Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 18
Presentation Time: 8:00 AM-5:00 PM


MCDONALD, Lois M., Geosciences Department, Boise State Univ, P.O. Box 1075, Meridian, ID 83680,

In the 1990's, community leaders and a prospective developer in North Idaho removed the pilings from the Spirit Lake mill pond resulting in a water level drop of 10 feet for the entire lake. The resulting loss of resort activity pushed the local community to find a remedy and reestablish the former lake level. The most recent method was to line 40 acres of the 57 acre Mill Pond with a plastic 30-mil sheet and replace the dark organic layer and overlying sediments.

The cause of the water loss can be traced to the regional story of Glacial Lake Missoula Floods (15,000 to 12,000 years ago).

The Geologic Time Scale suggest a long overview: "Three major geologic events define the creation of the Spokane Valley-Rathdrum Prairie Aquifer. The first event was the emplacement, metamorphism, and erosion of the Precambrian basement rock; the second event was the eruption of Tertiary (Miocene) flood basalts that created the Columbia Plateau; and, the third event was the glaciation in the Quternary Period that first eroded, then filled the Spokane Valley-Rathdrum Prairie area with coarse sediments and gravel to create the Aquifer."

Spirit Lake is located in the Purcell Trench lobe area of the Cordilleron Ice Sheet. Spirit Lake was formed by a terminal moraine at the southeast end of the lobe when ice retreated. The entire Purcell Trench was filled with hundreds of feet of Lake Missoula Flood outwash gravels. Drainage sediments from Mt. Spokane provided a lot of the sediment layer which is mixed with clay (rock flour deposits left from the glacier). This clayey layer forms a seal which normally limits the drainage from the lake. The lake itself, has only one small surface stream which disappears into the ground within a relatively short distance.

As a larger system, the Rathdrum Prairie Aquifer includes many small and large lakes (including Lakes Pend Oreille and Coeur d'Alene), all of which touch the flood outwash gravels and contribute to the aquifer. The highway between Spirit Lake and the southern end of Lake Pend Oreille (town of Athol) passes through ripple marks from the flood. Lake Pend Oreille is the 5th deepest lake in the United States and was used during World War II as Farragut Naval Training Base, a "boot camp" and submarine training site. The Navy still conducts sonar research near Bayview, at the south end of the lake.