why londoners keep black rivers in the tower of london

first of all, not Rivers but Ravens (вороны).

Londoners keep black ravens in the Tower of London because there is an old legend that says if the ravens ever leave The Tower of London - The White Tower would crumble and a great disaster shall befall Kingdom. That is why people who work there, take care of ravens. I also can add that the ravens' wings are kept short so they couldn't fly away.