The Palouse River Zig-Zags

At first glance the straight river paths of the Palouse River may not seem apparent due to steep basalt cliffs. However, at a second glance, the river makes numerous sharp turns at specific angles. This is especially evident looking to the south at the Fryxell Overlook gazebo, (see photographs). Walking north along the Palouse River, this is also obvious due to the linear appearance of rapids just north of the falls.

This linear and “zig zag” nature is thought to come from a concept termed stream capture or stream piracy. Numerous fractures are present in the Palouse Falls area. The fractures occurred after the basalt cooled around 15 million years ago and were originally zones of broken rock, which were more easily eroded than the surrounding basalt- especially by the Ice Age Floods. Once the fractures had been eroded, the river quickly moved to the lower elevation and became confined to fractures.

Earlier papers suggested these fractures occurred during folding of the basalt, but recent studies seem to agree that the fractures were produced by movement along faults in the underlying basement rocks. Evidence for this is how the fractures often meet each other at 15- to 30-degree angles in map view, which is very common in areas with lateral faults.

“Palouse River today is detoured from its former course along now streamless Washtucna Coulee into a striking, joint-determined scabland canyon which cuts through the pre-glacial divide.” (Bretz, Smith and Neff, 1956)

Images

Map