Scientific evidence indicates that the region has alternately been covered by shallow seas and reshaped by volcanic episodes. Vast amounts of lava flow have layered the area forming what is known as the Central Oregon Plateau. The river has carved through sedimentary and igneous rocks ranging in age from more than 65 million years old to as young as 1,300 years old. The vast majority of these rocks are volcanic in origin. Most of the dark colored cliff walls consist of basalt from the large scale Columbia basin basalt flows (17 14 million years ago) and younger more localized Deschutes and Dalles formations (10 6 million years ago). The Deschutes canyon formed between 4 1 million years ago through a series of volcanic, tectonic and erosional processes. The current appearance of the canyon was also influenced by the legendary Missoula floods as well as periodic flooding and erosional processes during glacial periods. The combination of these events that scoured and gouged the land, revealing the pages that time had written during the last 65 million years.
Large scale flooding created many major rapids including White River, Harris, Washout and Colorado. The event that created Washout rapids in 1995 was an intense, isolated thunderstorm that washed debris from a small tributary into the river.
A final note of geologic interest is the sand and small gravel at the left bank confluence of the White River. This is a relic of a lahar (mudflow) from the Old Maid eruptive sequence of Mt. Hood around 1800 A.D. Visit the Lower Deschutes visitor contact center in Maupin to learn more about the unique geology of the Deschutes River.