Original: The Salt River runs westward through the city of Phoenix; the riverbed is often dry or a trickle due to large irrigation diversions, except after the area's infrequent rainstorms or when more water is released from upstream dams.
Simplified: The riverbed is often dry or just a trickle due to large irrigation usage. The river is full after infrequent rainstorms or when more water is released from upstream dams.