How Geology Creates a River’s Identity

By Jake Sahl
Contributing Writer for the Deschutes River Conservancy

This past weekend, I took a visit to Smith Rock State Park to do some rock climbing with a friend. As we walked and climbed around, talking about the different rock types on display and their geologic origins, I couldn’t help but think about how the geology of Central Oregon has shaped the Deschutes River and its tributaries (like the Crooked River, which runs through Smith Rock).

It’s important to realize that geology is one of the major controlling factors for any river’s identity. For example, in a mountainous region, streams are steep and will flow in more-or-less straight lines because they have enough energy to move through most obstacles. Conversely, in areas with less extreme topography, streams meander all over their floodplains, forming huge S-curves as they make their way along the path of least resistance.

In the Deschutes River Basin, the local geology controls everything from channel shape to water temperature to summer flow levels. To explore this relationship a bit deeper, we need to review some basic geologic history.

The portion of the Cascades encompassing the Deschutes River basin is one of the most volcanically active regions in North America and the volcanic rocks that underlie the Deschutes River Basin were formed from the Cascadian volcanoes. At Smith Rock, for example, we can see two distinct types of volcanic rocks, a 30 million year old tuff (formed from an ash deposit following an explosive volcanic eruption) and a 400,000 year old basalt (formed from lava flows).

Because of the fairly extreme topography created by all of the volcanic activity in the area, the Deschutes River is steep—dropping over 4,500 feet from its headwaters before reaching the Columbia River. For comparison, the Columbia River only drops 2,700 feet over its 1,200 mile length. This steep flow path gives the Deschutes a lot of erosive power, allowing it to cut into bedrock and form the many steep, deep canyons it travels through on its way to the Columbia.

The volcanic rock in the region is also responsible for regulating the temperature and flow levels in the Deschutes River throughout the year. Because the rock is so porous, it absorbs a lot of rainfall and snowmelt, releasing it into the river at a slow rate over the course of the year from underground springs. The water from these springs stays at a nearly constant temperature and keeps a low level of flow (known as ‘baseflow’) in the Deschutes and its tributaries regardless of how much rainfall or snowmelt is occurring. These baseflows are very important for fish, especially those which depend on having cool pools to over-summer in (like steelhead).

Perhaps the biggest geologic impact on the area has to do with the climate. If it wasn’t for the Cascade Mountain Range, the climate of Central Oregon (along with the landscape, flora, and fauna) would be completely unrecognizable. But that’s a topic for next time…

About Jake Sahl:
Born and raised in California, Jake moved to Bend in early 2017. As a hydrologist, he is excited to absorb all the local knowledge of the Deschutes River system!
Jake has a BS in Geohydrology from UC Santa Barbara and a MESM in Water Resources from the Bren School of Environmental Science and Management. As a consultant, he strives to mitigate human impacts on the freshwater systems upon which we rely, and to restore natural function to highly-urbanized watersheds. Jake spends too much of his free time rock climbing, but also gets out boating and cycling whenever possible.