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Warmer water spells trouble for salmon
Anglers fish from the Mossdale Crossing railroad bridge pier on the San Joaquin River in Lathrop. - photo by HIME ROMERO/The Bulletin


The Bulletin

Temperatures are running 5 degrees warmer in the San Joaquin River at Vernalis 10 miles southwest of Manteca due to low water flows.

This isn’t good news for the Chinook salmon that are struggling to survive in the face of non-native predators in the Delta as well as the continuing California drought.

United States Geological Survey data shows temperatures reached 84 degrees on June 30 and then pushed 86 degrees July 6-8 before dropping to 82.5 degrees on July 12. That is a full five degrees warmer than the same time in 2014 when the San Joaquin River was dealing with the third year of the current drought. The ideal temperature the Chinook salmon is considered 65 degrees.

The water flow at Vernalis is 80 cubic feet per second.

“That’s a very slow flow for the San Joaquin River,” noted South San Joaquin Irrigation District General Manager Jeff Shields.

Shallower water means higher temperatures.

The combination of less water and air temperatures that typically exceed 100 degrees through much of July and August is concerning many whose agencies are responsible for fish health on the San Joaquin River. That includes the SSJID that belongs to the San Joaquin River Tributaries group of water districts.

The critical time for the Chinook salmon will come in October and November during spawning season. The best water temperature for spawning is 56 degrees.

But the fact New Melones is expected to have dropped to near 200,000 acre feet out of a capacity of 2.4 million acre feet by then. That means what water that can be released will be much warmer than usual. Also the flows will be significantly below normal to additionally elevate water temperatures.

The SSJID and Oakdale Irrigation District have spent more than $1 million over the past 10 years improving fish habitat and studying the Chinook salmon on the Stanislaus River.

Water temperature is considered to be the single most important factor that affects the distribution and survival of Chinook salmon. Chinook are cold water species and respond to small water temperature variations during different life stages. Unlike most mammals whose body temperature stays nearly constant, Chinook are poikilotherms. That means their body temperature varies with the temperature of their environment such as water temperature.


To contact Dennis Wyatt, email