Lake Roosevelt’s semi-arid climate lies to the west of the Cascades and east of the Rockies.
In comparison to Western Washington it is much drier, with summers that are hotter and winters that are colder. Average January temperatures in eastern Washington range from less than -7°C (20°F) to -1°C (30°F), often dropping down to -18°C (0°F). July averages are from 18° to 24°C (65° to 75°F). However, daytime temperatures are often above 32°C (90°F).
In addition, the climate within the Lake Roosevelt watershed varies a great deal from one end of the lake to the other. The southern portion of the lake near Grand Coulee is hotter and dryer. Average annual precipitation near the dam is about 10 inches. Area vegetation includes shrub steppe species like sagebrush and bitterbrush.
The areas around the northern end of Lake Roosevelt receive considerably more precipitation. The region around Colville for instance receives about 17 inches a year, enough to support ponderosa pine and Douglas-fir. As the lake gets closer to the Canadian border precipitation continues to increase, coming close to 20 inches annually at the Canadian border.
At higher elevations precipitation also increases dramatically, for instance Chewelah Mountain, which is about eight miles west of Chewelah, receives well over 100 inches of snow annually.
The southern end of Lake Roosevelt is largely sage-steppe and irrigated agricultural lands.
In the mid-lake area there is a transition from sage-steppe to second-growth ponderosa pine. Bluebunch, wheatgrass, hard fescue, and forbs such as balsamroot, northern buckwheat, brittle prickly pear, alum root and lupine are common here. Shrubs such as sagebrush, bitterbrush, rabbitbrush, snowberry, greasewood, and service berry characterize this region. Trees in this portion of the watershed include: black cottonwood, ponderosa pine and Douglas-fir.
In the upper lake area, vegetation includes dense growths of ponderosa pine, Douglas-fir and grasslands, Alder, willow, hazelnut, and black cottonwood are common along waterways. Common shrubs include chokecherry, serviceberry and wild rose.