A bell chain to warn when a hill stream suddenly rises

When it rains upstream in the hills, the stream below can rise suddenly while the village downstream is still in sunshine. People bathing or working in the stream are caught before they understand. Pick one person at several points along the stream who rings a bell when water turns muddy or rises fast. Each point rings in turn so the warning passes down to the village. Put a board by the stream saying: when you hear the bell, leave the stream and climb higher. You need a few bells or gongs, responsible people and the elders' agreement.