An ordinary watercourse is any river, brook, stream, culvert or passage through which water flows which is not designated as a Main River. Ordinary watercourses are also commonly not recorded on maps.
If the section of the watercourse you would like to alter is part of a Main River, you would need to contact the Environment Agency to obtain an Environment Agency Environmental Permit. You can check if the watercourse is a Main River by checking the Environment Agency’s Main River Map.