The lake is a former channel of the Tualatin River, carved in basalt to the Willamette River. Eventually, the river changed course and abandoned the Oswego route.
About 13,000 to 15,000 years ago, the ice dam that contained Glacial Lake Missoula ruptured, resulting in the Missoula Floods, which backed theColumbia River up the Willamette River. The flooding created an underwater vortex called a kolk, which scoured out and enlarged the old Oswego channel, creating a natural lake. The rocks and boulders were flung by the kolk up to a mile away to present-day Durham and Tualatin, where they were quarried for many years before the site was converted to the Bridgeport Village shopping center.
A kolk is an underwater vortex created when rapidly rushing water passes an underwater obstacle in boundary areas of high shear. High velocity gradients produce a violently rotating column of water, similar to a tornado. Kolks can pluck multi-ton blocks of rock and transport them in suspension for thousands of metres.
Kolks leave clear evidence in the form of plucked-bedrock pits, called rock-cut basins or kolk lakes and downstream deposits of gravel-supported blocks that show percussion but no rounding. These characteristics cannot be seen here due to the human interventions in the lake area.
Though the lake is naturally occurring (a former channel of the Tualatin River), it has been significantly altered because of the concrete dam that has increased its size to 395 acres.
To log this cache please email the cache owner with your observations regarding at least two steps which have been take to control erosion at the lake's edge in order to maximize development.