A canal climbs a hill in steps. Each step is a pool of still water. Between two pools sits a lock: a watertight chamber with a gate at each end. To go uphill a boat drives into the chamber from the low pool, the gates close, water pours in from the pool above until the chamber is level with it, and the boat drives out into the higher pool. Going downhill is the same in reverse.
Every fill takes water out of the pool above, and every emptying puts it into the pool below, so water only ever runs downhill; a small stream at the top is the only new supply. A keeper has to be standing at the lock to work it, and the keepers can only walk. Nothing opens at night. Most waiting on this canal comes down to three things: a keeper is elsewhere, the chamber is at the wrong level, or the pool above is too low to fill from.