The northern areas of Lake Roosevelt receive about 17 inches of rain a year.

This rainfall supports dense growths of Ponderosa Pine and Douglas-fir. Grasslands, Alder, willow, hazelnut, and black cottonwood are common along the river and its tributaries.

Moving south, however, the climate becomes far more arid. In the mid-lake area there is a transition from Ponderosa Pine to sage-steppe. Bluebunch, wheatgrass, hard fescue, and forbs such as balsamroot, northern buckwheat, brittle prickly pear, alum root and lupine are common here.

By the time the river/lake reaches Grand Coulee dam, average annual precipitation is only 10 inches. This precipitation occurs mostly in the winter and spring. The summers are hot and dry. So instead of dense forests, the environment is that of a high desert where shrub steppe species like sagebrush and bitterbrush predominate.