Main rivers (Welsh: prif afonydd) are a statutory type of watercourse in England and Wales, usually larger streams and rivers, but also some smaller watercourses. A main river is designated by being marked as such on a main river map, and can include any structure or appliance for controlling or regulating the flow of water in, into or out of a main river. Every other open watercourse in England and Wales is determined by statute as an 'ordinary watercourse'.