Geologic History of the Owyhee River

by Audrey Gehlhausen

Why is the Owyhee River geology cool?

The geology of the Lower Owyhee River is unique for several reasons. First, the oldest rocks on the Owyhee are around 16 million years old (Ma). This may seem old; however, geologically speaking this is very young. To give some perspective, the youngest rocks in the Grand Canyon are 245 Ma and the oldest are about 1.8 billion years old; the Appalachian Mountains are around 250 Ma; and the Rocky Mountains started growing about 70 Ma. The oldest rocks of the Owyhee are a mere 16 Ma, with some volcanic rocks as young as 70,000 years old. That’s pretty darn young!

Another thing that’s special about the Owyhee area is how active the river basin has been. Over the last 2 Ma, multiple dams have formed slowing, stopping, and moving the river, but the river fights back and flows on. It’s an ongoing battle with evidence of punches and kicks throughout the river corridor.

And finally, it looks stinkin’ cool! The tall sheer cliff of pink rhyolite, blocky columnar basalt, black and white layers of lavas and sediments, thin deep red layers of baked clay, green, red, tan, purple, grey, black, white… on and on. Geology jargon aside, it’s beautiful to look at!

Definitions of Geology Lingo

Before we get started, here are a few definitions to help understand some geology words.

Volcanic - A rock that was once magma in the earth, rose up through the crust, and cooled on the surface of the Earth. Volcanic is one of two types of Igneous Rocks.  The second type is plutonic, which is magma that then cooled inside the Earth.

Basalt - A type of volcanic rock that is usually dark in color and often breaks into parallel hexagonal columns while cooling.  It is richer in iron, magnesium, and calcium.  It also flows easier and usually travels further from the caldera.

Rhyolite - Another type of volcanic rock that is usually lighter in color, ranging from cream to tan to pink.  Rhyolite is richer in silica, potassium, and sodium, and is more viscous or sticky and does not flow well or far.

Miocene - Miocene Epoch is a section of geologic time from roughly 23 Ma - 5 Ma.  Geologic time is broken down into smaller and smaller groups of time. The largest time periods are Eons, then Eras, Periods, and the smallest are Epochs. Throughout this article many rock layers will simply be described as Miocene to give an age estimate.

Hotspot - A hotspot is source of magma rising up from the mantle and erupting on the surface. Slowly over time, plate tectonics move the land over this stationary faucet of magma and a chain of volcanoes result.  Yellowstone and the Hawaiian Islands are great examples.

Subduction - A geologic process where oceanic crust slides under continental crust (land).  Tens to Thousands of miles of oceanic crust can be subducted into the mantle.  A chain of volcanoes are produced on the continental crust and any island in the ocean will get jammed on to the continental crust.

Fault - A fault is when there is a fracture through the crust and the land on either side moves relative to each other parallel to the fracture.  In a normal fault the land is spreading and dropping down verses a reverse fault where the land is compressed and getting pushed up.

Basement Rocks

Oregon was created from different islands and landmasses colliding onto the western side of the North American plate through subduction starting around 400 million years ago (Ma). The base rock of Oregon is a huge compilation of all sorts of rocks crammed one next to the other, growing westward eventually creating present-day Oregon. These basement rocks are a canvas for further geologic processes to paint on and create the landscapes we see today. In southeastern Oregon, this basement rock is completely buried and thus the exact rock type is unknown. Never the less, it is there and is a platform for creating what we see now.

Yellowstone Hotspot (17 - 15 Ma)

Yellowstone National Park is an incredible, worldwide known location created by the Yellowstone Hotspot, but lesser known and still being discovered is that the magma creating Yellowstone N.P. originally erupted in eastern Oregon.

15 – 17 Ma the Yellowstone Hotspot first poked its head up on land. The volcanoes created were some of the largest to occur anywhere on earth with some flows traveling 400 miles and single flows covering 10,000 square miles. This created a huge area of volcanoes throughout Washington, Oregon, and Idaho; and created the famous Columbia River Basalts of Columbia Gorge. More importantly, it produced volcanic rhyolite flows in southeastern Oregon that we see along the Owyhee River, most notably Iron Point Canyon. Rhyolite most likely covers a much larger area in southeastern Oregon than seen, but because it has since been buried; only small pockets are exposed from the river cutting down through it.

Rhyolite walls of Iron Point Canyon

Figure 1. Rhyolite walls of Iron Point Canyon, river mile 32-35. Photo from Montgomery Rapid, mile 32.5.

Although there is still debate, it is thought that the Yellowstone Hotspot sort of looked like a large upside down tear drop rising up through the earth. So when it reached the surface, it created a large area of volcanoes. Once the large head was erupted, the much smaller tail was left and continued to produce magma. As the tectonic plates moved westward, this tail of magma is what created the trail of volcanoes from the Owyhee area through southern Idaho and up to its present day location of Yellowstone N.P.

