# Why does the San Luis Reservoir level swing so much each year? > Because pumps fill it, not a river. The federal and state water projects pump surplus Delta water into San Luis Reservoir in winter and spring and draw it down through summer for farms and cities south of the Delta. _Updated Sep 24, 2026._ San Luis Reservoir's level swings because pumps fill it, not a river. It sits off-stream on the west side of the San Joaquin Valley, and the federal Central Valley Project and the State Water Project pump surplus water from the Sacramento-San Joaquin Delta uphill into it in winter and spring, then release it through the summer to farms and cities south of the Delta. Over 1991-2020 its storage averaged highest in March and lowest in August, and in a run of dry and wet years the surface can move nearly 200 feet. [Chart: San Luis Reservoir storage](https://the-waterline.com/reservoir/san-luis-reservoir) ([data](https://the-waterline.com/reservoir/san-luis-reservoir.md) — chart image: https://the-waterline.com/charts/reservoir/san-luis-reservoir/chart.png) The surface stands at 498.51 feet above sea level on Oct 7, 2026, with 1.49M af in storage. ## How water gets in and out San Luis is the largest off-stream reservoir in the United States, held behind B.F. Sisk Dam, a 382-foot-high earthfill embankment more than 3.5 miles long about 12 miles west of Los Banos, according to Reclamation. Water reaches it from the Delta along two canals: the State Water Project's California Aqueduct and the federal Delta-Mendota Canal. They meet at O'Neill Forebay, and the Gianelli Pumping-Generating Plant lifts water from the forebay up into the reservoir. When demand south of the Delta runs ahead of what can be pumped straight from the Delta, the California Department of Water Resources sends water back out through the same plant, generating power on the way down. ## Two projects, one lake Reclamation owns the dam and DWR operates it. DWR puts total capacity at 2,027,840 acre-feet, of which 1,062,180 acre-feet is the state's share and 965,660 acre-feet the federal share, the 55 and 45 percent split Reclamation cites. The percent-of-capacity figure on [San Luis Reservoir water level](/reservoir/san-luis-reservoir) uses the 2,041,000 acre-foot capacity the California Data Exchange Center lists for the station. ## The yearly cycle Reclamation describes the job plainly: San Luis stores "excess winter and spring flows diverted from the Delta until the water is needed later in the year" by south-of-Delta contractors. CDEC's 1991-2020 average storage by month shows the shape of that job: - January: 1,527,220 acre-feet - March, the average peak: 1,766,453 acre-feet - May: 1,409,458 acre-feet - July: 876,375 acre-feet - August, the average low: 781,740 acre-feet - October: 913,409 acre-feet - December: 1,277,307 acre-feet In an average year that is a swing of about 985,000 acre-feet between March and August, close to half the reservoir. ## Worked example: five years of readings The site's daily readings from CDEC show how far the swing can run. In the dry autumn of 2021 San Luis fell to 192,050 acre-feet on Oct 23, 2021, under a tenth of capacity, with the surface at 350.95 feet above sea level. After the wet winter of 2023 it held 2,028,344 acre-feet on Apr 10, 2023, with the surface at 543.04 feet, a foot below the 544-foot top of the active conservation pool Reclamation lists for the dam. The surface climbed 192 feet in about a year and a half. This water year the reservoir peaked at 1,815,496 acre-feet on Mar 25, 2026 and drew down to 1,137,206 acre-feet by Jul 28, 2026, a net drawdown of 678,290 acre-feet in four months. Storage turned upward again at the end of July and stood at 1,419,553 acre-feet on Sep 23, 2026, well above the 870,014 acre-foot September average. ## Why the level matters beyond the lake Because both projects bank their south-of-Delta supply here, a full reservoir in spring is summer water for the contractors those projects serve. The percentages they receive are announced separately, by Reclamation for the CVP and by DWR for the SWP: see [CVP and SWP allocations](/allocation/cvp-south-of-delta-agricultural) for every class's current figure and [CVP vs SWP](/guides/cvp-vs-swp) for how the two projects differ. ## Related data + guides - [San Luis Reservoir water level](https://the-waterline.com/reservoir/san-luis-reservoir) - [CVP and SWP allocations](https://the-waterline.com/allocation/cvp-south-of-delta-agricultural) - [CVP vs SWP, explained](https://the-waterline.com/guides/cvp-vs-swp) ## Common questions ### How full is San Luis Reservoir right now? It holds 1.49M af, with the surface at 498.51 feet above sea level on Oct 7, 2026, per the California Data Exchange Center. [San Luis Reservoir water level](/reservoir/san-luis-reservoir) carries the daily chart, the level in feet and the comparison to its five-year median for the date. ### Why is San Luis Reservoir low in the summer? Summer is when it gives water back. San Luis stores surplus Delta water pumped in during winter and spring and releases it when demand south of the Delta outruns what can be pumped directly, so its 1991-2020 average storage bottoms out in August at 781,740 acre-feet, per CDEC. ### Who owns San Luis Reservoir? The Bureau of Reclamation owns B.F. Sisk Dam and the California Department of Water Resources operates it. The storage is shared, about 55 percent state and 45 percent federal. ### Is San Luis Reservoir part of the CVP or the SWP? Both. DWR describes it as part of the San Luis Joint-Use Complex, which serves the State Water Project and the federal Central Valley Project, and each project has its own share of the storage. ### How big is San Luis Reservoir? DWR lists its capacity at 2,027,840 acre-feet and CDEC at 2,041,000 acre-feet. Reclamation puts the top of its active conservation pool at 544 feet above sea level, and its October 2023 fact sheet describes a plan to raise B.F. Sisk Dam 10 feet to add about 130,000 acre-feet of storage. --- Canonical URL: https://the-waterline.com/guides/why-san-luis-reservoir-level-swings