Similar in meaning:
* Channel: This is a very close synonym, often used for natural waterways as well as man-made ones.
* Ditch: A smaller, shallower waterway, often used for drainage.
* Moat: A canal surrounding a castle or other fortified area.
Similar in structure:
* Aqueduct: This refers to a structure that carries water over land, often using a series of arches. While not exactly the same, it shares the function of moving water over a distance.
* Flume: A channel for transporting water, often used for irrigation or mining.
The best word to use depends on the specific situation. For example, "channel" is a good general synonym, while "ditch" is more specific to a smaller, artificial waterway.