The Rockies

The lush green slopes of the Rocky Mountains are the source of almost all the water that keeps the American Southwest alive. The winter snowpack slowly melts over the year, guaranteeing a steady supply of water for its many rivers of which the Colorado is the most important. If temperatures rise through climate change, there may not be any snow in a few decades. The winter precipitation will immediately run off, leaving the summers dry and hot without enough water. Photos: Ronald de Hommel

The Colorado River delta from the air

Agriculture is the main industry in the Mexicali valley that straddles the vast Colorado River delta. The 1.5 million acre-feet of water that the USA delivers across the border into Mexico are used to water these fields and to supply the major cities Tijuana and Mexicali. Most of it is used to irrigate the vast agricultural area that covers the northern part of the delta.