The reason the North Cascades contain such an abundance of glaciers is because western Washington receives a lot of snow, especially in the mountains. Weather moves from west to east across North America, so clouds that pick up moisture in the nearby North Pacific must rise to get over the mountains. As clouds gain altitude the temperature drops, causing the water vapor to condense and fall to earth as rain and snow. By the time the clouds cross into the rain shadow of the eastern side of the range, they are mostly spent and contain significantly less moisture. The average precipitation on the western slope is 112 in., but the Pasayten Wilderness on the east side of the range averages only 12 in.
Water is the life force of the North Cascades.
It falls from the sky, trickles off mountains, replenishes lakes, and flows to the sea. Within the Puget Sound watershed, the Skagit is the largest river. With its 2,900 streams, the Skagit River accounts for 20 percent of the water that empties into Puget Sound. In odd-numbered years, as many as one million pink salmon spawn in the Skagit. In some years, as many as 500 bald eagles spend the winter along the Skagit River.