PORTLAND — Geologists have developed a new computer model to simulate the massive Lake Missoula floods that repeatedly carved the Northwest landscape between 15,000 and 18,000 years ago.

The simulation shows the likely timing and detailed events of one of the floods, starting with the collapse of an ice dam and the draining of 200-mile-long Lake Missoula. It was developed by Roger Denlinger with the U.S. Geological Survey in Vancouver, Wash., and Colorado-based geophysicist Daniel O’Connell.

Filling in the gaps

Geologists told The Oregonian that it helps fill in gaps in the scientific explanations of how the prehistoric floods scarred the landscape during the last ice age.

“It’s just really powerful visualization that gives a sense of the scale of the floods, how they came down through the channel system and backed up the big tributary valleys,” said Jim O’Connor, a hydrologist with the U.S. Geological Survey in Portland who has written extensively on the floods.

The computer modeling pro- vides the first “really good information” on the timing of events, he said.

Calculating the path of such a massive flood requires an immense amount of number crunching. Simulating one flood requires more than eight months of computer time.

Scientists say Lake Missoula, which is up to 2,100 feet deep in places, formed behind a huge ice dam stretching from what is now western Montana to eastern Washington. The dam collapsed and reformed dozens of times over a span of 3,000 years, sending megafloods loaded with icebergs, truck-sized boulders and other debris smashing through Montana, Idaho, Ore-gon and Washington.

In the simulation of one of the largest possible floods, water quickly overwhelms the hills near Spokane and races overland to the south and west. The intense, overland flows carve miles of scab lands between Spokane and Pasco, Wash.

The water then slams into the narrow Wallula Gap of the Columbia River, reaching 850 feet deep and flowing a thousand times greater than the Columbia’s average flow today. All of Lake Missoula’s water — 550 cubic miles — drains in less than three days, according to the model.

The simulation shows that seven days after the dam burst, floodwaters peak in the Willamette Valley, with water reaching as far south as Eugene.

Another source?

Some scientists argue that the floods must have had multiple sources. John Shaw of the University of Alberta in Edmonton, for instance, has proposed that an enormous reservoir beneath the ice sheet over much of central British Columbia boosted the flooding.

“It is conceivable that other valleys in southern British Columbia contributed water to the scab lands but the field evidence necessary to test these possibilities has not been fully documented,” said Jerome-Etienne Lesemann at the University of Aarhus in Denmark.

At Wallula Gap, water levels in the simulation fell short by as much as 130 feet of physical indications, raising the possibility of another water source.