2 SLO County reservoirs are spilling after storms. Here’s a look

The recent atmospheric river storms pushed San Luis Obispo County reservoirs over the edge — literally.

Lopez Lake and Santa Margarita Lake were both overflowing as of Thursday morning, according to the San Luis Obispo County Public Works Department.

The Salinas Dam at Santa Margarita Lake began spilling at about 8:30 a.m. on Monday, the county’s data show.

Santa Margarita Lake spills on Feb. 6, 2024.
Santa Margarita Lake spills on Feb. 6, 2024.

Between Sunday and Monday morning, the reservoir in central San Luis Obispo County received about 3 inches of rain and gained more than 601 million gallons of water to push water levels over its spillway, according to the county’s data.

The reservoir feeds into the Salinas River, which runs north through Templeton, Atascadero and Paso Robles before making its journey through Salinas and to the ocean at Monterey Bay.

Water flows in the Salinas River under the East Garrison Bridge in Bradley in February 2024.
Water flows in the Salinas River under the East Garrison Bridge in Bradley in February 2024.

Lopez Lake spills for second year in 25 years

Lopez Lake, northeast of Arroyo Grande, began trickling over its spillway on Wednesday afternoon, according to the county.

By Friday, water was fully flowing down the spillway, Public Works shared in a video post on X.

Between Sunday morning and Wednesday evening, the reservoir received about 4.3 inches of rain, the county’s data show. During that same period, the lake gained roughly 437 million gallons of water to push it past capacity.

The last time the reservoir spilled was March 2023. That had marked the first time the reservoir spilled in 25 years.

A second year of Lopez Lake spilling doesn’t necessarily come as a surprise, however, as the large storms helped the reservoir to regain water storage lost during seismic retrofits to the dam more than two decades ago.

Lopez Lake is located along Arroyo Grande Creek, which runs southeast through Arroyo Grande and empties into an estuary adjacent to the Oceano Lagoon.

When Lopez Lake spills, it can increase Arroyo Grande Creek’s flows by 20% to 40%, according to county officials. Historically, those peak flows coupled with torrential storms have caused the Arroyo Grande Creek levee to breach and flood parts of Oceano.

However, repairs to the levee completed in the fall should fend off levee breakage during large storm events, according to San Luis Obispo County water resources division manager Courtney Howard.

Lopez Lake trickles over its spillway on Feb. 7, 2024, after atmospheric river storms dumped several inches of rain on the reservoir’s watershed.
Lopez Lake trickles over its spillway on Feb. 7, 2024, after atmospheric river storms dumped several inches of rain on the reservoir’s watershed.

Will Whale Rock Reservoir spill this year?

Whale Rock Reservoir, located just outside of Cayucos, is close to spilling as well.

As of Wednesday, the reservoir was at about 97% capacity with water levels about 2 feet below its spillway, according to the City of San Luis Obispo.

Whale Rock has a much smaller watershed than the other two reservoirs, making it slower to gain water even from big storms. Although it’s difficult to predict when the reservoir may spill, Noah Evans, the Whale Rock supervisor for the city, surmises it could happen this season should more large storms hit.

“We still have two months remaining in the wet season, and the chances of receiving additional storms that may fill the reservoir are high,” Evans wrote in an email to The Tribune. “I am confident that Whale Rock will spill after receiving another storm providing significant rainfall.”

Whale Rock spilled last March for the first time in 18 years.

Lake Nacimiento, a reservoir in northern San Luis Obispo County fed by the Nacimiento River, remains far below spilling.

The large reservoir was at 73% capacity as of Thursday, according to the Monterey County Water Resources Agency, which manages the lake’s water although it lays within San Luis Obispo County.

During the latest storm from Sunday through Wednesday night, the lake gained nearly 12 billion gallons of water, according to Monterey County’s data.