Complex sentence: 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.
Split into simpler sentences: The Salt River runs westward through the city of Phoenix, and the riverbed is often dry or contains a little water due to large irrigation diversions. It will have a heavier flow of water after the area's infrequent rainstorms or when more water is released from upstream dams.