Knickpoint Retreat and Stream Channel Morphology in the Columbia River Gorge

Background:

The location and appearance of waterfalls results from several geomorphic and geologic processes. Waterfalls can be formed by varying rates of rock cooling, rock jointing, and mountain building from tectonic forces or volcanism. Geomorphic processes like weathering and erosion control where waterfalls are located in a watershed as well as waterfall appearance (Hudson, 2012). Waterfalls tend to be concentrated in mountainous areas along streams with steep gradient but can also occur along major rivers when there is a sharp change in channel slope (Stachelrodt, 1971). These points of extreme relief along a stream channel are called “knickpoints” and often correspond with waterfall location.  Knickpoints propagate upstream as they erode, creating distinct boundaries between adjusting and relict topography. Rates of propagation depend on rock strength, rock type, and layering (Lamb, 2009). Streams and waterfall erosion are also greatly controlled by climate. This can be expressed directly through precipitation or indirectly through influence of soils, weathering, and vegetation on stream discharge (Wohl, 2000).

Basalt comprises the bedrock for all waterfalls in the Columbia River Gorge. The striking basalt formations seen in the gorge are part of the Columbia River Basalt Group (CRBG), a series of lava flows that erupted from fissures almost 17 million years ago in what is now eastern Washington. These lava flows comprise a large igneous province of basalt that stretches throughout much of eastern Oregon, eastern Washington and western Idaho. A bulk of these flows spilled through the Columbia River Gorge, scraped the northern end of the Willamette Valley and even made it to the Pacific Ocean (Reidel, 1989). Lava flows from the columbia river basalt group are diverse in texture due to the typical layering of several flows with different chemical compositions and varying grain size (Stachelrodt, 1971). The wide variety of waterfalls in the gorge are due to the diversity of basaltic cooling rates. Although the CRBG laid the underlying bedrock of waterfalls in the gorge, the true reason for so many waterfalls in the Columbia Gorge is differential rates of erosion between the Columbia River and it’s smaller tributaries (Allen, 1979). Smaller tributaries erode at a much slower rate than the Columbia River, forcing them to fall over hanging valleys and layers of lava flows. Steep drainages that contain creeks of lower discharge typically result in poor sorting and a cascade or step pool morphology (Knighton, 2009).

Framing Question: How does knickpoint retreat affect stream channel morphology in mountain watersheds?

Situated Context:

The Columbia River Gorge is a massive canyon carved by the Columbia River that spans 80 miles from Troutdale to Hood River, Oregon. A wide range of topography and elevation makes the gorge an extremely ecologically diverse place, transitioning from lush woodlands to shrubby grasslands in eighty miles (Topik, 1982). A combination of steep topography and generous rainfall makes the Columbia River Gorge a waterfall hotspot (Allen, 1979). Many of the Gorge’s waterfalls lie within the Columbia River Gorge National Scenic Area (NSA) that is managed by the Columbia River Gorge Commission and the United States Forest Service (USFS, 1992). The gorge holds dozens of watersheds that contain many knickpoints but are currently inaccessible due to the Eagle Creek Fire. The Bridal Veil Creek watershed towards the western end of the gorge remains largely open and contains at least three consecutive knickpoints along it’s main stem. The Bridal Veil Creek watershed is also an especially fluvially active place. Sediment transport from logging in the early 20th century completely changed the appearance of Lower Bridal Veil Falls by filling in the pool between the two tiers (Cowling, 2001). Such active sediment transport has interesting effects on stream morphology and waterfall erosion. 

Focus Question: How does knickpoint retreat rate and stream channel grain size affect landscape development at Bridal Veil Falls?

Methodology:

Conduct a pebble count above and below Lower Bridal Veil Falls in the Columbia River Gorge to draw possible conclusions on differences in stream channel composition. Count and measure 100 pebbles directly below the waterfall, then another half- mile downstream. Do the same for at the top of the waterfall than conduct another pebble count half -mile upstream. It would be difficult to impossible to conduct a pebble count further than a half-mile upstream of the lower falls as the middle falls blocks any upstream progress. Find average pebble size for above and below a waterfall to see if what (if any) effects the drop has on grain size.

I would also investigate how to measure knickpoint retreat rate by reviewing case studies and scholarly literature on the topic. This would include research on what specific type of basalt is present at Bridal Veil Falls and how quickly it may erode in proportion to Bridal Veil Creek’s average discharge. Landscape response to knickpoint retreat will also be important to consider with regards the Columbia River Gorge Scenic Highway crossing Bridal Veil Creek almost directly above the falls. Upon gathering sufficient information, I would attempt to find a correlation between the rate of knickpoint retreat and grain size in the stream channel.

Broader Implications

Pebble count findings would shed light on how knickpoints influence grain size along different sections of stream in the Bridal Veil Creek watershed. This would also test the significance of waterfalls as points of transition along a stream’s course. If stream bed morphology changes significantly above or below a waterfall, this has far reaching implications for the surrounding ecosystem. Grain size in a stream is essential for the survival of riparian ecosystems and the morphology they create controls salmon migration. A greater understanding of stream bed morphology also helps to make decisions regarding the construction of stream side infrastructure like roads, trails, or bridges. Understanding rates of knickpoint retreat also aids in determining how close to the brink of a waterfall one should build. 

 

Works Cited:

Allen, John Eliot. The Magnificent Gateway: A Layman’s Guide to the Geology of the Columbia River Gorge. Scenic Trips to the Northwest’s Geologic past; No. 1. Forest Grove, Or.: Timber Press, 1979.

Cowling, Tom, 2001, “Stories of Bridal Veil, A Company Mill Town, 1886-1960

Knighton, D. 1998. Fluvial forms and processes: A new perspective. 3d ed. London: Routledge.

Lamb, Michael P., and William E. Dietrich. 2009. “The persistence of waterfalls in fractured rock.” GSA Bulletin 121 (7-8): 1123–34

Reidel, Stephen P., and Hooper, Peter R. Volcanism and Tectonism in the Columbia River Flood-basalt Province. Special Papers (Geological Society of America) ; 239. Boulder, Colo.: Geological Society of America, 1989.

Stachelrodt, Chris. 1971. “A Geomorphic Study of Waterfalls and Basalt Jointing in the Columbia River Gorge.”

Topik, C., Wagner, D., Cook, S., Udovic, J., & Frank, P. (1982). Columbia River Gorge. Science, 745.

Wohl, Ellen E. Mountain Rivers. Water Resources Monograph; 14. Washington, D.C.: American Geophysical Union, 2000.

United States. Forest Service. Management Plan for the Columbia River Gorge National Scenic Area. Washington, D.C.?]: [USDA Forest Service], 1992.

 

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