A water gate is used to stop a flow of water. Water gates are adjustable gates used to control water flow by either shrinking the stream, or by stopping it altogether. They can be made quite easily with redstone and pistons.

• 

  A Water gate open and closed.

[edit] Construction

There are three parts to a water gate: The water source, the water channel, and the gate. The water source, obviously, provides the water. The channel is a small trench that guides the water where you want it to go. The gate itself is a device made of pistons that blocks the water flow. To create a water gate, the first thing to do is to create the channel. This can be done by digging it out of the ground or by placing walls on the ground. The water source can be a lake, ocean, pond, or even just a bucket of water dumped at the beginning of the channel. The gate is simply two pistons facing into the channel that, when extended, occupy the channel and prevent water from flowing. Sticky pistons with an additional block stuck to them can be used for wider channels. The pistons are then wired to a redstone signal that controls when they are on or off.

[edit] Uses

There are a few things you can do with a floodgate. One of the most useful ways to use a floodgate is to harvest crops by channeling the flow of water over your mature crop and it will automatically pick your crops for you. By channeling the water you can also designate a pickup area for the already harvested crops. More uses for floodgates are mob traps, the all known watervator, a safe passage up and down a cliff side.