River Tay
The River Tay emerges from Loch Tay at Kenmore, and flows from there to Perth which, in historical times, was its lowest bridging point. Below Perth the river becomes tidal and enters the Firth of Tay. The largest city on the river, Dundee, lies on the north bank of the Firth. On reaching the North Sea, the River Tay has flowed from west to east across central Scotland.
Firth
A firth is generally the result of ice age glaciation and is very often associated with a large river, where erosion caused by the tidal effects of incoming sea water passing upriver has widened the riverbed into an estuary. Demarcation can be rather vague. The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow, while locally the change is held to be at the Tail of the Bank where the river crosses a sandbar off Greenock at the junction to the Gare Loch, or even further west at Gourock point.