Flood gates to control

When they talk about opening the Red River floodgates what are the floodgates? How do they work?

Barry's Response - South of Winnipeg is a floodway, designed to provide an alternative channel for excess water to flow around the city instead of through it. It is over 40 km long and diverts water around the east side of Winnipeg and dumps it back into the river near Selkirk.

Duff's Ditch has saved the city's neck on several occasions. It uses flood gates to control the flow rate of water into the channel and this year they plan to close them once again to force water to divert.