(The following is meant to give my journal Columbia River Gorge: Geology, Myth & Legend context rather than to be an exhaustive study.)
The Geology of the Columbia River Gorge has shaped a startling beautiful place. It remains as a reminder of the volatile forces that have been Earth‘s geologic history.
Rising far to the north at Lake Columbia in British Columbia the Columbia River, a Washington and Oregon’s boundary for most of the length of both State, travels 1214-miles before joining the Pacific Ocean at Astoria, Oregon. Meandering a southwesterly course to confluence with the Snake River at Pasco, Washington, near Kennewick it turns westwards to run through a gorge, 100-miles-long and 3,000-feet-deep, created by the incessant movement of the water over millennia.
Water, and ice, and plate tectonics, and geologic uplift, and volcanism … and time.
This is a volcanic landscape, even where it's hidden beneath a mantle of trees, moss, or fields, or water. Beginning about 17-million-years-ago some 41,000-cubic-miles of basaltic flows aided the creation of the much of the Northwest, changing the course of the river a number of times. Uplifting along geologic faults as crustal plates push against each other and the volcanic flow created mountains, giant folds in the land. The land splits and the gorge is formed.
The Gorge bisects the High Cascades Mountains; an extension of the Sierra Range running from the tip of South America well into Canada. They are large, angular, snowy, and due to plate tectonics and volcanism still growing. The most famous would probably be Mount St. Helens (8,364-feet tall) whose explosive 1980 eruption was a reminder of the forces involved. The tallest of the Mountains are: in Oregon Mt Hood (11,239 feet) and in Washington Mount Rainier (14,410), and Mount Adams (12,276). All four are visible at some point when traveling from Portland up the gorge.
[The resulting landslide when the north face of St Helens blew off had a volume of over half (0.56) a cubic mile, reducing the mountain 1,300-feet in height. The cloud of ejected debris, smoke, and dust hung on the horizon looking like a nuclear blast.]
Permanent glaciers dot the range today. During the Pleistocene, the last ice-age, a thick glacier covered much of North America. Glaciers are not large smooth continuous chunks of ice, but craggy and broken, with deep crevasses. The shortest distance is a straight line doesn’t apply here. In some places the ice acted as a dam retaining water as a lake, a tricky balance in temperature between frozen and non-frozen water. A number of these lakes existed throughout the world, some for extensive periods of time. The volume of these lakes increased, sometimes dramatically, as the ice melted.
What remains of Lake Bonneville is the Great Salt Lake of Utah. At one time it covered almost 20,000-square-miles. About 14,000 years ago the earthen dam at Red Rock Pass, Idaho, eroded to the point of collapse. Water seeks the course of lowest resistance. Over a period of several weeks the lake’s level dropped 300-feet as the water rushed out. It flowed into the Snake River and filled the gorge known as Hell’s Canyon lying between Oregon and Idaho. Peak flow is estimated at 33 million cubic feet per second. The Snake then empties into the Columbia, which carried the flood through the gorge.
[The ten-mile wide Hell’s Canyon, world’s deepest, is 8,000-feet at its deepest and averages 5,500-feet. The flood did not carve it but did enhance it.]
Ancient Lake Missoula stretching over 200-miles, primarily in modern day Montana, contained 520-cubic-miles of water, maintained by a glacier ice-dam blocking the Clark Fork River. Periodically the ice-dam failed and floods ensued. About 13,000 years ago the ice-dam broke for the last time. And although this time period, as the last ice-age came to a close, saw global flooding of low-lying regions, the Missoula Floods were, probably, the largest ever seen. Moving at speeds of up to 65-miles-per-hour the water scoured topsoil and rock in its path, leaving behind what is known as channeled scabland, an indelible mark on the landscape. Reaching depths of 800-feet the swiftly moving water submerged Crown Point, as it moved into and down the narrow Columbia River Gorge. The lake is estimated to have drained in about 48-hours.
[The cliff-face at Dry Falls, Washington, once supported the world’s largest waterfalls.]
[Upon reaching the upper end of the Willamette Valley flood waters spread out and as the water slowed in filling this wider space the soil dropped out, settling to the valley floor to enhance the top-soil already there. The Willamette Valley, named for the river running through it, has some of the richest farmland in the world.]
Five-hundred years ago part of Table Mountain, on Columbia’s north bank, broke off, the massive landslide creating a 5-mile 200-foot tall natural dam, a lake 80-miles long and perhaps 100-feet deep forming behind. Gradual erosion undermined it to the point where -- that’s right -- it flooded, a wall of water up to 50-feet high broke through sweeping everything in its path, shifting the riverbed south. What remained of the natural dam, a series of large boulders and wild white-water rapids, is the source of the name for the surrounding mountain chain: Cascades.
These events gave rise to myths about the creation and destruction of the "Bridge of the Gods" and a number of the Cascade Mountains. Some, especially those involving Loowit, saw a popular resurgence after the eruption of Mt St. Helens in 1980.
Beacon Rock marked the end of the rapids. Now a State Park, at 848-feet tall is the world’s second largest monolith, behind Gibraltar, and the core of an ancient volcano. Featured in William Clark's journal, it was at here that they noted the tidal influence and knew they were close to the ocean. However, some scientists claim the tidal influence isn’t actually measurable here and that their "observation" was a surmise based upon what they expected to find due to other information they had. Hmmm … whatever.
In 1896 the Cascade Locks were built to ease progress down river. This was followed in 1937 by the Bonneville Dam a primary power source for the region, allowed ships to proceed 100-miles further up river than was originally possible. The river runs deep and placid compared with the wicked and mercurial channel that Lewis & Clark, as well as all those early along the Oregon Trail, had to navigate. The cascades are now all submerged behind the dam.
People lived in the flood zone, for all three events, but the physical evidence in the immediate affected area was destroyed by each catastrophic inundation. As it stands, the oldest physical evidence for human occupation in Oregon is at Fort Rock Cave, in eastern Oregon, where a pair of sandals dating to 9000bce have been discovered. The anecdotal evidence comes from the verbal histories of the various tribes mostly expressed in their mythology. So we know they were there when the amazing and amazingly blue Crater Lake, in southern Oregon, was created in an explosive event within Mount Mazama such as the one that truncated Mount St. Helens.
There are about 70 waterfalls located in the Gorge, the largest concentration of waterfalls found anywhere in the world. Most are on the Oregon side where the gorge walls are steeper and more abrupt. Some are seasonal, run-off from snowmelt, and some are lost beneath the waters beyond the dam. The basaltic flow layers are clearly visible in the cliff walls of such places as Multnomah Falls.
Viewing the Scablands from the air:
Flying in or out of Portland, coming from or going to the east, try to get a seat on the side that will let you see out a window looking north. Also, an early (7-8am) flight leaving or late afternoon/early evening -- do your math if your coming the other direction so that you’ll be in the air above eastern Oregon/Washington and Idaho around 8-9am or during the corresponding later time periods. On a clear day the oblique light will throw the scablands below into high-relief allowing you to get the big picture and truly appreciate how amazing and beautiful they are -- and what a large area they cover.
--Beautiful but slow loading Relief Map without counties overlaid or with.
--Cascadia Geology (Nice set of links with descriptions.)
--Learn More About Oregon Geology
--Oregon Department of Geology & Mineral Industries
--USGS: Cascades Volcanic Observatory (An absolute wealth of information and photos.)
--U.S. Geological Survey: Oregon
--Geologic Time (Global geology.)
--The Geologic History of the Columbia River Gorge (pdf)
--Columbia Plateau-Channeled Scablands: Field Course, Internet Resources
--Glossary of Technical Terms Related to Ice-Age Floods