Fluvial Processes in the Willamette Valley
/Recap of the GSOC Willamette Valley Transect Geotour - June 18, 2022
Willamette Valley Transect Trip stops overlaying a geology map of the Willamette Valley. Base from online DOGAMI interactive geology map.
Thirteen happy participants of 2022’s first GSOC field trip piled into the back room of the Bear Creek Pizza parlor in Molalla this past Saturday night to eat dinner and sketch up their conclusions from the day’s journey across the Willamette Valley. GSOC Past President and Programming Director Sheila Alfsen led the group from Erratic Rock State Park on the west side of the valley to the Molalla Rosette to the east of the valley in more or less a straight line. The purpose of the trip was to observe the landforms of the Willamette Valley, concentrating on the fluvial processes at work in the sediments of the valley floor.
The geotour started at Erratic Rock State Park near Bellevue, Oregon. Here, perched upon the valley sides composed of weathered Eocene marine sandstone, a giant chunk of argillite, or metamorphosed shale, from perhaps Montana, landed after the iceberg that floated it down the Columbia River system came to rest and melted. The rock now sits at approximately 140 feet below the crest of the Missoula Flood which bore it to its present location. Alfsen usually conducts this geotour for her Geology of the Pacific Northwest students at Chemeketa Community College, and one of the things they do for the class is make a tiny iceberg that will float an argillite pebble, and compute the ratio of the volumes of ice and rock to determine the minimum size of glacier that floated the rock. She says minimum because rock from this boulder was used in pioneer times to construct barn floors in nearby farms, and so it has considerably less volume than in pre-pioneer days. The argillite has a distinct cleavage which makes it a source of very nice flooring material. Also, part of the rock is buried beneath the soil that makes a nearby vineyard flourish.
The argillite erratic rock near Bellevue.
Looking east from argillite erratic into Yamhill River Valley.
Alfsen also mentioned that vineyards are located on the hillsides around this area to take advantage of the nutrient poor soils that evolved from weathering of both Miocene basalts (Jory-type soils) and Eocene sandstones (Willakenzie-type soils). Vineyards are not located on the valley floor in the nutrient rich sediments derived from the depositions of the Missoula Floods (loess) and rivers carrying sediments from the Cascade Mountains.
From the Erratic Rock hilltop, GSOCers were able to look out across the Yamhill River Basin, which forms the westernmost arm of the Willamette Valley between the Coast Range and the Amity Hills. The valley floor is about one hundred sixty feet below the rock, and trees bordering the floodplain of the Yamhill could be seen as well as the Amity Hills beyond. Alfsen had the participants review a diagram of the anatomy of a typical river basin so that they would be able to name the parts of the Yamhill and Willamette Rivers as we saw them in the field. The names correspond to the numbers on Alfsen’s diagram as follows:
Sheila Alfsen and the “biodegradable thalweg detector”.
Terrace
Floodplain
Levee
Thalweg (pronounced TALLveg)
Cut bank
Meander
Channel
Channel bar
Oxbow lake
Point bar
Next stop was on a bridge spanning the Yamhill River on Oregon Highway 153 east of Bellevue. At this location, the group could view the terrace slopes on either side of the river, the floodplain, a point bar and cut bank of a meander in the river and the thalweg (or swiftest path down the river). Alfsen had the participants drop “biodegradable thalweg detectors” (i.e., watermelon rind discs) from the upriver side of the bridge in various points across the river width and time how many seconds it took for them to pop out on the downriver side of the bridge. The disc that took the fewest seconds had the fastest path, which therefore was the thalweg.
The group then proceeded to the turnoff for Brigittine Monastery off Broadmead Road for stop 3. Here the group could overlook the Yamhill floodplain from the edge of the terrace on the valley floor. The river has cut down approximately 30 feet from the top of the terrace, which is the elevation of the valley floor. Garry Oaks grow along the terrace edge, survivors from the clearing that took place when European settlers prepared fields for growing crops. Alfsen said Bill Orr told her that exclusive stands of Garry Oaks can mark locations of carbonate reefs from when the valley was an embayment of the ocean during the Eocene. Only oaks can survive in the pH of the soils in these locations.
Terrace edge of the Yamhill River floodplain.
Fossil in the 310’ depth sample at Amity Hills quarry.
The group then headed north to Oregon 153, then east on 153 towards the Wheatland Ferry. On the way to the ferry, the road transverses the Amity Hills, and stop 4 was at the quarry site there. The back wall of the quarry is a cross section through two flows of the Columbia River Basalt Group (CRBG), the Grand Ronde below and the Wanapum above. Sheila toted along a number of core sections given to her by the quarry owner. The first sample was at 25 ft. depth and showed vesicular basalt with large air bubble holes. Down at the 245 ft. depth the sample is of solid basalt. The most interesting were the samples at 300 ft. and 310 ft. At 300 ft. the sample is of river sediments which are well cemented, implying the ancient path of the Willamette River as it was displaced by the volume of lava pouring into the valley. The sample at 310 ft. is of oceanic bay sediments complete with a fossil gastropod, Bruclarkia, a testament to the shallow seafloor that existed during the Oligocene.
The Amity Hills, the northern segment of the Eola Hills, rise above the valley floor. Most of the CRBG lies below the Quaternary sediments of the valley floor. The Amity Hills stick up in relief due to the erosion of the softer surrounding sandstone, a process producing what is called, inverted, or reverse topography.
