Climate varies widely across the watershed. Mean monthly high temperatures are in the upper basin and in the lower basin, and lows average , respectively. Annual precipitation averages , ranging from over in some areas of the Rockies to just along the Mexican reach of the river. The upper basin generally receives snow and rain during the winter and early spring, while precipitation in the lower basin falls mainly during intense but infrequent summer thunderstorms brought on by the North American Monsoon.