As noted previously when I stumbled on Mullan Road, I was looking for the spot where the Palouse River leaves its old valley and takes an abrupt turn through a deep narrow slot of a canyon on its way to Palouse Falls and the on to the Snake River. Easy enough to spot on Google Earth.
The sharp left turn of the solid blue arrows marks the spot where the Palouse River leaves its old valley for the Palouse Canyon.
Palouse River Routes
Upstream of the abrupt turn in the river, the Palouse is a meandering river in its valley passing through scab land stripped of its soil by the massive Missoula Floods. Irrigated fields line the valley bottom and are used to augment the scant feed the scab land grazing provides.
Palouse River Valley with irrigate fields next to the river. The foreground is a gravel bar from the Ice Age floods.
The flood waters completely filled the valley and surged south across a plateau between the Palouse River and the Snake River carving deep holes into the joint sets within the underlying Columbia River Basalt Group. When the flood receded the Palouse remained within one of those deep channels leaving its former valley as a mostly dry coulee.
The old river valley, now Washtucna Coulee
The valley floor is filled with sand and gravel from the flood at this point
Spot where the Palouse leaves the valley/coulee
The former route of Mullan Road passed just to the right of the low area.
There were schemes to divert the Palouse River back into its old valley via a canal. A canal was started but never came close to completion. It would be possible on a topographic basis to dam the current canyon and spill water westward down the now dry coulee. I have found no reference as to that ever being considered. Perhaps the area potentially irrigated was too small or the sidewalls of the canyon would be not hold water very well. Regardless, the lower end of the old valley a bit west of Connell is now well irrigated with Columbia Basin water via the system from Grand Coulee Dam.