Lake Lewis

LAKE LEWIS

When Lake Missoula's ice dam failed, floodwaters gushing to the southwest quickly overwhelmed or bypassed everything that stood in their way--until these torrents funneled into the Pasco Basin. The result was Lake Lewis. Named for explorer Meriwether Lewis, this lake might be described as an Ice Age version of a gigantic stormwater runoff retention pond. This huge pond filled and emptied dozens of times.

The choke point which created Lake Lewis was the Horse Heaven Hills region just south of present-day Pasco, Wash. This range of basalt hills, which includes ridges whose elevations reach 2,000 feet or higher, is breached by the Columbia River at Wallula Gap. The gap is the only natural outlet for stream flows from the entire Columbia River Basin of eastern Washington. These terrain features were in place long before the Lake Missoula floods of 15,000-20,000 years ago. [The National Park Service has named Wallula Gap a National Natural Landmark, owing to its geographic significance.]

During each of the Lake Missoula flood episodes, as much as 200 cubic miles of water per day might accumulate in the low country north of Wallula Gap. This created a temporary lake level of 1,200 feet elevation. Since Wallula Gap could only disgorge about 40 cubic miles of water daily, one or two weeks might elapse before Lake Lewis flood water was drained from the basin. Outflows from the gap also were retardedas teh Columbia River was slowed at similar choke points downstream--such as the narrows at Crown Point, east of Troutdale, Ore.

Lake Lewis covered the entire Pasco Basin, most of the Quincy Basin, and Yakima Valley, much of the Ahtanum Valley, and the valley of the Walla Walla River and its main tributaries. Floodwaters also backed up the Snake River well beyond present-day Lewiston, Idaho. An estimated 3,000 square miles of land were under water.

The locations later occupied by the region's principal cities all were submerged. Pasco and Richland would have been under 800 feet of water, and the only dry ground in the nearby area was atop a handful of promontories--Badger, Candy, and Red mountains, as well as Goose Hill. [Geologists refer to these high points as the "Lake Lewis Isles".]

Today's Walla Walla would have been covered by 50 feet of water, and Yakima would have been inundated by overflows backing up through Union Gap into the Ahtanum Valley. The vicinities of Quincy and Ephrata would have marked the temporary Lake's northern limit, while much of the Saddle Mountains and Rattlesnake Hills, along with the upper elevations of Frenchman Hills, remained above the water line.

Geologic evidence suggests that Lake Missoula floods occurred at least 40 times, and Lake Lewis would have functioned as a temporary impoundment during most if not all of these events. But many geologists believe that there were many more floods--some of them from sources other than Lake Missoula, and others taking place at much earlier dates than the Lake Missoula floods. If so, Lake Lewis probably existed at these times as well, because the Columbia Basin, its rivers, the Horse Heaven Hills, and Wallula Gap all predated the Pleistocene Ice Age.