Borders quite often follow streams or rivers. I guess they formed natural borders in an age when bridges (or complete culverting) was a lot rarer than both are now.
The Rubicon, when there is a heavy rainfall, often gets diverted at about where Parkend Rd joins Borough Rd. into an overflow that runs back up Borough rd to Everest Rd where it runs, deep below Everest Rd, Under the ridge and Victoria Park under St Pauls Rd and eventually runs into the Mersey from an outfall under the oil jetty. This was constructed in the 1950s to alleviate frequent flooding.