Primarily, because place names need not follow rules of grammar. They just have to suggest some 'unique' characteristic.
In Central Manchester it is the River Irk and River Medlock both of which then flow into the River Irwell.
The correct name for the 'River Mersey' is 'The River of Mercy' given at the time of slavery when the so called cargo was deemed as sick or the prices needed to be increased due to an over supply....
The River Mersey is in North West England.
The Irish Sea between the Wirral & Liverpool.