Over last weekend, El Niño finally delivered on its promised potential, soaking most of Northern California with heavy, sustained rain and snow.  Creeks and arroyos began to flow, filling reservoirs and turning rivers into churning muddy torrents.  By Monday morning, every reservoir in Marin County had reached full capacity.
Reservoirs further east also rose quickly, but they are still quite a ways from full.  So when the operator of Folsom Dam opened its spillway for the first time since 2011, public reaction was immediate:  why are they releasing water when we are still in a drought?
Many dams, especially large ones, are dual-purpose.  They protect areas downstream from flooding, and they provide water for human needs.  The agencies that manage them, whether federal or state, are tasked with balancing these two goals.
In the heirarchy of natural disasters, floods place much higher than droughts.  They are instantaneously catastrophic as opposed to slow and insidious.  They cause immediate death and destruction.  For this reason, preventing flooding always takes precedence over providing water.  But that doesn’t mean that decisions come easily.
There is a spiderweb of public and semi-public agencies involved in managing dams and the water they store.  Many, if not most of them, are water agencies whose funding comes entirely from selling water.  Funding for flood control comes from taxes.  During long droughts, many water agencies see their budgets shrink as water sales drop.  When it starts raining, and the reservoirs state filling, water agencies literally see money falling from the sky.
Enter the System Operator of the dam, legally required to prevent flooding at all cost.  They are watching water pour into a reservoir like Folsom Lake, and using an array of calculations to figure out how fast it will fill.  They need to decide when to start releasing water, and how much to release.  The timing of their decision affects whether or not downstream communities get flooded during a storm, and how badly.  But it also affects how much water gets stored for the dry season.
If you live near a dam or have watched the way this plays out, it can seem counterintuitive.    We farm right along Putah Creek, the outlet for Solano County main reservoir Lake Berryessa. So we get to see it all the time.  When it rains, they don’t release much water:  the watershed itself can cause the creek to flood in a storm, and they don’t want to add any water to the flow if they don’t have to.  But when a storm ends, they often let lots of water out, to make room for more in the reservoir in case another big storm hits.  As a result, Putah Creek sometimes runs higher in between storms than during them.
In Yolo County, our most important water source is Clear Lake — a natural lake that we share with the residents of Lake County, who also mainly live around its shores just a few feet above water level.  To guarantee their water supply, we are prohibited from taking any water from the lake until it reaches a certain height.  But once it gets to that height, we are required by law to take the water in order to prevent flooding…in their county.  If it causes flooding in our county, that’s our problem.
During large storms like the one last weekend, water comes into Clear Lake from the surrounding watershed four times faster than it can be released out. If the operators of dam wait until the lake is completely full to start releasing water, it can quickly overflow and flood the surrounding communities. Since they cannot allow that to happen, the operators open the spillway ahead of time.
In the end, dam operators must all base their decisions on one critical and notoriously unreliable dataset:  the long-term weather forecast.  When wet weather arrived in December of 2013, system operators for several of the state’s largest reservoirs released a tremendous amount of water.  They were anticipating an El Niño winter that year as well.  In the end, this turned out to be a disastrously bad call in what turned out to be one of the driest years in recent memory.
I think all Californians should reserve the right to question and criticize the decisions made by the people that oversee our flood prevention and water supply infrastructure.  But we should also cut them some slack.  Now about about those weather forecasters…