The Geology of The Deschutes Basin
Fifty million years of geologic history are dramatically displayed on the canyon walls of the middle Deschutes River and lower Crooked Rivers. Volcanic eruptions that occurred over thousands of years created a large basin dramatized by colorful layers of basalt, ash and sedimentary formations. The most significant contributor to the outstandingly remarkable geologic resource is the unique intra-canyon basalt formations created by recurring volcanic and hydrologic activities.
Mt. Jefferson Lahar
|Along the lower 50 kilometers of the river, - gravelly lahar deposits approximately 15 meters above the current river level contain pumice from a 100 ka eruption of Mt Jefferson. This lahar traveled at least 200 kilometers from its source, apparently a record for the Cascade Range U.E. O`Connor and A.M. Sarna-Wojcicki, unpub data, 2000). Pumice and ash-rich lahar deposits from the Mt Jefferson eruption are also interbedded in fluvial gravel 50 meters above river level near the Warm Springs River confluence
Pleistocene Landslide Dams and Outburst Floods: 40,000-10,000 Years Ago
|Although undoubtedly impressive at the time, volcanic eruptions during the Quaternary have left far less of a mark on the modem channel of the Deschutes River than have landslides from the canyon walls. Uplift of the Mutton Mountains, along with 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 soft sediment 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 segment of the river. Many of these blocks likely have been slowly slumping throughout incision of the river, but several landslides were rapid enough to dam it during the Pleistocene. These include 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, but they all seem to have occurred during the Pleistocene, based on the amount of surface erosion and weathering of boulders in the deposits. Whitehorse Rapids Landslide seems to be the youngest large mass movement, and its persistent influence on the channel is difficult 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 airfall 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 prior to that time. Radiocar- bon dates from flood deposits downstream from Whitehorse Rapids indicate that the landslide dam breached catastrophically at least once between about 40 ka and 3.8 ka (O`Connor, Curran, and others, in press).
|Missoula Floods 15,000-12,000 Years Ago When ice dams impounding Glacial Lake Missoula failed repeatedly in the Pleistocene, floodwater traveled down the Columbia River and backed up in tributaries to an altitude of approximately 300 meters above sea level in the vicinity of the Deschutes River. Slackwater silt and ice-rafted boulders were deposited on the banks of the Deschutes River as far south as Maupin (Orr and others, 1992). The effects of the Missoula Floods are not evident upstream from Maupin, but its tan silt up to several meters thick mantles the banks of the Deschutes River downstream from about RM 50.
Old Maid Lahar
|Old Maid Lahar: Approx AD 1800 Mt Hood`s Old Maid eruptive cycle about AD 1800 produced at least one lahar that entered the Deschutes River via the White River. Its volume was meager compared to lahars from the Bend-Tumalo and Mt Jefferson eruptions, but remnant gray pebbly sands lie near the White River confluence at RM 47, well above the stages of floods during the past 200 years.
Outhouse Flood: 4100-2800 Years Ago
|Outhouse Flood: 4100-2800 Years Ago Bouldery cobble bars, massive sand deposits, and stripped bedrock surfaces 5 to 19 meters above summer low flow stages along the lower Deschutes River were left by at least one exceptional Holocene flood that was substantially larger than any historic flow (Figs. 8,9). The Outhouse Flood, named for Bureau of Land Management toilet facilities built on many of its bouldery deposits, 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 largest historic flows and the discharge indicated by the high and coarse-grained Outhouse Flood deposits, we originally 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 obvious 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 I increased substantially downstream in a manner similar to historical storm floods Hence our current interpretation of these features is that they were formed by an exceptional meteorological flood with a discharge 2-3 times as great as any flood of record
Historical Flood: AD 1861
|Historical Flood: AD 1861 According to written accounts and geomorphic evidence, the rain-on-snow flood in December 1861 was the largest historical flood in the lower Deschutes Basin (Anonymous, 1861). Studies of the Crooked River suggest that the 1861 flood was the biggest flood 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 larger than recent floods in 1964 and 1996, which were regulated-upstream by dams
Dams from 1920 to 1964
|Ochoco Dam: AD 1920 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: AD 1956 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: AD 1960 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: AD 1964 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. Presently, the dam is operated for hydropower and recreation but does not provide flood control because it is kept full.
Historical Flood: AD 1996
|Historical Flood: AD 1996 The February 1996 flood was caused by a rain-on-snow event similar to the one in 1964. Although this flood was regulated by the Ochoco and Bowman Dams, Lake Billy Chinook was already full, and the flood wave passed through the Pelton-Round Butte Dam Complex without attenuation. The 1996 and 1964 floods were similar in discharge, and they are calculated to have a recurrence interval of about 100 years in light of both the gaged record and the stratigraphic record of flooding during the past 2000 years