The Geology of Lake Roosevelt National Recreation Area

Lake Roosevelt. Image from the National Park Service's page.

Lake Roosevelt National Recreation Area in northeastern Washington protects the largest reservoir in the Pacific Northwest. This summary draws on the National Park Service's Geodiversity Atlas, part of the Geologic Resources Inventory.

The park

• Franklin D. Roosevelt Lake formed behind Grand Coulee Dam, built in 1941, and stretches 214 km (133 mi) from the dam to Onion Creek through Ferry, Grant, Lincoln and Stevens Counties.
• The park was established as Coulee Dam Recreational Area on December 18, 1946, and renamed on January 1, 1997.
• It covers about 40,626 hectares (100,390 acres), protecting the natural and cultural resources of the Upper Columbia River Basin behind the dam, with more than 483 km (300 mi) of public shoreline for boating, fishing, camping, picnicking and sightseeing.
• The lake lies where the Okanogan Highlands meet the Columbia Basin.

Four forces that shaped the land

The landscape was carved by a rare sequence of geologic events:

1. Tectonic plate convergence. Metamorphic, igneous and sedimentary rocks around the lake record ancient plate collisions. Near Kettle Falls in the north are metasedimentary rocks that were once seafloor.
2. Volcanism. Grand Coulee Dam stands on granite formed by volcanism tied to subduction. In the Miocene, vast lava flows built the Columbia River Basalt Group, whose dark, layered basalt lines the lake's southern edge from Fort Spokane to the Grand Coulee Dam area.
3. Continental glaciation. Pleistocene ice sheets scoured the mountains in the north of the park.
4. Cataclysmic Ice Age floods. In the south, glacially dammed lakes repeatedly burst, unleashing immense floods that carved the broad, box-like canyons of the Channeled Scabland. Signs of these floods are everywhere around the lake, in carved channels called coulees and in layered sediments called rhythmites.

A narrow arm of Lake Roosevelt. Image from the National Park Service's page.

Regional setting and further data

The park lies in the Columbia Plateau and Northern Rocky Mountains physiographic provinces and shares their geologic history. The Geologic Resources Inventory has produced geologic maps and reports for the park, and a Soil Resources Inventory has been completed; both are on the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Lake Roosevelt National Recreation Area, Washington."
• Works it cites: National Park Service (2015, 2016).