Hold back the flood. Manage a river of reservoirs against a storm you can only forecast.
You control the spillway gates of a chain of reservoirs along one river, from the headwaters down to the towns at the mouth. Each day you set how much each dam releases; rain and runoff pour into the top reservoir, and every release cascades downstream to the next. A seeded storm crests partway through the season, and you only ever see a forecast whose uncertainty narrows as it approaches — so you must pre-release to make headroom while you're still unsure. Release too much and you flood the towns; too little and they run dry; let a reservoir overtop and the dam breaches.
Spillway is grounded in real reservoir and flood-control operations, where dam operators lower water levels ahead of a forecast storm to create room for the surge, then release it gradually so downstream rivers carry the flow without flooding — all while keeping enough water in store for supply. It's a decision-under-uncertainty problem: the forecast is never perfect, and the cost of acting too late is a spillway at capacity. This game distills that tension into a tense, deterministic daily run.
Spillway is an original, free browser simulation game with no download and no sign-up. It is a game, not a hydrology tool; the river is simplified for fun. Play the shared Daily storm to climb the leaderboard, or jump into Practice for endless runs.