51°47′47″N 0°04′49″W / 51.796393°N 0.080298°W
Hertford Castle Weir is a weir located in Hertford near to Hertford Castle and next to Hertford Theatre.
Its function is to connect the upper River Lea to the canalised section that runs through Hertfordshire, Essex and London, to the River Thames. The section of the river above Castle Weir is not deep enough to support barges or narrow boats, but is navigable by row boats, canoes and kayaks.
The weir marks the start of the River Lee Navigation. It is overlooked by a function room at the Hertford Theatre (previously called Castle Hall).