Duk-Rodkin found rocks along the Anderson River that were of the same type as rocks found near the Snake River in the Wernecke Mountains. This was a puzzling find as now there is no link between these two drainages.

The Snake empties into the Peel River, which flows north to the Mackenzie. The Anderson River empties into the Beaufort Sea east of the Mackenzie Delta. Duk-Rodkin found that the Laurentide ice sheet, flowing north out of the Mackenzie Mountains about 30,000 years ago, had rearranged the landscape. The ice covered the Mackenzie Delta area, extending north along the coast towards Herschel Island and blocking drainages throughout the area.

When the climate warmed and the Laurentide ice sheet began to retreat, meltwater channels formed along its western edge. One river after another was created in this way. The Peel, Snake, Arctic Red, and Mountain Rivers all formed at the edge of the ice sheet, following its margin north to the Beaufort Sea. This history makes the Mackenzie the largest glacially diverted river system in the world.

Her work added to the story that other researchers had already established as glacial ice was known to have blocked the northward flow of the Porcupine River, sending it west toward the Yukon River. The Laurentide ice also dammed the Peel River and most of its tributaries to form glacial Lake Hughes in the middle Peel Valley.

This lake drained into the Eagle River, which flowed on to another huge glacial lake in the Old Crow Basin area. Glacial Lake Old Crow drained south and west, capturing the Porcupine River, and eventually joining the Yukon River.

Duk-Rodkin was able to map the ebbs and flows of the later glaciers on air photos. The most recent glaciation occurred towards the end of the Pleistocene Epoch, and lasted from about 30,000 to 10,000 years ago. A more extensive glaciation, called the Reid, occurred about 200,000 years ago.

The most recent glaciations have left their marks on the landscape, carving U-shaped valleys and basins called cirques found high in the mountains. Moraines and a host of landforms formed by glaciers are easy to identify, even from air photos.

To map the earlier glaciations, called the Pre-Reid, Duk-Rodkin looked for glacial erratics, rocks that have been transported by glaciers from a distant source. She found that these rocks told a consistent story, and helped prove that Pleistocene glaciers had ground through the Yukon much earlier than had previously been thought.

Duk-Rodkin is now working on a final draft of the glacial limits map, which will be available through the Geological Survey of Canada when finished. Knowing how glaciers have moved and how rivers once drained is important to geologists, so she hopes that her work will help with mineral exploration. The glacial limits information, along with other geological data, will eventually be available on a CD-ROM.