Ice, floods created Rathdrum Prairie we see

The Rathdrum Prairie has been a tormented place, geologically speaking. Now it’s a tame and picturesque landscape, much in demand for both agriculture and housing. However, you wouldn’t have wanted to buy land there 15,000 years ago. For one thing, you wouldn’t have been able to get flood insurance.

But I’m getting ahead of myself. Actually, things were quite different here 15,000 years ago. It was the peak of the last great ice age and British Columbia, except for some of the taller mountains, was covered in a sheet of ice.

Actually, “sheet” doesn’t do it justice. Blanket doesn’t either. The ice was over two miles thick in some places. Think of it, two miles, straight up. That’s over 10,000 feet thick.

During this time, a large glacier from the British Columbia ice sheet moved down the Purcell Trench, the valley that extends south from Canada and contains Bonner’s Ferry and Sandpoint. In the vicinity of Sandpoint, the glacier was 30 miles wide, east to west.

Kootenai County was not inundated by this ice, but it came close. Sandpoint was covered, as was Lake Pend Oreille. The Clark Fork River existed back then, but it was blocked by ice just west of the Montana border.

Kootenai County had its own problems, though. After the glacial era reached its peak 15,000 years ago, there was a sudden period of global warming. Ice sheets that had taken 30,000 years to form melted in 3,000 years.

During that time of massive glacial melting, the runoff created huge lakes. One is called Glacial Lake Columbia, and covered an area from present-day Lake Coeur d’Alene, west through the Spokane Valley, through the present Spokane River lowlands, to Grand Coulee.

Lake Coeur d’Alene didn’t actually exist back then. The prehistoric St. Joe River, before Glacial Lake Columbia formed, flowed unimpeded through Post Falls and into the Spokane Valley.

Meanwhile, as Glacial Lake Columbia was covering Kootenai and Spokane Counties, and a whole lot more, Montana was having its own flood-control problems. When the Purcell Trench glacier dammed the Clark Fork River, it backed up and formed gigantic Glacial Lake Missoula.

From current Darby, Mont., in the south of the Bitterroot Valley, north through Missoula to Polson, to Thompson Falls on the west and north just into Idaho at the ice dam, every valley was flooded. In its entirety, Glacial Lake Missoula formed a body of water half the volume of Lake Michigan. In the Missoula Valley it was over 900 feet deep. In North Idaho, at the ice dam, it was 2,000 feet deep. It contained over 550 square miles of water.

How do I know all of this? I read a lot. Most recently I read “Glacial Lake Missoula and it’s Humongous Floods,” written by David Alt and published in 2001 by Mountain Press Publishing Company. It was he who gathered all these facts.

What is the point of all these facts? Well, for me it was astounding to think of all that has occurred here in the past few tens of thousands of years. Glaciers have come and gone. Lakes have formed and disappeared, and new lakes have formed. There have been huge environmental changes right here where we live.

Think about how grand we consider our Lake Coeur d’Alene. But it’s tiny compared to Glacial Lakes Columbia and Missoula. And it’s a Johnny-come-lately, geologically. Remember how I said that the St. Joe River once crossed into Washington? Well, during the period of warming, massive floods of meltwater from the northern glacier traveled down the U.S. Highway 95 corridor from Sandpoint to Coeur d’Alene, and into Glacial Lake Columbia.

Those floods carried large amounts of gravel that were dumped on the present Rathdrum Prairie and buried the channel of the St. Joe between Coeur d’Alene and Post Falls. This was not your average driveway-type gravel. To put it in an Idaho perspective, Alt referred to the rocks as baking potato-sized.

When Glacial Lake Columbia eventually drained, the St. Joe and Coeur d’Alene Rivers backed up behind the Rathdrum Prairie gravel dam to form Lake Coeur d’Alene. The lake, in turn, overflowed the dam and cut a channel that was to become the Spokane River between Coeur d’Alene and Post Falls.

That same process of flood-gravel deposition cut off and formed a number of smaller lakes along the corridor. Thus we have Hayden to the east; Spirit, Twin, Hauser and Newman to the west; and Liberty to the south. These lakes do not have rivers leaving them, but drain via seepage through the gravel.

Overlying this huge bed of gravel – note the numerous open-pit gravel quarries in the vicinity of the Rathdrum Prairie – is a thin layer of rich soil. Mud being lightest, it’s the last to settle out in the wake of flooding. Thus have we come to our current Rathdrum Prairie. If you doubt my words, simply drive the prairie and note the numerous rock piles at the edges of agricultural fields.

Just think of it. We live on a gigantic gravel pit that filled in the whole area between our mountains during glacial-melt flooding, and that also formed our lake. I’ll never be able to look at those rock piles, or baking potatoes, quite the same again.

All that’s in our most recent past. The real floods were back before the ice started to melt. But that’s a story for another time.