A Sluice

1. An artificial passage for water, fitted with a valve or gate, for example in a canal lock or a mill stream, for stopping or regulating the flow. Sluice - Wikipedia
2. A water gate or floodgate.
3. Hence, an opening or channel through which anything flows; a source of supply.
4. The stream flowing through a floodgate.
5. (mining) A long box or trough through which water flows, used for washing auriferous earth.
6. (linguistics) An instance of wh-stranding ellipsis, or sluicing.