The Upper Klamath Basin, defined by the drainage area of the Klamath River above Iron Gate Dam, is a unique transitional area between the Cascade Range to the west and the Basin and Range Province of the northern Great Basin to the southeast.
This region extends from the southern Lower Klamath Lake area into the Lost River and Upper Klamath Lake basins.
Crustal stretching and block faulting created a topography with characteristics similar to both regions.
Almost the entire basin is a graben region, bearing basin and range characteristics, formed by uplifting and subsidence along several north–south faults.

Pre-Quaternary, igneous and sedimentary rock compose the Yonna Formation, which crosses much of the region and rises above the surface in large outcroppings of solid rock in many of the ridges.
Underlying rocks are generally younger from east to west.
The many ridges crossing the upper Klamath Basin divide it into valleys with up to 330 feet (100 m) of vertical relief, and drainage patterns generally follow the topography.
An extensive geothermal system occurs deep underground within the upper basin, creating hot springs and artesian springs, but is not well understood.
Further south, in the Shasta River area in Siskiyou County, much of the underlying rock is composed of lava flows issuing from the Mount Shasta volcanic region.

The same age pattern is true in the Cascade Range and Klamath Mountains that cover the western half of the Klamath River watershed.
As the North American Plate moved slowly southwestward over the past 10 million years, successive oceanic terranes dating from the Cambrian to the late Jurassic were added to the bulk of the North American continent.
There are four distinct terranes from west to east.
While the coastal mountains date to less than 3 million years ago, the farther inland High Cascades are as old as 7.5 million years.
Granite batholiths, overlying sedimentary rock, and volcanic rock were crumpled into the massif of southwestern Oregon and northwestern California.
 Instead of being diverted southwards, however, the Klamath continued to flow westward and created a steep-walled gorge through the rising range.
One of these terranes brought with it a long north–south running band of easily eroded mica that now lies about 30 miles (48 km) inland from the Pacific coast of Northern California.
When the Klamath encountered this layer, it began cutting its canyon along the mica instead of continuing southwest to the Pacific, resulting in the sharp northward bend where the Trinity River joins.
The lower Trinity also follows portions of the mica and its south fork as well.
The Upper Klamath Basin, defined by the drainage area of the Klamath River above Iron Gate Dam, is a unique transitional area between the Cascade Range to the west and the Basin and Range Province of the northern Great Basin to the southeast. This region extends from the southern Lower Klamath Lake area into the Lost River and Upper Klamath Lake basins. Crustal stretching and block faulting created a topography with characteristics similar to both regions. Almost the entire basin is a graben region, bearing basin and range characteristics, formed by uplifting and subsidence along several north–south faults.
Pre-Quaternary, igneous and sedimentary rock compose the Yonna Formation, which crosses much of the region and rises above the surface in large outcroppings of solid rock in many of the ridges. Underlying rocks are generally younger from east to west. The many ridges crossing the upper Klamath Basin divide it into valleys with up to 330 feet (100 m) of vertical relief, and drainage patterns generally follow the topography. An extensive geothermal system occurs deep underground within the upper basin, creating hot springs and artesian springs, but is not well understood. Further south, in the Shasta River area in Siskiyou County, much of the underlying rock is composed of lava flows issuing from the Mount Shasta volcanic region.
The same age pattern is true in the Cascade Range and Klamath Mountains that cover the western half of the Klamath River watershed. As the North American Plate moved slowly southwestward over the past 10 million years, successive oceanic terranes dating from the Cambrian to the late Jurassic were added to the bulk of the North American continent. There are four distinct terranes from west to east. While the coastal mountains date to less than 3 million years ago, the farther inland High Cascades are as old as 7.5 million years. Granite batholiths, overlying sedimentary rock, and volcanic rock were crumpled into the massif of southwestern Oregon and northwestern California.  Instead of being diverted southwards, however, the Klamath continued to flow westward and created a steep-walled gorge through the rising range. One of these terranes brought with it a long north–south running band of easily eroded mica that now lies about 30 miles (48 km) inland from the Pacific coast of Northern California. When the Klamath encountered this layer, it began cutting its canyon along the mica instead of continuing southwest to the Pacific, resulting in the sharp northward bend where the Trinity River joins. The lower Trinity also follows portions of the mica and its south fork as well.
Klamath River