Owyhee River Landslides

For a Google Earth .kmz file to explore Owyhee River Landslides, scroll to the very bottom of the page.

The Owyhee River is located in the very southeastern portion of Oregon State. It hugs the borders of Oregon, Nevada, and Idaho. The river mainly runs through a very narrow canyon, but sometimes opens up into vast valleys. The area is mainly composed of horizontally bedded rhyolite tuffs and sedimentary rocks like conglomerates, sandstones, shales, and claystones, that are overlain by basaltic lava flows (Othus, 2008). The Owyhee River has cut down through the dense basalt and then through the underlying weaker sedimentary rocks. As the river cut through the dense overlying rock, it wasn’t really able to meander back and forth, so the canyon remained very narrow. However, as the river cut down past the dense basalt and into the weaker underlying sedimentary rock, the river began to undercut the canyon walls causing landslides to begin forming, and some of these landslides are huge. There are two very large landslides that are great examples of the types of landslides, or mass wasting events, that can be seen along the Owyhee River Canyon (See Figure 1). These events include large slumps and large earthflow. In fact, it seems in some areas there are mainly slump events and as you get near the confluence of the Owyhee River with the Snake River these large landslide events turn into large earthflows. So, lets start with the events upstream. In a portion of the river referred to as Artillery Reach there is a series of large slump events that line the right side of the river. These events seem to get younger as you move upstream, almost as if they form through headward erosion. If you travel down into the youngest landslide, named Artillery Landslide, immediately you begin to see different backward rotating blocks. What are these exactly? Well, when a slump beings sliding it tends to move in discrete packages along a curved plain (see Figure 2).

Many times slumps move in several sliding events and not just one single event. When you get down into Artillery Landslide you can begin to see some of the sediments underlying the flat basalt cap that include layered conglomerates and sandstones. The sediments show old near stream and in channel deposits, basically showing an old stream system was in the area prior to the formation of the basalt that caps the whole section. These deposits are different from those that can be found further downstream and could be one of the reasons for the differing types of landslides. One interesting aspect of the landslides along Artillery Reach is that, more likely than not, when the landslides moved their mass blocked the river channel. When this happened it would have created a natural dam and over time an impounded lake would have been formed upstream of the dam. As the lake filled with water it would have also filled with sediments. Eventually the landslide dam would have burst either because the lake reached the top of the dam, over-topping the structure and washing it away or just because the lake was pushing so hard on the dam it burst. A burst or compromised dam would have initiated a large outburst flood, and when the water drained from the lake behind the dam it would have sent boulders and other sediments downstream of the dam. Upstream of the dam the flow of the river would have eroded down into the sediments that accumulated behind the dam. This erosion upstream of the dam would form fill terraces, and downstream of the

dam huge boulders can be seen sitting on top of older fill terraces. The youngest landslide in this reach is Artillery Landslide and is about 0.62 mi by 0.5 mi (1 km by 0.8 km) in dimension. Artillery Landslide has been aged at occurring near but before the time of Mt. Mazama’s catastrophic eruption based on ash found near surface in the depression behind the highest backward rotating block of the landslide. Also nearest the river channel you can see evidence of a terrace cut into the toe of the landslide suggesting this landslide is now inactive and the remnants of a rock topple of a basalt from the other side of the river channel (see Figure 3). In this particular stretch of the river there are only rotational slides, rock falls and rock topples and the river channel is steep and narrow. Yet if you go only about 10 km downstream the canyon opens up into a wide valley that is known as The Hole in the Ground, an area where landslide characteristics change drastically.

The Hole in the Ground (not to be confused with Hole in the Ground, another geologic feature in Oregon. I linked a 1961 Ore Bin .pdf about it with pictures and a discussion of whether it is volcanic feature or meteorite crater. Enjoy!) is pretty amazing to see if you are in the area because if you were going to float the Owyhee River from upstream at Rome, Oregon down to The Hole in the Ground you would find a mostly narrow canyon but The Hole in the Ground opens up into a wide area with an old homestead right at the base, with a single towering mesa, several large fill terrace surfaces, and even an area at the very downstream end with hot springs. Its a strange little oasis just up from Owyhee Reservoir and Birch Creek that doesn’t get nearly enough attention. So why does the canyon immediately open up so wide in this area? Well, it most likely has to do with the sediments that underlie the basalt cap in this region. Remember, upstream there was this thick basalt cap overlying more fluvial (or river) sediments; however, in this area the sediments underlying the basalt cap seem to be more lacustrine (generated by lakes) in nature, more fine grained and thinly laminated (Othus, 2008). These sediments are totally different and seem more localized only to this particular area. The resulting landslide from this change are more landslide complexes, starting with slump events near the walls of the canyon and turning into earthflows as the underlying sediments are exposed to the elements. The most clearly seen landslide in this area is East Spring Earth Flow. This landslide has its origin a little over 3 mi (5 km) from the current river channel, and as it flowed from its origin it was squeezed through a small area of rocks that are still in-situ (meaning they are presumably where

they were originally deposited) and finally fans out over .62 mi (1 km) at its base, pinning the river channel up against the opposite wall of the canyon (see Figure 4). This landslide too must have blocked the channel and we can see evidence of this dam both upstream and downstream of the landslide. Upstream there are several fill terraces that most likely were formed as the lake drained and downstream there is an entire bar of flood deposited boulders called Greeley Bar. Also, just on top of the toe of East Spring Earthflow is a channel with rounded boulders, suggesting that water once flowed over the top of the landslide at a far higher elevation than the current river level.

What makes these landslides so important is that they show how large landslides can block rivers and create flooding both up and downstream of the landslide. This is important to understand because landslides can happen along rivers that are more populated. Landslides and flooding resulting from a landslide entering a river channel can be devastating, as we recently saw with the Oso Landslide. These landslides are also useful because they can give us an example of what can happen during the evolution of a river valley when landslides occur frequently. Using the information garnered from studies of landslides like this can help geologists to better identify areas of mass wasting as well as what can results before, during, and after a large mass wasting event.