In some of the world’s rivers, an incoming high tide can arrive as a breaking wave, or as a smooth jump decorated by waves. An approximate theory illustrates hamiltonians in quantum mechanics, and an analogy with Hawking radiation in relativity. This dramatic phenomenon, in
which the river flows backwards, illustrates the first unification in physics: the same force - gravity - keeps us on the ground, holds the moon in its orbit, and pulls the tides.