In England, the Environment Agency decides which watercourses are main rivers. It consults with other risk management authorities and the public before making these decisions. The main river map is then updated to reflect these changes.
This is where the Environment Agency amends sections of the main river map to reflect changes in the environment or to correct inaccuracies. These changes only correct the line of the main river and do not change who has legal power to carry out flood risk management work.
Statutory guidance to the Environment Agency
This guidance sets out the basis on which the Environment Agency should decide whether or not a river or watercourse is treated as a ‘main river’.