The Sanpoil River Arm is a section of Lake Roosevelt in Washington state, formed by the flooding of the lower Sanpoil River when Grand Coulee Dam was built. This arm extends approximately 12 miles upstream from where the Sanpoil River joins the Columbia River. The area offers a mix of open water paddling on Lake Roosevelt and a more riverine experience further upstream.

Access to the Sanpoil Arm is available via State Route 21, which follows the west shore. Keller Park Campground, located beside the lake, provides a potential put-in point for paddlers. The surrounding landscape features rolling shrub steppe, riparian forest along the river, and ponderosa pine forest extending from the waterline to mountain slopes.

Paddlers can expect to encounter a variety of wildlife. The area hosts numerous fish species, including rainbow trout, kokanee salmon, smallmouth bass, and walleye. Bird watchers may spot various passerines, woodpeckers, waterfowl, and other waterbirds, particularly during spring and summer.

It's important to note that the Sanpoil Arm is part of the Colville Indian Reservation. Paddlers should be aware of and respect any tribal regulations that may apply to water access and use. Additionally, the presence of non-native predatory fish like smallmouth bass and walleye at the river/lake interface may impact native fish populations.

Water conditions can vary seasonally. Spring snowmelt and heavy rains have caused significant flooding in the past, as evidenced by a major flood event in 2017. Paddlers should be prepared for potential wind, especially in open areas of Lake Roosevelt.

The Sanpoil Arm offers a unique paddling experience that combines the history of the Columbia River dam system, the cultural significance of the Sanpoil people, and the diverse ecosystems of eastern Washington.