Original sentence: The "Big Ditch" (as it is said to be known to locals) consists of the River Irwell and River Mersey made navigable to Manchester for seagoing ships leaving the Mersey Estuary at Eastham Locks on the south east side of the Wirral Peninsula
Edited sentence: The "Big Ditch" (as it is said to be known to locals) consists of the River Irwell and River Mersey made navigable to the UK's third city of Manchester for seagoing ships leaving the Mersey Estuary at Eastham Locks on the south east side of the Wirral Peninsula