The Columbia River basin is a complex tapestry of mountains, high plateaus, desert basins, river valleys, rolling uplands, and deep gorges woven together by the Columbia River and its tributaries. Mountains are a major and dramatic presence in the Columbia River Basin. There are a number of mountain ranges in the basin, including the volcanic Cascades forming the western border and the Rocky Mountains on the basin's eastern border.
Much of the Columbia river basin experiences the rain shadow effect of the north-south mountain ranges that block moist winds from the Pacific Ocean. As the air rises, it cools, losing ability to retain moisture. Rain and snow fall on the western side, while lands east of the mountains remain arid. Rain shadows are particularly evident where major mountain ranges are perpendicular to the direction of prevailing winds and storm tracks coming from the ocean, as with the Cascades. One of the most dramatic climate changes occurs here where the lush, moist old-growth forests on the western side give way to an arid shrub-steppe environment on the eastern side. Climate varies greatly throughout the basin, depending upon elevations ranging from just above sea level to over 3,900 m. Annual precipitation can vary from about 25 cm to 250 cm.
A common feature of the Columbia River Basin is the shrub-steppe ecosystem where moisture is scarce (coming mostly from snow in winter), the wind is persistent, and temperatures vary from 38C in summer to well below freezing in winter.