I live in South Manchester and the River Mersey is 10 minutes walk from my house. So, as far as I am concerned, I currently live on 'Merseyside'.

Historically, the River Mersey was the boundary between the historic counties of Lancashire and Cheshire. I am an amateur botanist and can tell you that, for botanical recording purposes, the north bank of the Mersey marks the southern boundary of the 'vice county' of South Lancashire (VC 59) and the south bank the northern boundary of the vice county of Cheshire (VC 58).