Extending from arid eastern Oregon to the cold and rainy Northern California coast, the Klamath River watershed drains parts of three Oregon counties and five counties in California and includes a diversity of landscapes.
The northernmost part of the watershed is high desert country drained by the Williamson River and the Sprague River, both which flow generally southwest into Upper Klamath Lake.
The middle basin is characterized by extensive wetland, grassland, and agricultural areas, and is partially filled by two major bodies of water: Upper and Lower Klamath Lake.
The extensive lower basin, which encompasses over one half of the 15,689-square-mile (40,630 km²) watershed, is composed mainly of rugged mountains, forests and canyons.

Several other West Coast and interior drainage basins border on that of the Klamath River.
On the northwest are the Rogue River and Umpqua River in Oregon and the Smith River in California.
On the east there is the closed Harney Basin and a small portion of the Great Basin.
The south side of the Klamath River watershed is bounded by the Sacramento River and its upper tributaries, including the Pit River, and on the southwest side are the Mad River and Redwood Creek.
The western boundary of the upper Klamath Basin is formed by the High Cascades and the Klamath Mountains, one of the southernmost extents of the Cascade Range, and the California Coast Ranges cover the southwestern watershed.
The Klamath is one of only three rivers that begins east of the Cascades and flows into the Pacific Ocean; the other two are the Columbia and the Fraser.

Most human use of the watershed is limited to the upper basin.
Despite the semiarid climate, dams have been built, irrigation water has been supplied from the Klamath and Lost rivers, and plentiful groundwater has been drawn to transform most of the upper Klamath Basin to farmland.
At least 11,000 years ago, Lower Klamath and Tule Lakes in the rainy season would combine into one giant freshwater marsh that was nearly 290 square miles (750 km²) large.
This, combined with the over 100 square miles (260 km²) of Upper Klamath Lake, formed a temporary habitat for millions of migratory birds.
These lakes are all remnants of a large Ice Age lake, Lake Modoc, that covered about 1,100 square miles (2,800 km²).
Although all of the marshlands have been developed with the exception of Upper Klamath Lake, about 3.7 million migrating birds still pass through the watershed each year.

Despite its plentiful flow in California, the Klamath does not supply significant amounts of water to irrigators and municipal users in central and southern portions of the state.
The Klamath Reclamation Project in the  Klamath Falls area supplies water to local irrigators, and the Central Valley Project diverts water from the Trinity River to supply irrigation water to the Sacramento Valley.
Other tributaries of the Klamath, including the Lost and Shasta rivers, are also diverted for irrigation.
Water use of the lower Klamath—one of the last relatively free-flowing rivers in the state of California—has been debated for decades among conservationists, tribes, irrigators, and government agencies, and its eventual fate is still unclear.
Extending from arid eastern Oregon to the cold and rainy Northern California coast, the Klamath River watershed drains parts of three Oregon counties and five counties in California and includes a diversity of landscapes. The northernmost part of the watershed is high desert country drained by the Williamson River and the Sprague River, both which flow generally southwest into Upper Klamath Lake. The middle basin is characterized by extensive wetland, grassland, and agricultural areas, and is partially filled by two major bodies of water: Upper and Lower Klamath Lake. The extensive lower basin, which encompasses over one half of the 15,689-square-mile (40,630 km²) watershed, is composed mainly of rugged mountains, forests and canyons.
Several other West Coast and interior drainage basins border on that of the Klamath River. On the northwest are the Rogue River and Umpqua River in Oregon and the Smith River in California. On the east there is the closed Harney Basin and a small portion of the Great Basin. The south side of the Klamath River watershed is bounded by the Sacramento River and its upper tributaries, including the Pit River, and on the southwest side are the Mad River and Redwood Creek. The western boundary of the upper Klamath Basin is formed by the High Cascades and the Klamath Mountains, one of the southernmost extents of the Cascade Range, and the California Coast Ranges cover the southwestern watershed. The Klamath is one of only three rivers that begins east of the Cascades and flows into the Pacific Ocean; the other two are the Columbia and the Fraser.
Most human use of the watershed is limited to the upper basin. Despite the semiarid climate, dams have been built, irrigation water has been supplied from the Klamath and Lost rivers, and plentiful groundwater has been drawn to transform most of the upper Klamath Basin to farmland. At least 11,000 years ago, Lower Klamath and Tule Lakes in the rainy season would combine into one giant freshwater marsh that was nearly 290 square miles (750 km²) large. This, combined with the over 100 square miles (260 km²) of Upper Klamath Lake, formed a temporary habitat for millions of migratory birds. These lakes are all remnants of a large Ice Age lake, Lake Modoc, that covered about 1,100 square miles (2,800 km²). Although all of the marshlands have been developed with the exception of Upper Klamath Lake, about 3.7 million migrating birds still pass through the watershed each year.
Despite its plentiful flow in California, the Klamath does not supply significant amounts of water to irrigators and municipal users in central and southern portions of the state. The Klamath Reclamation Project in the  Klamath Falls area supplies water to local irrigators, and the Central Valley Project diverts water from the Trinity River to supply irrigation water to the Sacramento Valley. Other tributaries of the Klamath, including the Lost and Shasta rivers, are also diverted for irrigation. Water use of the lower Klamath—one of the last relatively free-flowing rivers in the state of California—has been debated for decades among conservationists, tribes, irrigators, and government agencies, and its eventual fate is still unclear.
Klamath River