A Geological History of Sullivan’s Gulch

a2009-009-1458-sullivans-gulch-1932

The City of Portland Archives recently publicized this wonderful image from 1932. This is Sullivan’s Gulch before the construction of Interstate 84. Way before, in fact. The construction taking place in this image is of a golf course that once resided here, not the interstate which was still decades away.

The gulch was an awkward locale; prone to flooding, as you might imagine. So, to make best use of the land, in the early 1950s Portland acquired the gulch as a thoroughfare that became the western terminus of I 84.

The gulch itself made a natural pathway from the prominence of Rocky Butte to the Willamette River and, after being partly filled in for the construction of the railroad, the flooding was largely controlled. A small stream did once flow through this drainage, but it was not sufficiently competent to have carved the gulch itself. The obvious question that arises for a geologist then is, how did this huge gash in east Portland form?

Sulivan's Gulch Lidar

The above Lidar image (courtesy of DOGAMI) shows northeast Portland. Center-right is an old cinder cone volcano called Rocky Butte. Partially visible near bottom-right is the northern tip of another volcano, Mount Tabor. Portland’s west-side city center is at bottom-left on the other side of the Willamette River. Sullivan’s Gulch is the depression stretching from the south side of Rocky Butte to the Willamette.

If you go back about 15,000 years to the end of the last ice age and manage to score a view from the vantage point of the map above, you’ll witness something amazing. In fact, if you’re willing to watch for a few thousand years, you’ll witness it upwards of 90 times.

A massive rush of water would come pouring out from the west end of the Columbia River Gorge and enter your field of view at the top-right of the map. These powerful flood waters, in their haste to fill the Willamette Valley, had to pass by Rocky Butte first. The volcanic deposits on the east side of the butte were stripped from the slopes of this relatively young volcano. The gulch was carved in this and subsequent Missoula Flood events at the end of the Pleistocene (the last ice age). Behind Rocky Butte a pendant bar formed which we now refer to as Alameda Ridge.

To me, it’s a beautiful thing that all present-day westbound travelers on I 84 are making this same speedy rush into the Willamette Valley. The highway flows out of the Columbia River Gorge and bends sharply around the remnants of the old volcano before being ushered by the flood-carved topography into the gulch and toward the Willamette River.

Google Maps

I love finding little slices of history like the image that inspired this post. If you’re similarly a history buff, I encourage you to attend the 2nd annual Oregon Archives Crawl on October 15th. It’s the best opportunity you’ll have all year to learn how to navigate Portland’s rich historical archives. As is typical in Portland, attendees are meeting for beer after the event.

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6 thoughts on “A Geological History of Sullivan’s Gulch

  1. Great post! I knew about the pendant bar on the N side of Rocky Butte, and the eroded swale on the E side is a feature I love pointing out to people. But I was unaware of Alameda Ridge, and just assumed, without much thought, that “Sullivan’s Gulch” started as a smaller stream valley, which was subsequently enlarged, widened, and entrenched for the express purpose of putting I-84 in it. Ever since I moved here, it’s been so artificial looking- entirely encased in concrete- that I just assumed it was a dominantly man-made landform. Tres kewl! Thanks.

  2. My mother has told me her father and her grandfather operated a gravel pit near the base of Rocky Butte and near the end of Sandy Blvd. I’m guessing it was between 1880 and 1930 or so. I have never seen the site and am curious if it’s location is still detectable. Any opinions on whether or not anything on the Lidar image in that area looks man made?

  3. Hmm, I’m not sure the dates fit. My mother has memories of an active pit in that area and she wasn’t born until 1925, She says my grandfather lost the business during the Great Depression. Maybe it was located nearby. Thanks for the interesting history of The Grotto.

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