How far do you have to travel before you reach a different watershed? Can you draw the boundaries of yours? Q7 of The Big Here Quiz

Using Environment Canada's Know Your Watershed, I now know that I live in the Northern Lake Erie - St. Claire watershed (WSHED_ID=620)

I'm trying to figure out if there is a particular feature like a river that defines my watershed's Eastern border on the Thames River Watershed. I think it may be the Ruscom River which is approximately 30 km away.