Query: What river borders Plymouth on the west?
Context: Plymouth (i/ˈplɪməθ/) is a city on the south coast of Devon, England, about 37 miles (60 km) south-west of Exeter and 190 miles (310 km) west-south-west of London, between the mouths of the rivers Plym to the east and Tamar to the west where they join Plymouth Sound to form the boundary with Cornwall.