Geology of the Owyhee River Region
The rocks found in the Owyhee River Canyon are relatively young in geologic terms. The oldest are those of the Miocene Sucker Creek Formation, which were deposited about 16 million years ago. In general, the Owyhee rocks are composed of sediments deposited in shallow lake basins, interspersed with volcanic deposits. Fossils preserved here include leaves, pollen, and wood, as well as numerous fish and mammals.
The rocks of the Owyhee Plateau are predominantly flat lying. However, in some areas faulting has broken the landscape up into a series of blocks, creating a rugged landscape.
The Owyhee River from Rome to Owyhee Reservoir cuts neatly through the strata of the Owyhee Plateau, beginning with the youngest rock — the Rome Beds — and ending with the oldest — the Sucker Creek Formation. To float down river is in effect to travel progressively back in time, from rocks 10 million year old in Rome, to rocks 16 million years old at the reservoir. This amounts to a backward step in time of roughly 100,000 years with each river mile.
Owyhee Geology at River Level
In Rome, the buff to tan colored Rome Beds are visible on the skyline in every direction. The evenly layered nature of these sediments is the hallmark of an ancient lake of considerable size. Between the mud/clay layers one sees coarse sand and gravel from the old lake shore.
The fossils preserved in this old lake include a diverse cross-section of mammals, including rhinoceros, camels, horses, bears, peccaries, beaver, antelope, rabbits, and otters. In addition, the bones and scales of many species of fish are preserved. All of these fossils have been dated as late Miocene in age — about 10 million years old. The abundance of huge lake fish and the presence and size of aquatic mammals confirm that the lake was quite large. Toward the northern end of these lake sediments, about 5 miles downstream from Rome, the river cuts directly across the axis of a gentle basin, or “syncline”. Here the Rome Beds have been folded to form a shallow north/south trough.
Less than 6 miles down river from the put-in, the river abruptly leaves its wide-open valley and enters a narrow canyon composed of rocks of the Jump Creek Formation. The formation consists of a volcanic rock called rhyolite. Although rhyolite weathers to a black or gray color, fresh exposures are pink. When they are hot, rhyolite lavas are characteristically stiff, viscous low-temperature flows. The lava’s dough-like consistency, which appears in swirling patterns like that of a marble cake, can easily be seen in the sheer canyon walls.
Downstream, the river moves into a series of very narrow canyons, cut into rocks of the Deer Butte Formation. The Deer Butte consists of alternating thin, fossil-bearing lake and stream sediments, and thicker lava flows. (Fossils in the Deer Butte — rodents, carnivores, and hoofed mammals — are 15 million years old.) For the next 6 to 10 miles the river crosses a series of fractures in the rock, which were probably created during large earthquakes.
Farther downstream, the river canyon opens up gradually, as more and more soft sediment is exposed along the banks. Lambert Rocks, some 25 miles downstream from the put-in, is a fantastically eroded monolith of great beauty. Part of what makes Lambert Rocks so striking is a series of black lava flows that provide sharp contrast to the lighter colored lake sediments. Volcanism has lent other color and drama to Lambert Rocks, as well. As lava flowed rapidly over moist lake flats, clay in the muddy flats was baked by the intense heat, and turned into a natural, red brick. This brick layer is visible below each of the lava flows, and it is particularly resistant to erosion. Each of the picturesque columnar rock formations in the Lambert Rocks badlands is capped or armored by a layer of the brick. The hard, erosion resistant nature of the brick is in fact responsible for the columnar topography.
In the dry washes emerging from Lambert Rocks, opal may be found. The opal is white, glassy where broken, and shows a pearly luster. The quartz in the opal is derived from the dissolved volcanic ash of volcanoes.
A fascinating geologic feature found below Lambert Rocks is the presence of “intercanyon lava flows”. About 10,000 years ago, a series of volcanoes on the Owyhee Plateau poured hot, runny lava into the river canyon. Although these lava flows did not entirely fill the canyon, they did disrupt the river’s flow for a time. Faced with a wall of rock in its path (the cooled lava), the river began the slow process of cutting around the blockage. Generally, the easiest route was along one of the old canyon walls.
Intercanyon flows are visible today where the two canyon walls don’t match. On one canyon wall, lake or stream sediments are exposed in gentle slopes, and on the opposite wall, blocky lavas form sheer cliffs.
Volcanic breccias are common along this stretch of river. Breccia forms where volcanoes turn explosive due to the sudden release of water and gas. The result is rock composed of yellow volcanic ash studded with small blocks of lava.
Faults are also common along the river. Whistling Bird Rapid, perhaps the Owyhee’s most challenging, was formed by a large block of rock and debris that slid down a steep fault line on the east side of the river.
After Whistling Bird Rapid the river takes an abrupt eastward turn into Green Dragon Canyon (aka Iron Point Canyon). Incredibly sheer walls make this canyon one of the most picturesque sections of the river. The rocks here are predominantly banded rhyolite, and the varicolored pinks and grays characteristic of this rock are spectacular.
Downstream from Green Dragon Canyon a boater encounters Montgomery Rapid, another of the river’s noteworthy drops. Below Montgomery, the river flows past old lake bed, coarse sand and gravel stained in multiple hues by oxidized iron in the sediments.
For the last 6 to 10 miles before reaching the Owyhee Reservoir, the rock seen in the canyon is of the Sucker Creek Formation. This brightly colored rock formed in an ancient stream and lake environment that experienced a series of volcanic ash flows. The Sucker Creek Formation contains scattered mammal fossils dating from the early Miocene (16 million years old), but it is most famous for its rich deposits of fossil leaves. Plant fossils have been used to estimate local rainfall during the Miocene (43 inches per year). Regional altitude, latitude, and minimum temperatures have all been calculated as well, based on the fossil plant record.
From the takeout — at Leslie Gulch on Owyhee Reservoir — the road back to Jordan Valley winds up the Gulch, through a beautiful, colorful landscape typical of the Sucker Creek Formation, and onto the Owyhee Plateau.