Water as a weapon

I knew water is causing problems, but I didn't think of it as a weapon itself - until I saw this from FP Passport:

North Korea appears to have deliberately opened a dam that sent water surging down a river into South Korea, a top South Korean official said Wednesday, but he stopped short of calling it an attack.

Six South Koreans camping and fishing at the river were swept to their deaths last weekend after North Korea unleashed an estimated 40 million tons of water from a new dam into a river at the border without providing prior notice.

This does seem like an anomalous situation; it is much easier to imagine countries expressing belligerence by refusing to release water to their downstream neighbors than by releasing it too quickly and without notice, as North Korea did here.