Sweetwater Canyon (mile 10.5)

The Owyhee River incised through 150 feet of basalt lava and 300 feet of underlying soft sediment to create the narrow-walled, 450-foot-deep Sweetwater Canyon, an excellent example of a high lava plains canyon. It’s nearly impossible to hike out of the rugged canyon until reaching Hike Out Camp at mile 14.5.

Multiple lava flows:

The spectacular 150-foot-thick basalt lava caprock protects the underlying layers of soft sediment from erosion and preserves the canyon’s scenic, narrow aspect. From a distance the canyon walls appear to be a massive single lava flow, but up close one can see the walls consist of dozens of relatively thin layers of basalt lava. The individual layers may be from different lava flows of different ages that may have pooled here at a low spot in the topography. Sometimes enough time went by between eruptions for sediment to accumulate between the layers. The geology hike from Hike Out Camp to the rimrock leads to an example of sediment sandwiched between two lava flows.

Age and source:

The lava likely erupted from one of the volcanic vents at Owyhee Butte on the lava plateau about three miles west. Geologists think Owyhee Butte probably erupted multiple times. The most recent eruption occurred 1.9 million years ago, the only laboratory-dated one.

Lava plateau:

The high lava plateau on both sides of the river is covered with dozens of volcanic vents (now extinct) and overlapping basalt lava flows of various ages, only a handful of which have been laboratory-dated and mapped. The dated lava flows range in age from a few thousand years to over 10 million years old, and new flows bury older ones, making study difficult.

Ancient rivers:

Deposits of fluvial (carried by water) conglomerate up to 100 feet thick are scattered on both sides of the rimrock—evidence that after the lava flow(s) occurred and before the modern Owyhee Canyon existed, the ancestral Owyhee River flowed on top of the plateau, in roughly this same location.
The base of the lava rimrock sits at approximately the same elevation as traces of an ancient riverbed in the Rome valley, so it’s plausible that when the lava flow(s) arrived, the lava may have dammed a through-flowing river. Beneath the lava are hundreds of feet of light-colored lake and stream sediment like those found in the Rome valley upstream.

Baked sediment & hydrothermally affected lava (mile 11.3)

Most of the underlying sediment in the canyon wall is covered by talus but in places it’s visible and in contact with the overlying basalt lava. Notice the sediment is often an unusual pink color, not tan or white. This is a classic thermal alteration feature that geologists look for in the field. It means the underlying sediment in Sweetwater Canyon was saturated or covered with ground water when the lava flow arrived.

The heat of the incoming lava was so intense that it baked the underlying sediment into hard, pink-colored clay-like bricks, creating the distinctive, pink-colored band that underlies the canyon rimrock. The overlying lava flow was also affected by the intense hydrothermal condition. In this case, it altered the glassy basalt base into a zone of brown-colored palagonite—a mixture of iron-rich clay minerals, iron oxides and hydrated silica.

These features are best seen from river level with binoculars, climbing up the steep talus covered slopes in this reach of the canyon is hazardous. However, the geology hike trail from Hike Out Camp leads to a similar exposure above the camp and about a quarter mile downstream.

Bullseye Landslide and Rapid (mile 13.4)

At the 90-degree bend in the river, Bullseye landslide descended from the rimrock and blocked the river with an earth and rock dam up to 150 feet high. After water infiltrated and overtopped the dam, the dam catastrophically failed. The resulting outburst flood carried boulders and debris downstream and deposited it on a boulder-bar that creates today’s Bullseye rapid, just around the corner. There’s a chance the landslide was caused by the Alvord Lake outburst flood (described earlier), but that would need to be confirmed by future studies. There are dozens of these landslides on the river.

Sources:

(Evans, 1991), (Ely, et al., 2012), (Skilling, et al., 2002), (Godchaux & Bonnichsen, 2002), (Bondre, 2006)