Dam

From the Introduction:

For the longest time the villages in the valleys below have been threatened by the regular spring flood when a hard winters worth of snow starts to thaw. Thus a number of small valley like this one have been dammed up, to accumulate most of the melt water and release it at a more sedate pace after spring. The dam itself has been built from heavy stone blocks and it is crowned by a simple tarred road. Tunnels through the rock at both ends connect it to civilization and other small dams like it.