Why is Folsom Dam releasing so much water? Flood protection.
Invariably, whenever the Bureau of Reclamation releases water stored behind Folsom Dam during the winter before the reservoir is full, people across the region accuse the Bureau of mismanagement, incompetence, and wasting water. The management of Folsom reservoir is complicated. But during the winter months, the main priority of Folsom Dam is flood control. That is why the dam was built in the first place.
Folsom Dam Built For Flood Control
Folsom Dam and reservoir were originally authorized under the Flood Control Act of 1944. The Folsom dam project was reauthorized in 1949 as a multi-purpose facility and its storage capacity was increased to just under 1 million acre feet. Folsom reservoir is relatively small compared to Shasta and Oroville which provide some flood control but were designed primarily for water storage.
| Reservoir | Capacity | Watershed |
| Acre Feet | Square Miles | |
| Shasta | 4.5 | 6421 |
| Oroville | 3.5 | 3607 |
| Folsom | 0.975 | 1875 |
The average runoff into Folsom reservoir (rain water and snow melt) is 2.7 million acre feet annually. That means that Folsom Lake can almost be drained twice and still be almost full going into summer. The bulk of the 2.7 million acre feet of water flowing into Folsom occurs between November and March, a 5 month window.
Folsom Lake Can Fill Up Fast
Under the existing rules, Folsom Lake must reserve approximately 400,000 acre feet of capacity for flood control between November and March. That is almost half of the total capacity of the lake. While keeping Folsom only half full during the winter for flood control may sound ridiculous, remember, those are the months when Folsom can receive over 1 million acre of water.
Army Corp of Engineers Rule Curve for available storage capacity at Folsom Lake. Between November and March, depending on weather and forecast conditions, Folsom Lake should have no more than 610,000 acre feet of water. This means the Bureau of Reclamation must release water during the winter to maintain acceptable flood control capacity.
Between December 9th and 13th of 2016, Folsom Lake filled with an additional 150,000 acre feet of water.
In early December 2016 after a power rain storm, Folsom Lake received almost 250,000 acre feet of water. The Bureau of Reclamation promptly started releasing water to maintain the proper flood storage capacity. Note the green line of 1982 – 1983 when more water was kept in Folsom and rain storms pushed storage over the maximum level.
As of January 4, 2017, the Bureau of Reclamation had released over 250,000 acre feet of water. Had they not released all that water, there is little doubt that Folsom Lake would be nearly full with still 3 more months of the rainy season remaining. Once Folsom Lake is full, there is no room to store unexpected flood surges. The water must be released, potentially into a Central Valley that might already be swollen and at flood stage.
The parking lot at Beeks Bight at Folsom Lake is surrounded by water in May of 2012 as Folsom Lake is allowed to fill with melting snow pack and is no longer under the Army Corp of Engineers rules to maintain flood storage capacity.
Army Corp of Engineers Sets Folsom Lake Winter Storage Rules
Even though Folsom Dam is operated by the Bureau of Reclamation, the flood control aspect is specified by the Army Corp of Engineers. The Army Corp determines the rules for how empty Folsom reservoir must be during the winter to capture potential flood surges of water. In addition to the Army Corp rule curve, the Bureau of Reclamation has an agreement with the Sacramento Area Flood Control Agency to keep even less water in Folsom during the winter. While the Army Corp rule is no more than 577,000 acre feet during winter months, the SAFCA agreement may pull Folsom Lake down to 400,000 acre feet of storage in some situations. When it comes to flood control, the Army Corp talks about river flow. In other words, what volume of water is the American River and Sacramento River capable of handling before it goes over the top of the levees.
Cubic Feet Per Second and Acre Feet
The flow is always expressed as cubic feet per second (cfs). There are 43,560 square feet of water to one acre foot of water. That is one square acre of surface filled to a depth of 1 foot, or 12 inches of water. On December 16, 2016, after a pretty good rainstorm, the full natural flow into Folsom Lake was 56,362 cfs. To put it another way, there was 1.29 acre feet of water entering Folsom Lake EVERY SINGLE SECOND. That is over 60 acre feet per minute – 3,600 acre feet per hour. Within a stretch of 10 hours, overnight, Folsom Lake could receive enough water to meet its 400,000 acre feet of flood control storage.
When it comes to potential flooding projections, the Army Corps refers to the maximum flows a system or river can handle before imminent flooding. A release of 115,000 cfs from Folsom could trigger the probability of a 100-year flood downstream. A controlled release of 160,000 cfs could result in a 200-year flooding event. It’s also important to remember when the releases from Folsom might occur. If Folsom were to release 125,000 cfs in the middle of summer, when there is plenty of capacity in the Central Valley to absorb the water, more than likely, there would be no flood.
