Unlike the Great Plains of the midwest, the Snake River Plain is basaltic and is a good source of rechargable groundwater. There may be areas where extraction and recharge don't balance (especially in dry years), but on the whole the aquifer is recharged by runoff from the mountains (that disappears once streams intersect the basalt), and seepage from the river and lakes formed by dams on the river. I look at the area as one gigantic sponge. The Great Plains, OTOH, receive little recharge except direct precipitation, which is generally sparse (though some low-flow rivers such as the Platte, Arkansas, and Canadian run through them).