To add a bit more,had a look at the Google map,terrain and the M53 goes through a light green low level narrow area from Upton to Woodchurch rd Upton it does go wider as it gets closer to the coast,does anybody have any terrain maps?
These may help (with full acknowledgement to Stephen J. Roberts - A History of Wirral).
The map below is 1792, land lost to the sea, I've been trying to imagine the course of the Main Fender, in my mind, if it was a fender (sea defence) it would have been breached in places, not only that, would the said defences taken the correct course, I could be totally wrong, thoughts on a postcard.
God help us, Come yourself, Don't send Jesus, This is no place for children.
...just to clarify one small point davew3, the Liverpool Authorities did not own any of the land around North Wirral until it acquired Wallasey Pool in 1829/30. Even then, it only bought the land to prevent the original Birkenhead dock scheme from going ahead and threatening it's monopoly as a port.
Most of the lowlying land in North Wirral was owned by the Lords of Bidston Manor. Warrington's Bridge at the upper reaches of Wallasey Pool was just one of the local attempts at flood control instigated by these landowners back in the 1730's.
There are records that show tenant farmers were obliged by their landlords to maintain their sections of The Fender or risk fines for failing to do so. The very construction of Warrington's Bridge itself was a condition of tenancy imposed upon a Mr.George Warrington who wanted to farm the land in that area.
If you can find a copy of Canon Hume's book on Ancient Meols, there's a Victorian map in the front which is a composite of those above. It shows the relative positions of the watercourses to the flood areas and lands lost to the sea.
Sorry, but I don't have a copy of the book myself. It's a very old book, but most reference libraries should have it. Just be warned, not all copies have the map because they sometimes fall out from the front or get nicked.
It looks increasingly that the aforementioned 'Fender' was more of a coastal defensive measure as opposed to raised defences along the course of a river. That being so we are again challenged to understand or accept why a river would be named after a coastal protection construction, that was raised at some late date, centuries after the river was probably accorded an original name.
On the subject of maps, would it not be reasonable to expect such flood prevention structures to be marked? Are there existing, maps showing such constructions marked as Fender?
I disagree with part of what you say Erainn about The Fender being purely a coastal defence. Cust's property (which was by far the most important building along the coast) was situated in front of The Fender (i.e - between The Fender & the Sea). Also, before the construction of Leasowe Road the only dry road connecting Wallasey Village with Leasowe and Meols was Green Lane. This ancient road, often refered to as 'Old Road' on early maps was also in front of The Fender.
The sand dunes were the main defence along the coast. So, wouldn't you think that the fendering of the inland watercourses was more likely to be connected with preventing flooding from Wallasey Pool?