How Much Water Is Already In The Central Valley?
But Folsom usually has to release water during the rainy season, during rain storms, when all of the other dams, creeks and rivers are pouring water into the Central Valley. If the Yolo Bypass is full, the Sacramento River is full, creeks and streams are adding more water into the Central Valley, AND, you might be experiencing a high tidal event, you don’t want Folsom dumping 100,000 cfs or more into the system if you can avoid it.
How Much Rain And Melted Snow Will Runoff Into Folsom?
Another consideration for drawing down the Folsom Lake level by releasing water during the winter is if an anticipated storm may have abnormally high snow elevations. A storm with snow levels above 7,000 feet, rain below that elevation, means a significant amount of the accumulated snow pack may be turned into runoff. These warm storms are generally the ones that create the high flows into Folsom Lake because there is the rain AND melting snow pack. These situations can easily create flows above 100,000 cfs entering into Folsom.
In this scenario, where the Central Valley is over flowing with rain water, you want Folsom to be holding water back. But Folsom can only hold back water if it has the storage capacity to absorb monstrous flows above 100,000 cfs coming into it. Consequently, water must routinely be released to free up capacity for future rain storms and snow melt.
New Years Day 2006, a high precipitation event created a flooding event around my house in Granite Bay.
Stand On Levee
I have lived in the City of Sacramento. I have stood on the levee in the Pocket area and watched as the Sacramento River, only a few feet from the top of the levee, flowed by. At that point, I was praying that Folsom and the other dams were holding back water. I didn’t care if there would be no water in the lake during the summer. At that moment in time, I just didn’t want my house to flood. Even though I moved to Granite Bay in 2003, my house, as I later learned, is in the Linda Creek Flood Plain. I have witnessed water flowing around my house during high precipitation events. While the flood capacity of Folsom Lake has no bearing on my situation as I am upstream of the dam, I still must live with the dread of flooding like so many downstream of Folsom Dam experience during large winter storms.
I love Folsom Lake. I’m out there every weekend. To all those who gripe and complain that the Bureau of Reclamation is releasing too much water during the winter, try living in Sacramento, Rancho Cordova, North Natomas, West Sacramento, or any of the other communities that are protected by levees and depend on the flood control Folsom Dam provides. Go stand on those levees and look down at all the homes and business that could be flooded if it weren’t for Folsom reservoir.
Folsom Lake hit record low water levels in 2015 exposing much of the dam.
We Survived Folsom Lake At 15%
The Sacramento region lived through the drought of 2015 when Folsom Lake was drawn down to 15% of its capacity. No one died of thirst. There was still plenty of water for normal daily living activities. So even if Folsom Lake is only half-full after the rainy season because too much water was released, we will survive. I would rather have a valley full of dead brown grass because we have to stop irrigating than have thousands of homes and business being rebuilt because Folsom couldn’t hold back water and flooding occurred downstream.
Even though the Folsom project was enlarged from just a purely flood control structure, it still retains a core flood control storage element of approximately 400,000 acre feet. The Army Corp of Engineers designed and built Folsom Dam because part of their mission is to build flood control projects. The Bureau of Reclamation builds dams to provide irrigation water. The Bureau of Reclamation manages the Central Valley Project, which includes Shasta Dam, to provide irrigation water to the Central Valley of California. The Army Corp of Engineers transferred the operation of Folsom Dam over to the Bureau of Reclamation. However, the Army Corp of Engineers still sets the rules governing Folsom’s flood control operations.
Below are some Frequently Asked Questions and Answers provided by the Bureau of Reclamation regarding the operation of Folsom Dam.
- Q – What is the function of Folsom Dam – why was it built?
Completed in 1956 by the U.S. Army Corps of Engineers, flood damage reduction is an authorized purpose for Folsom Dam and Reservoir. Sacramento and surrounding areas reside in a flood plain that relies on a state-federal flood control system, designed to keep flood waters away from people and property. The project is operated by the Department of the Interior’s Bureau of Reclamation as part of the Central Valley Project serving water supply demands in the state of California.
Additionally, Folsom Dam and Reservoir produces hydroelectricity, helps to maintain water quality in the Bay Delta, provides recreation and water for local municipal agencies.
- Q – Who is responsible for Folsom Reservoir releases?
A – Reclamation is responsible for daily operations and releases at Folsom Reservoir. Releases are typically for seasonal water supply, water right requirements, biological considerations, regulatory requirements, storage and flood management operations.