Period of Extension (15 - 2 Ma)

Starting around 15 Ma, southeastern Oregon begins stretching east to west. The exact causes of this still are not certain, but there are two main hypotheses.

(1) That residual magma from the hotspot is pushing up and stretching the land.  

(2) That it is the very northern part of the Basin and Range Province of Nevada and Utah.

The Basin and Range Province is large area of east-west extension created by processes that would take a full research paper to even attempt to explain.  Either way the results are the same for southeastern Oregon - north-south running mountains and valleys.

The land, now dotted with rhyolite lava flows, and most likely also spots of basalt, starts to rift, or break apart. North-south normal faults cut through the land creating north-south running mountains and valleys.  During this time hundreds of volcanoes break through and cover the southeast third of the state. There is everything from small cinder cones to calderas many miles across and lava flows of all types, although along the Owyhee River, primarily basalt is seen.

The newly formed valleys also served as great places for water to hold up and hundreds of small lakes evolved during this time. The lakes and volcanoes were constantly interacting. Lakes formed in the valleys, lava flowed into them and forced them to drain, move, or change. New low spots would form and thus new lakes and then eventually lava would flow in again. Often when this hot lava poured onto the bottom of the lakes it would bake the clays of the lakebeds into a hard almost metamorphosed red brick layer. This cycle is seen again and again represented by interbedding layers of lakebed sediments (siltstones, sandstones, gravels) and basalt lava flows.  For the sake of simplicity, these layers will be lumped together and referred to as Miocene lake sediments and lava.  This is very obvious in the Lambert Rocks area and is also seen in several spots throughout the river.

Lambert Dome

Figure 2. Lambert Dome, Mile 25, river left.

This entire section roughly river mile 24-28, on river left, was deposited in the Miocene (prior to the Owyhee river), buried, and later exposed by the river. (While all of the basalts on river right are West Crater Lavas, 70,000 years old.)

More Basalt Volcanoes (2Ma – Present)

This is where the story starts getting a little more complicated. There are a series of primarily basalt volcanoes popping up around the Owyhee valley in the last 2 million years. Although lots of them are not seen along the river, several of them are. Unlike sedimentary rocks which naturally lie younger on top of older, lava flows over whatever topography is present and like water flows downhill and fills low points. It is not uncommon to see much younger basalt appearing to be under older sediments or lava.

In general when floating down the river, it is easier to lump all of these younger basalts together and lump all the older Miocene basalts together and attempt to identify the two groups. However, with several recent research projects, if one wanted, the younger basalt can be divided up and identified.  Five of the more commonly seen flows and their ages are as follows: Bogus Basin (1.8 Ma), Deer Park (780 ka), Clark’s Butte (215 ka), Saddle Butte (144 ka), and West Crater (70 ka).  The main point in mentioning this is give an idea of how complicated the geology really is.

In general, if you see rock that looks more weathered or broken down and appears as though it's only visible because the river carved down and exposed it, most likely it's Miocene basalt or sediments. If rocks appear to be sitting on top, or on new ridges in the canyon, and have a stronger structure (less weathering) most likely they are the younger basalt.

View of Jackson Hole, mile 37, river right. Showing the ridges of Bogus Basin Lava and nub of Clark’s Butte Lava.

Figure 3. View of Jackson Hole, mile 37, river right. Showing the ridges of Bogus Basin Lava and nub of Clark’s Butte Lava.

On the take out drive from Birch Creek, there are the Jordan Craters which cover hundreds of square miles and are only 3,200 years old. A string of these very young volcanoes dot southeastern Oregon. Although there are no currently active volcanoes in the area, it is fair to assume that at some point in the next few thousands of years the Owyhee will be active again.

The Emerging River, Lava Dams, and Landslide Dams (2Ma - Present)

The exact age of when the Owyhee River began is unknown, but it’s believed to have originated within the last 2 million years. Since it started to flow, just like its preceding Miocene lakes, it too has been in an ongoing battle with the lava. As the river starts to cut down, lava flows come and plug it up. Over time the river either fills the reservoir and slowly starts to cut through the dam, or the river finds a more easily erodible area, such as older lakebed sediments, and finds a new path around the dam.  These dams are more of a recent discovery and really ground-breaking for the geology of the area.  It's truly amazing how you can see geology happening here - the fight between river erosion and lava filling in.

When a lake is formed from a dam, deposition occurs forming new lakebed sediments and adding another component to the story. This adds layers of more recent sediments into the mix. From river level it is difficult to determine if the sediments are Miocene or within the more recent last two million years.

