On April 29, 36 members of the Puget Lobe Chapter enjoyed a day-long bus trip to Frenchman Coulee, Potholes Coulee, Babcock Bench, and the mouth of Lynch Coulee at Trinidad where we had grand views of the Columbia Valley. Led by Puget Lobe Chapter president Dale Lehman we left Mercer Island at 7 a.m., heading toward Snoqualmie Pass on I-90. Along the way Dale explained formation of Glacial Lakes Washington, Sammamish, and Snoqualmie as the Puget Lobe advanced and retreated, creating outwash channels in the Bellevue, Issaquah, and North Bend areas and damming the South Fork Snoqualmie and other western Cascade rivers with lateral moraines at the eastern margin of the ice.
After coffee break at Indian John Hill rest stop we entered the Ellensburg valley where Dale explained movement of alpine glaciers east from the Cascade crest, which left large moraines in the west end of the valley. Crossing the Columbia River at Vantage, we soon arrived at 2-1/2 mile long Frenchman Coulee via the North Alcove. At the Middle Alcove, where “The Feathers” attract rock climbers, Dale reviewed the massive Columbia basalt flows which occurred from 17 to 6 million years ago, topped by the distinctive red Roza member seen in Frenchman Coulee. He explained how Ice Age floodwaters moving south through Quincy Basin were diverted west by the Frenchman Hills toward the Columbia Valley. As floods passed through Frenchman coulee water level dropped from about 1400 feet elevation upstream of the cataracts to 800-1200 feet where floodwater exited the Coulee into the Columbia River. Dale showed how basalt entablature and colonnades were eroded by cavitation in fast-moving water to form cataracts which migrate upstream as the basalt erodes away.
We hiked from the Middle Alcove up onto the rock ridge which separates Frenchman Coulee from Echo Basin. At the lower cataract above Echo Basin we enjoyed a fine view west through Echo Basin to the Columbia River. Then we continued north and west along the ridge to a viewpoint overlooking the Columbia River Valley. At the viewpoint we admired flat-topped basalt columns 3-4 feet in diameter. Returning to Middle Alcove, we completed our 3-mile hike, then went by bus down to the lower end of old Vantage Highway (US Hwy 10) where it meets the Columbia River, now pooled by Wanapum Dam at the boat ramp. There we enjoyed a good lunch.
We next drove a few miles north to Potholes Coulee where Brent Cunderla, president of the Wenatchee Erratics Chapter, explained the cataract system of Potholes Coulee, more extensive than that of Frenchman Coulee. Four lakes, Stan Coffin, Quincy, Burke, and Evergreen, lie in the upstream ends of the cataracts. We hiked a short distant to an overlook where we could see Dusty Lake a half-mile below the cataracts.
From Potholes Coulee we drove north again to an overview of Babcock Bench where Road 9 NW turns south to become Ancient Lake Road. There, floodwaters have passed both east and west through a spillover channel at 1425 feet elevation between the Columbia Valley and Quincy Basin. We saw two 3-5 foot diameter ice-rafted granitic erratics which had been dropped in the spillover channel above Babcock Bench.
Our last stop was at the home of IAFI board member Ken Lacy, located on the west bank of Lynch Coulee where it joins the Columbia Valley. There we were treated to a BBQ dinner and fine views of the Columbia Valley downstream from the Lacy home. Ken first gave us a petrified wood lesson. Many of his dozen samples were agatized or opalized; just two were real petrified wood. From Ken’s back yard we could see the giant current ripples across the river at West Bar, and we could look south down-river to Potholes Coulee where a large gravel bar remains in the Columbia River, left by floods exiting Potholes Coulee some 20,000 to 12,000 years ago. Ken explained how the first five Ice Age floods were the largest, with remaining floods (40 to 100) decreasing in size as the Cordilleran ice sheet melted back.
We enjoyed an evening of conversation about Ice Age floods and geology together with good food, wine, and great scenery. We returned by bus to Mercer Island by 10:30 p.m.
For more complete site descriptions, see Bruce Bjornstad’s On the Trail of the Ice Age Floods.