The Thames is so strongly tidal (and tidal further inland than it used to be -- now the tide goes as far as Teddington, but apparently only to Putney in Viking times) that I suspect most of its tributaries -- such as Deptford Creek, into which the Ravensbourne flows at Deptford Bridge -- are tidal simply by having Thames water forced into them. At a rough estimate, I'd say the Ravensbourne's tidal limit is about a mile to a mile and a half from the Thames -- and some of that's culverted, which forces the water further and faster in both directions.