Thursday March 19, 2026
California Policy Center —
The salmon decline in the delta has been attributed to the impact of water withdrawals into the California Aqueduct and the Delta Mendota Canal. But something else happened at the same time as the pumps began operating; dredging in most channels in the delta virtually ceased. For the last 50 years, especially in the south delta, silt has been accumulating. This has harmed an aquatic environment that, for the century prior to the 1970s, had a thriving salmon population that was not adversely affected by dredging.
Many delta channels that had been maintained at depths of 12 feet or more up until the 1970s are now less than two feet deep. The introduced non-native species of bass that prefer warm shallow water are now free to prey on salmon and other pelagic fish that prefer cooler and deeper water. Even without bass predation, salmon can’t possibly migrate as effectively in water that is so much warmer, and the water isn’t warmer because of climate change, it’s mostly warmer because the channels are so shallow.