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Local geologist breaks down the details about West Linn's beloved hydro-magic site

PAMPLIN MEDIA GROUP: ALVARO FONTAN - Nearly 40 percent of the precipitation that falls on the edges of the Willamette Valley tumbles over Willamette Falls. Living in West Linn or Oregon City almost guarantees that you are familiar with Willamette Falls.

Surprisingly, I never heard of this particular waterfall until I moved here several years ago. It is surprising because Willamette Falls is the largest waterfall by volume in the Northwest and it is the second largest in North America behind Niagara Falls. The Willamette Falls Legacy Project is seeking to remedy this "unknown" status with projects, including a riverwalk, which will transform public access to the falls.

A community vision to develop the riverfront around the waterfall is exciting. For the past 150 years, Willamette Falls has been an industrial workhorse for our area. Various business ventures have left the falls surrounded by a sprawling complex of massive, industrial structures, making it difficult to get a close-up view of the falls and truly experience its size and grandeur. Willamette Falls becomes even more interesting when you understand what Mother Nature has done to provide us with our own "natural wonder."

What creates the waterfall?

A waterfall develops when a ledge forms that elevates the upstream portion of a river from the downstream portion. Constantly flowing water will normally erode away any ledges in a riverbed, so it takes unusual conditions to develop a waterfall on a scale as grand as Willamette Falls.

The first condition is that, after a waterfall initially forms, the geologic time frame must be short enough that the river doesn't have time to erode away the ledge. The second condition is that the rock foundations of the river bed must erode in a fashion that preserves the rapid elevation change. A 42-foot change in elevation over a mile creates a set of rapids, but a 42-foot drop over a 10-foot distance creates a waterfall.

What about Willamette Falls?

The rocks that form Willamette Falls are the Grande Ronde Basalts. They originated in the Miocene Epoch about 16 million years ago as part of the more extensive Columbia River Basalts. These basalts developed from lava flowing out of large fissures in Eastern Washington and Oregon. Miles-long fissures formed cracks in the earth's crust where molten lava from the interior poured onto the surface and flowed like water.

The lavas didn't mound up to build volcanoes. Instead, they flowed as sheets of liquid rock covering the landscape. Up to 36,000 cubic miles of lava came out of the earth to produce the Grande Ronde Basalts. The weight of the lava was so great that it depressed the crust of the earth beneath it.

A total of 120 separate lava flows compose the Grande Ronde Basalts. The flows stack on top of each other like sheets of paper and individual flows can be hundreds of feet thick. These flows cover large portions of Western Idaho, Southeastern Washington and Northern Oregon. The Grande Ronde Basalt flows stretch from Idaho all the way to the coast where the Columbia River flows into the Pacific Ocean.

Willamette Falls is formed from a ledge that developed between two Grande Ronde Basalt flows. The river bed rocks at the top of the waterfall belong to the Sentinel Bluffs basalt flow. At the base of the falls the water lands on the Winter Water basalt flow. The actual ledge is there because erosion has removed the Sentinel Bluffs rocks downriver from the falls.

The removal of the Sentinel Bluffs basalts via erosion has caused the location of the falls to migrate upriver as increasing volumes of these rocks eroded away. The original position of Willamette Falls was probably just under a mile north of the current location at about where 11th street in Oregon City would intersect the river. This original location is where the Bolton Fault crosses the river. The waterfalls initially formed when the rocks on the south side of the fault were pushed up higher than the rocks on the north side, creating a ledge.

Initially, the falls probably developed as a roughly linear feature across the river. Over time, as more erosion occurred in the center of the river than on the edges, it has developed its current arcuate, horseshoe shape.

Where does all the water come from?

The average daily flow rate of water over Willamette Falls throughout the year is 27,200 cubic feet per second (cfs). This flow rate translates into 17.5 billion gallons of water per day flowing across the falls. The actual daily water flow varies by season with the lowest flow being in the summer dry season. Water flow can dramatically increase during the wet season, and during the 1964 floods, approximately 270 billion gallons of water a day were passing over the falls.

All of that water originates upriver in the Willamette Valley. The Willamette River, which traverses 187 miles of the valley, carries the water to West Linn. The river's watershed drains nearly 12 percent of Oregon and contains 70 percent of the human population of Oregon. The massive flow of water that we see cascading over the waterfalls at West Linn is all from annual rain and snow in the Willamette Valley.

The valley acts like a big funnel to collect all of the precipitation between the west slopes of the High Cascades and the east slopes of the Coastal Range. That works out to be about 15 trillion gallons of water a year precipitating into the Willamette Valley watershed during an average year. That is enough precipitation to bury the city of Portland under 500 feet of water each year. Some of the water soaks into the ground, some is lost to evapotranspiration, and what is left (about 40 percent) flows over Willamette Falls on its way to the Columbia River and ultimately to the Pacific Ocean.

The Willamette Falls Legacy Project envisions a vibrant riverfront environment that will provide us with great opportunities to more closely observe our own "natural wonder." Understanding the earth science processes that create Willamette Falls will make our riverfront visits even more interesting. Enjoy the viewing and don't get too wet.

William House is an earth scientist and writer in West Linn.

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