Scientists aim to increase their data set from spring pulse flows
By Gary Pitzer, Bureau of Reclamation
Prior to the development of the Central Valley Project, the advent of spring would unleash a flood of cold snowmelt into the Northern California watershed – signaling to young Chinook salmon that it was time to begin the long, arduous journey down the Sacramento River, through the Delta, and out to the Pacific Ocean to grow and mature.
While that runoff is now largely managed through storage, it’s still possible to recreate those jolts of water through the system. Thanks to ample storage in Shasta Reservoir, Reclamation has been working to implement spring pulse flows on the Sacramento River.
Pulse flows are rapid increases and decreases occurring during a short time frame. A rim-full Shasta Reservoir prohibited a full-fledged pulse flow exercise, but it’s hoped that enough additional water was added to the system to create favorable conditions for fish while facilitating storage management.
Spring pulse flows, which were called for in the 2019 Biological Opinions for the Coordinated Long-Term Operation of the Central Valley Project and State Water Project, help fish because the ramped-up flows stir particulates (increasing concealment from predators) and increase the travel speed for the two-inch fish.
“These conditions probably mean that the fish are better able to avoid predators on their way to the ocean,” said Lisa Elliot, fish biologist with Reclamation’s Bay-Delta Office.
After spending more than two years at sea, the adult salmon return upstream to spawn.
Two pulse flows were originally scheduled for April 24 and May 8. Flows were indeed ramped up on April 24th, but the schedule was thereafter altered from its initial iteration because of the overall volume of water in the watershed. Reclamation officials realized that more water needed to be released from Shasta for storage management purposes than originally predicted, eliminating a lower flow period between the pulses from which scientists hoped to glean data.
The pulse flow schedule was altered from its initial iteration because of the overall volume of water in the watershed. Reclamation officials realized that more water needed to be released from Shasta for storage management purposes than originally predicted, eliminating the lower flows from which scientists hoped to glean data.
“While the flow schedule alterations may slightly hinder our ability to measure the impact of pulse flows on salmon survival, most of us fisheries scientists would agree that it shouldn’t hinder their actual survival, if anything, it may improve it beyond the original schedule,” said Cyril Michel, assistant project scientist with University of California, Santa Cruz and NOAA Fisheries.
The quirks of the present flows notwithstanding, pulse flows are designed to give provide a glimpse of how fish react to the releases. Acoustic tags were surgically implanted into 1,250 juvenile salmon shortly before their release at Red Bluff to track their migration rate and survival down the river and to the ocean.
“Acoustic receivers will listen for these acoustic tags at fixed checkpoints along the river, similar to a FasTrak on the freeway, and we will be able to identify how many fish survive to different downstream checkpoints,” said Michel.
The 1,250 fish have been released in staggered batches of 250 to provide researchers the best possible portrayal of their response to the flows. To the extent possible, scientists expect to be able to compare the survival and migration rates of the different release groups to estimate the impact of the pulse flow on these metrics, according to Michel. He noted that turbidity sensors deployed in the river to provide researchers a better picture of the conditions encountered by the salmon during their outmigration.
“There’s a gap in turbidity monitoring on the Sacramento from Red Bluff down to the Delta, so we are going to try and fill some of those gaps to see how pulse flows might affect turbidity in the river, which is one of the presumed mechanisms for improving survival of salmon,” Michel said.
The pulse flows were written into the biological opinion to benefit spring-run Chinook salmon, because they are listed as threatened under the Endangered Species Act, said Michel, adding that because of the similarities in outmigration timing, the flows also benefit fall-run Chinook salmon, and, to a lesser degree, winter-run Chinook salmon and steelhead.
The progress of the fish released for the pulse flows can be tracked at https://oceanview.pfeg.noaa.gov/CalFishTrack/pageSpringPulse_2023.html.