Lava Dams

There are two types of dams that form in the Owyhee Canyon – lava dams and landslide dams.  Lava dams occur when flowing lava enters the river and blocks the water.  Usually once the lava enters the river, it starts to flow both up and downstream.  More lava flows in piling on top of itself until it has created a solid dam.  Lava dams thus occur at the time of the eruption.  These dams are hard to erode through and if possible, the river will erode around them and create a new path until it can get back to its original path.

Figure 4. Saddle Butte Lava Dam

Figure 4. Saddle Butte Lava Dam

When lava flows into water, either into the river or more common into the lake that it is forming, a very diagnostic appearance is created, as seen in Figure 4.  Sections of rock like this are seen in several locations along the river (and they're really cool!).  They signify lava flowing into water.  The bottom layer is a yellow-tan color (from weathering) with angled lines of basalt flowing in on top of each other.  The angle shows which direction the lava was flowing.  Above that section is a thin passage layer where the basalt is right at water level, which is then topped by subaerial lava or above the surface.

There are six lava dams on the river, four of which have known location dam sites which are as follows:

Figure 5. View from the top of Lambert Dome on river left, river mile 25, of West Crater Lavas, West Crater Lava Dam, and Bogus Basin Lavas and Canyon.

Figure 5. View from the top of Lambert Dome on river left, river mile 25, of West Crater Lavas, West Crater Lava Dam, and Bogus Basin Lavas and Canyon.

All of the basalt on river right through Lambert Rocks area is the West Crater Lavas at 70,000 years old. All of the rocks on river left are Miocene lakebed sediments and lavas. The Bogus Basin lavas flowed down Bogus Basin Canyon to reach the Owyhee River, creating the Bogus Basin Lava Dam.

Figure 6. Looking up on river right from river mile 44, evidence of Deer Park Lava Dam.

Figure 6. Looking up on river right from river mile 44, evidence of Deer Park Lava Dam.

Landslide Dams

In addition to lava dams there were also landslide dams. As the heavier layers of basalt sit on the more easily erodible lake sediments, at some point a layer in the sediments can slide creating large landslides. The exact catalyst that triggers the slides is unknown. There are multiple spots to see evidence of landslides along the river (Figure 7). These landslides occur sometime after the eruption, cooling, and eroding of the lava.  Landslide dams are more easily erodible than lava dams. The river can usually erode through these dams verses carving around them. Often landslide dams can break catastrophically, move large boulders, and erode or smooth out rocks downstream from the mass release of water. The gravel bar on river right, just below Artillery Rapid is an example of this from the Saddle Butte Landslide Dam. (Sorry, no photo)

Figure 7. Photo from about river mile 14 (just above Hike Out camp)

Figure 7. Photo from about river mile 14 (just above Hike Out camp), on river left, there are large chunks of basalt that are back angled from the river. These chucks of rock slid down from higher up, down to their current location. They are most likely Bogus Point Lava and these chunks continue for about three miles down.

There is evidence that some of these landslides have dammed up the river. Two examples of this are noted below.

Figure 8. Mile 13, River Left, there are several piles of basalt from landslides.

Figure 8. Mile 13, River Left, there are several piles of basalt from landslides.

Figure 9. Mile 13, River Right, two large chunks of basalt that slide down, most likely originated from river left.

Figure 9. Mile 13, River Right, two large chunks of basalt that slide down, most likely originated from river left.

Figure 10. A pinch at roughly around mile 20, about Read it and Weep Rapid.

Figure 10. A pinch at roughly around mile 20, about Read it and Weep Rapid. A section of Saddle Butte Lava slide from river left down and now is on river right creating the pinch. At one point this section was a landslide dam.

The Quick and Dirty Version

Ok, The Owyhee Geology is Cool... Now What?

Go check it out!! Go climb up the black and white Miocene layers of Lambert Dome, touch the 70,000 year old columns of basalt, crick your neck trying to see the top of Iron Point Canyon, discover the dam sites, camp along the persistent carving river, see the amazing multitudes of colors! Go embrace your inner geologist! You can get a crew of friends, boats, and gear and float the river on your own - permits are easy to get. If you're lacking in boats and gear, or just want a little extra help, you can go on a commercial rafting trip.  End of April and May are usually great times to go.  If you want to see more or rafting isn't quite for you, try a hike around Birch Creek Ranch, Jordan Craters, Leslie Gulch, or Succor Creek State Recreation Area. Enjoy!


Originally Published: | Updated on

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Author

Audrey Gehlhausen

Audrey is from the great state of Indiana where she is still famous for her academic exploits and track and field records. After earning a degree in geology from Depauw University she came west where she fell in love with the Middle Fork of the Salmon River. She is now famous among the guiding community for her colorful geology talk, glowing personality, and for being named the Idaho Guide of the Year in 2014.

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