- Q – What determines when/how much water is released from Folsom Reservoir?
A – Reclamation bases overall CVP reservoir operations and releases—including Folsom Reservoir—on a number of legal, contractual, and regulatory requirements and takes into consideration projected runoff forecasts throughout the Central Valley that cover a range of future wet and dry conditions. The latest weather forecasting information, current and forecasted reservoir storage and inflow data, and snowpack accumulation are all used to help determine appropriate reservoir releases at any given point in time.
In addition, flood management, seasonal water supply needs, biological considerations and storage management concerns can all factor into decisions regarding reservoir operations.
- Q – Who controls Folsom Dam during flood operations?
A – Reclamation is responsible for managing releases when reservoir levels go above seasonal thresholds, also called flood operations; however, several considerations which Reclamation takes into account include close coordination and agreements with other agencies.
The Corps knows the limitations of the levees downstream of Folsom Dam. The Corps, with input from local, state and federal agencies, developed a comprehensive “owner’s manual,” the Water Control Manual. Within the Manual is a diagram that defines seasonal reservoir storage volumes to leave empty in the fall, winter and spring, and reservoir release requirements in order to reduce flood risk.
A separate agreement and diagram exists between Reclamation and the Sacramento Area Flood Control Agency defining how much room to leave empty in the reservoir. The SAFCA diagram establishes an additional storage requirement for the city of Sacramento’s flood risk reduction and differs from the Corps’ Manual by considering available storage in three upstream reservoirs: French Meadows, Union Valley and Hell Hole.
Between the two agreements/diagrams is a built-in flexibility upon which Reclamation and the Corps collaborate when reservoir operations must consider dam safety guidelines. The goal is to strike a balance between multiple purposes without compromising public safety and the protection of property. Reclamation considers these manuals, along with CVP operational demands and current/forecasted conditions, to help inform its decisions.
- Q – We’re in a drought and water is still being let out of Folsom Reservoir—how come?
A – During drought, flows are scaled back to conserve storage when inflows are poor; however, water still needs to flow downstream to maintain the fishery habitat per federal and state regulatory requirements and for other downstream demands, which include seasonal water supply needs, water right requirements, biological considerations, regulatory requirements and storage management concerns.
Inflow from the Sierras needs to be appropriately managed as the snow melts in the spring and runs downstream to Folsom Reservoir. Basically, storage conditions at the reservoir need to be at an appropriate level in the fall to prepare for the upcoming rain or snow season.
Folsom is one of the smallest CVP reservoirs with one of the highest refill potentials; this means it can fill quickly and frequently. Enough water passes through the reservoir to fill and empty it 2-3 times in a wet year.
Folsom is operated as part of one, integrated CVP system, which includes Trinity, Shasta, New Melones, and Millerton reservoirs.
Reclamation releases water from Folsom for many reasons including water supply, water quality in the Sacramento-San Joaquin River Delta (primarily to prevent salinity intrusion from the Pacific Ocean), and for endangered and threatened species. Reclamation has contracts with the following agencies for water supply from Folsom Reservoir: El Dorado Irrigation District, City of Roseville, Sacramento County Water Agency, San Juan Water District, East Bay Municipal Utility District, Sacramento Municipal Utility District, Placer County Water Agency, and Sacramento County.
Determining how much water to release from Folsom and the other CVP reservoirs is constantly being assessed in coordination with numerous agencies including the California Department of Water Resources, State Water Resources Control Board, California Department of Fish and Wildlife, National Marine Fisheries Service, U.S. Fish and Wildlife Service, and the U.S. Army Corps of Engineers. Reclamation also coordinates CVP operations with 271 CVP contractors, 80 Preference Power customers, numerous environmental groups, and federally recognized tribes and Native American groups in an effort to jointly maximize the operational flexibility of both the CVP and California’s State Water Project.
- Q – Will Folsom Dam’s new spillway allow for greater storage at Folsom Reservoir?
A – The auxiliary spillway is not intended to replace any of the main dam’s current functions or increase the capacity of the reservoir; however, operators should have more flexibility to increase the space available for water supply storage during dry conditions. The new auxiliary spillway will allow the release of water not only in larger amounts but sooner based on forecasting.
- Q – When is the next update of the Folsom Dam Water Control Manual?
A – The Corps, in conjunction with SAFCA, the Central Valley Flood Protection Board through the California Department of Water Resources, Reclamation and others, is working on an updated Water Control Manual to incorporate the reservoir’s new auxiliary spillway and is expected to be completed in 2017.
Army Corp of Engineers Documents on Proposed Folsom Lake Flood Storage Rules Curve
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