There’s a rule in California that may seem bizarre in a drought-stricken state: in the winter, reservoirs aren’t allowed to fill up completely.

In fact, even as this post goes up, a handful of reservoirs are releasing water to maintain empty space. The practice, which has long inflamed combatants in California’s water wars, is due to a decades-old rule designed to protect public safety. If a major winter storm comes in, reservoirs need space to catch the runoff and prevent floods.