The plan of the tour was to proceed onto the Wheatland Ferry and cross to the east side of the Willamette River to stop 5, where the group was to view Mission Lake, an oxbow lake at Mission Lake State Park. Neither one of those happened on the June 2022 trip, however; the late spring rains had produced minor flood conditions. The Wheatland Ferry was closed and the road to the lake was flooded. The GSOC group had to content itself with detouring across the bridge at Salem and backtracking up to Mission Lake, where they found the access to the lake blocked. Fortunately, the picnic area was open and they ate lunch and proceeded up to the east side of the ferry to view the Willamette River and some of its features by its high rushing waters.
GSOC group alongside the Willamette River at the Wheatland Ferry dock. The sign on the first pylon denotes the height of the 1996 flood.
Stop 6 at the banks of the Willamette was a good place to observe river levees, which are formed as flood waters lose velocity as they leave the main channel to spread out over the floodplain. With the drop in velocity comes the dumping of fine grained sediments that previously had been held in suspension. River levees tend to have a rumply character as the deposition is rapid and uneven.
Terraces that flank Lake Labish.
Heading southeast on Brooklake Road from the ferry area, the group crossed Interstate 5 at Brooks and headed to a small turnoff, stop 7, near the intersection of 65th Ave. NE and Labish Center Road NE. What is known as “Lake Labish” is an abandoned channel of the Willamette River. Looking on the shaded relief map of the trip, one sees that this channel, which contains the Pudding River on its north end, is clearly a much more direct path between Salem and Canby than the current channel of the Willamette River. Fluvial dynamic processes are governed by tectonic uplift and shift, water speed and deposition of sediments; and rivers tend to wander back and forth across their valleys in looping paths that change over time. The Missoula Floods occurring between 15,000 and 12,000 years ago also deposited up to 200 feet of sediment in places, completely obliterating previous channels to be reformed in the time after they ran their course.
Base is shaded relief map by R. W. Givler and R. E. Wells, 2001.
The group headed from Lake Labish east along local roads to a stop east of the village of Downs on Downs Road NE. Here at stop 8 they had an unobstructed view of Mt. Angel, a lozenge-shaped hill of Frenchman Springs CRBG rising two to three hundred feet above the Willamette Valley floor. This hill was extruded out from between two crustal blocks along the Mt. Angel fault zone, which produced the Spring Break Earthquake in 1993. Alfsen told the story of the quake, which fortunately happened when school was out of session, because it toppled a large decorative parapet above the entry to Molalla High School. Tectonic movements of the Willamette Valley include down-warping of the valley in the forearc basin, and movement along faults accommodating the rotation of the crust in response to the nearby plate boundary forces.
View of Mt. Angel.
From stop 8, the GSOC group headed east to Oregon Hwy 213 and headed north into the town of Molalla, which was to be the location of the dinner restaurant. But meanwhile there was one more stop in the transect. Heading east from Molalla on S Feyrer Park Road, then south on S Dickey Prairie Rd, the group made its way up the Molalla River into the Molalla River Recreation Area, a gorge lined with Columbia River basalt. The last stop, stop 9, was at the Molalla Rosette, a swirling feature of basalt jointing patterns caused by the Frenchman Springs CRBG lava flowing into a narrow canyon in the ancestral Cascade Mountains. This year (2022) was a good year to view basalt, just upriver from the rosette, in the area which had recently been swept by a large forest fire.
The Molalla Rosette.
So, after visiting the Molalla canyon, the group was ready for a pizza party at Bear Creek Pizza in Molalla. While dinner was cooking, the participants grabbed a sheet of graph paper and plotted the elevations of the points in the transect, then filled in the supposed profile of the basin. The assumptions made in the map construction are that older layers, like the Eocene Siletzia basalt, underlie the younger layers in the valley. This can be confirmed by drill core samples and other imaging techniques done by geologists.
Colorized version of profile cartoon by author. Superposition of younger over older layers drives the order of units in this profile. Coloring scheme roughly corresponds to geology map.
Upon completing their cross sections, the participants celebrated the day by digging in to the good food provided by the restaurant.
REFERENCES
Jim E. O’Connor, Andrei Sarna-Wojcicki, Karl C. Wozniak, Danial J. Polette, and Robert J. Fleck, “Origin, Extent, and Thickness of Quaternary Geologic Units in the Willamette Valley, Oregon,” U.S. Geological Survey Professional Paper 1620, Prepared in cooperation with Oregon Water Resources Department, Reston, Virginia, 2001. https://pubs.usgs.gov/pp/1620/pdf/pp1620.pdf.
Interactive Maps & Geospatial Data - Oregon Department of Geology, https://www.oregongeology.org/gis
“Map showing faults, bedrock geology, and sediment thickness of the western half of the Oregon City 1:100,000 quadrangle, Washington, Multnomah, Clackamas, and Marion Counties, Oregon, by S. Burns and others, 1997. https://www.oregongeology.org/pubs/ims/IMS-004.pdf
“Color Shaded-Relief Map of the Willamette Valley, Oregon,” By. R. W. Givler and R. E. Wells. 2001. https://pubs.usgs.gov/of/2001/0294/pdf/wvc125.pdf
