The Ge0l0gy 0f The Deschutes Basin

Although undoubtedly impressive at the time, volcanic eruptions during the Quaternary have left far less of a mark on the modern channel of the Deschutes River than have landslides from the canyon walls.

Uplift of the Mutton Mountains and changes in base level and sediment delivery have caused the Deschutes River to cut a deep canyon through alternating resistant lava flows and soft volcaniclastic sediment. Between the Pelton-Round Butte Dam Complex and North Junction, the river cuts through the smooth deposition of the John Day Formation, capped by the lava of the John Day Formation, the Columbia River Basalt Group, and the Deschutes Formation. This sequence of lithologies is particularly susceptible to slope failure, and mass-movement deposits ranging from single slump blocks to hummocky landscapes covering 50 square kilometers dominate the valley walls along this river segment.

Many of these blocks likely have been slowly slumping throughout the incision of the river. Still, several landslides were rapid enough to dam it during the Pleistocene. These include the Whitehorse Rapids Landslide (The Pot) that created Whitehorse Rapids, the large debris flow near Dant, and the landslides that delivered boulders to Wapinitia and Boxcar Rapids at RM 55-53. Trout Creek Rapids may also be the remnant of a landslide dam or breach deposit. Few dates constrain these mass movements and resulting floods. Still, they all occurred during the Pleistocene, based on the amount of surface erosion and weathering of boulders in the deposits.

Whitehorse Rapids Landslide is the youngest significant mass movement, and its persistent influence on the channel is complex for boaters to ignore. Whitehorse Rapids, which drops 12 meters over less than 1 kilometer, is the remains of the landslide dam, and high bouldery deposits directly downstream from the rapids were deposited when the dam failed catastrophically. The presence of air fall tephra from the 7.6 ka eruption of Mt. Mazama (Zdanowicz and others, 1999) in a closed depression in the landslide hummocks suggests that the Whitehorse Rapids Landslide occurred before that time. Radiocar dates from flood deposits downstream from Whitehorse Rapids indicate that the landslide dam breached catastrophically at least once between 40 ka and 3.8 ka (O`Connor, Curran, and others, in press).

4100-2800 years ago, cobble bars, bedrock, and sand deposits surfaced 5 to 19 meters above the lower Deschutes River from a large Holocene flood. Much more significant than any historic flow, this flood was named The Outhouse Flood from the Bureau of Land Management toilet facilities built on many of its deposits. This occurred during the middle Holocene, between 4.1 and 2.8 ka (Beebee and O`Connor, in press). Because of the disparity in flow magnitude between the most significant historic flows and the discharge indicated by the high and coarse-grained Outhouse Flood deposits, we initially interpreted the Outhouse Flood to be the result of some sort of dam breach within the basin, similar to but more recent than that caused by the breaching of the Whitehorse Rapids Landslide.

However, no prominent middle Holocene breach site has been located, and step-backwater modeling of three reaches at River 1 (miles 82,65, and 11) indicates that discharge increased substantially downstream like historical storm floods. Hence, our current interpretation of these features is that they were formed by an exceptional meteorological flood with a release two to three times as significant as any flood of record.

According to written accounts and geomorphic evidence, the rain-on-snow flood in December 1861 was the most significant historical flood in the lower Deschutes Basin (Anonymous, 1861). Studies of the Crooked River suggest that the 1861 flood was the largest in that drainage during the entire Holocene (Levish and Ostenaa, 1996).

The stratigraphic position of deposits from the 1861 flood at the lower Deschutes River also shows that it was more significant than recent floods in 1964 and 1996, which were regulated upstream by dams.

The engineered Ochoco Dam near Prinede closed in 1920, providing flow regulation in the Crooked River part of the Deschutes Drainage Basin.

The Dalles Dam: The Dalles Dam on the Columbia River closed in 1956, flooding the mouth of the Deschutes River and Celilo Falls on the Columbia River.

Bowman Dam: Bowman Dam on the Crooked River closed in 1960, providing flood control and water storage in the Crooked River Basin.

Pelton-Round Butte Dam Complex and Flood: The Pelton-Round Butte Dam Complex was completed in 1964, creating Lake Billy Chinook. In December 1964, a rain-on-snow event produced record floods on the Crooked and Deschutes Rivers. Because Lake Billy Chinook was not yet full, the Pelton-Round Butte Dam Complex provided some flood control. The dam is operated for hydropower and recreation but only provides flood control if kept full.

In 1961, three years before Lake Billy Chinook was created, the University of Oregon archaeologist Luther S. Cressman surveyed the three river canyons in this area. Of notable interest was this massive, engraved basalt boulder along the west bank of the Crooked River. Because of its perceived historical value, the Crooked River Petroglyph was extracted in the winter of 1963 and placed at this site, approximately one mile from its original location, as a reminder of the area`s cultural history.

Similar ancient images and designs found worldwide are collectively known as "rock panels" by scientists. In this region, the appearance of these often-stylistic images varies among cultural groups, but the true meaning of the images is known only to those who originally made them.