Cache Elevation: 1140 ft
General Directions: From Kennewick take SR 397 through Finley. Turn south onto Piert Rd and then left at Meales Rd. Continue 4.5 miles before turning onto Ayers Rd. Follow Ayers Rd to the end and park above railroad and the Columbia River.
Wallula Gap Overlook
Wallula Gap, a natural water gap of the Columbia River, is one of the most significant and spectacular features created by the great Ice-Age floods. The National Park Service recognized this in 1980 by designating Wallula Gap a National Natural Landmark. Only one other flood feature in the Mid-Columbia Basin, Drumheller Channels, shares this distinction.
The story of Wallula Gap begins back in the Miocene Epoch, about 17 million years ago. At that time, lava flows of basalt ran out of giant fissures in the Earth’s crust east of here, near the Idaho-Oregon-Washington border. During basalt volcanism, the southern and western portions of the Columbia Plateau, including Wallula Gap, began to warp and fold under stresses deep in the Earth’s crust. The bending of the ancient lava flows is clearly visible in the folded layers of basalt exposed in the steep walls of the gap.
Early in the history of folding, the ridgecrest here was slightly lower than elsewhere along the ridge. This caused first the ancient Salmon-Clearwater River (precursor to the Snake River), and then the Columbia River to flow across the ridge over this low point. As the ridge continued to bend upward, river erosion kept pace, and a water gap developed. Until about 10 million years ago only the ancestral Salmon-Clearwater River flowed through Wallula Gap. It wasn’t until about 6 million years ago that the Columbia River joined in, where it has continued to flow ever since. Somewhere between 2 and 3 million years ago, the ancestral Salmon-Clearwater River captured the Snake River in the vicinity of Hell’s Canyon, along the Idaho-Oregon border; this added significantly to the amount of water draining through the gap.
Ice-Age floods, which began as early as 1 to 2 million years ago, took the path of least resistance. Since an opening across the Horse Heaven Hills already existed here, the floods naturally drained through Wallula Gap. No other place along the ridge was low enough to allow floodwaters to pass, so all the water was forced through this narrow opening, becoming a huge bottleneck. Floodwaters from Glacial Lake Missoula spread out over an area almost 100 miles wide across the Channeled Scabland, so its not difficult to understand why they backed up when suddenly forced to pass through an opening as small as two miles wide.
Geologists have calculated that during the Ice-Age floods, almost twice as much water entered the Gap than could pass through. The name for the body of water that backed up is Lake Lewis, a temporary lake that only lasted a week or less; the amount of time it took for all the water to drain through the gap. The water behind Wallula Gap also partially submerged a set of ridges that extend southeastward from Rattlesnake Mountain, forming the Lake Lewis Isles near the Tri Cities.
Behind the bottleneck, floodwaters in Lake Lewis rose up to 1220 feet. High spill-over channels that reach that elevation define the highest levels of Lake Lewis.
Ice-rafted erratics also are found to this elevation in the Pasco Basin, affirming the highest levels of Lake Lewis. (A number of light-colored, erratic boulders are visible along the route to the geocache). During the largest Ice-Age floods, water levels in Wallula Gap were almost 1000 feet deep! Downstream, the Columbia Gorge, also a narrow passage, caused floodwaters to back up, temporarily, to about 1100 feet below Wallula Gap. Floodwaters that backed up below Wallula Gap are referred to as Lake Condon.
The geocache is located at west side of Wallula Gap, at an incredible 850-ft drop off into the Columbia River below. Even at this height the largest Ice-Age floods still rose another 80 ft above the cache site.
To experience more incredible features left behind by the Ice-Age floods try finding these other geocaches placed by geologist Bruce Bjornstad:
Upper Goose Lake
Frenchman Coulee Rib
Rattlesnake Slope Erratics
Saddle Mountains Overlook (Earthcache)
West Bar Overlook
Lake Sacajawea Flood Bar