Sediments found in and around Flathead Lake are helping a University of Montana geologist piece together the history of glacial/post-glacial flooding and climate change in Western Montana.

Professor Marc Hendrix and his students started studying the lake sediments four years ago. They have compiled new geological maps, analyzed seismic profiles and collected more than a dozen lakebed core samples, all in an effort to learn what the deposits can tell them about past conditions.

"It's been a real interesting project," Hendrix said during a recent interview. "We're finding a lot of history there that's never been interpreted. We've found evidence of a number of things, including earthquakes and climate change."

A colleague and co-investigator,Johnnie Moore, previously collected some of the cores and the seismic profiles. Additional core samples, including 19 extended cores, were collected during the summer of 2000 and 2003, Hendrix said.

The longest measured 11.5 meters, or about 38 feet. Although it captured sediments that date back more than 14,150 years, to just before the end of the last ice age, it still represents less than 10 percent of the total lakebed deposits.

"There are about 135 meters of sediment in Flathead Lake," Hendrix said. "I think it probably contains one of the best, if not the best sedimentary record that's available of the ice age, post-ice age transition along the whole Cordilleran margin."

The Cordilleran Ice Sheet covered most of British Columbia during the last ice age, as well as the northern portions of Washington, Idaho and Western Montana.

By analyzing such characteristics as grain size and mineral content and by looking carefully at the various layers contained in the cores, Hendrix said geologists can surmise a great deal about how sediments were initially deposited and under what environmental conditions.

"You learn to read the details like a book," he said.

For example, the Flathead Lake sediments increase in size as they get younger, recording the change from a glacial environment to today's more temperate climate and stronger stream flows.

The trend in grain size was gradual until about 8,000 years ago, Hendrix said, when there was a sharp spike that lasted for 2,000 years.

Researchers have found similar spikes in Europe and in the oxygen isotope content of Greenland ice cores. This is considered evidence for a period of abrupt climate change, during which a massive melt-off of arctic ice took place.

Hendrix said the melt-off may have dumped so much cold, fresh water into the northern Atlantic that it deflected the warm Gulf Stream ocean current and actually promoted a cooling trend in Europe.

"This spike has never been documented before in the Western United States," he said.

The Flathead Lake cores also display regular, even layers of light and dark sediments, called "varves," that are commonly found in glacial lakes. The light layers typically contain larger sand and silt grains that were deposited during the summer melt; the dark layers contain finer particles that came out of suspension in winter, when the water currents slowed down.

Above the varves, there's a much thicker, more variable layer that starts out coarse and gradually transitions to smaller grains.

"We think this upward-fining bed records a single flood in the Flathead Basin, with large-volume flows through the drainage," Hendrix said.

Based on the relatively recent age of the core sediments, he said, it's unlikely that event was related to Western Montana's most famous deluge - the monstrous Glacial Lake Missoula floods (or flood), which scoured vast channels through the bedrock of Eastern Washington and the Columbia River Valley after a glacial dam near the Cabinet Gorge burst open.

Exactly how many times the dam burst, how many times the lake emptied and how many catastrophic floods were released is still a matter of debate among geologists.

"I think there's considerably more uncertainty to the story than most people are aware of," Hendrix said. "There's very little disagreement that Glacial Lake Missoula existed, and there's little doubt that huge floods swept across Eastern Washington - but the idea that the one necessarily led to the other hasn't been proven."

In fact, a team of University of Alberta researchers suggested in 1999 that at least one massive outburst flowed south, out from under the Cordilleran Ice Sheet. They estimated that a subglacial reservoir there held about five times as much water as Lake Missoula.

Hendrix said some of the largest flood channels in Washington point north, towards Canada, rather than to Western Montana. There's also evidence that the youngest flood deposits post-date Lake Missoula by several thousand years, based on estimates that the lake existed from about 19,200 to 16,000 years ago, compared to an age range of 19,095 to 13,695 for the various flood deposits.

However, testing the hypothesis that Glacial Lake Missoula was the sole source of the ice age floods hasn't been the main focus of Hendrix's research, or for his students, whom he credits with doing much of the "heavy lifting" and detailed sediment analysis.

"We can't ignore the Lake Missoula story, but right now we're just looking at the sediments that are here and trying to interpret what they tell us," he said. "We think we have a record of abrupt climate change in Flathead Lake, and a record of large post-glacial floods and drought. We'll start there and build up to a bigger picture."

Evidence of the post-glacial drought - and of some major earthquake activity - came from the seismic profiles of the lake bottom.

The profiles were created by bouncing sound waves off the underlying bedrock. They look something like a birthday cake sliced in half; based on subtle similarities in the seismic reflection patterns, separate "facies" or sedimentary layers can be seen.

Regressions between the layers are visible as well, where one facies pinched out farther from shore than the underlying sediments. Hendrix said that indicates that the lake level dropped substantially while the younger sediments were being deposited.

The pattern persisted over the course of many years, affecting several sedimentary facies, until the lake once again increased in size while depositing the uppermost layer.

There are also abrupt offsets in the profiles, where earthquakes raised or lowered entire blocks of sediments.

"Flathead Lake exists where it does because of structural downdrops along the Mission Fault," Hendrix said. "Some of these fault segments were quite active. Based on the offsets we can see, a few of the quakes probably had magnitudes greater than 7.0."

Hendrix said his immediate plans for the Flathead research include preparing the results for formal publication. After that, he wants to delve even deeper into the mud.

"I think there will be three or four papers coming out for peer review over the next several months," he said. "Ultimately, I'd like to secure funding to recover a complete set of cores from the lake. We've sampled less than 10 percent of what's available. There's a tremendous amount of information down there related to the last glacial maximum, and no one's taking advantage of it."

Reporter Bill Spence may be reached at 758-4459 or by e-mail at bspence@dailyinterlake.com