Floodgate (noun)

1. A gate or sluice used to control the flow of water.
2. A device for controlling the flow of water in a river or canal.
3. A situation or event that leads to a sudden outpouring or release of a particular emotion or action.

Origin:

Mid 17th century: from flood + gate.

Examples:

1. The floodgate was opened to release the excess water.
2. The floodgate was closed to prevent the river from overflowing.
3. The new policy was seen as the floodgate to a wave of changes.
4. The floodgate allowed the water to flow freely into the canal.
5. The tragedy was the floodgate to a outpouring of grief and condolences.