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Reversing Falls. The St. John river (apparently, the river's name is Abbreviated, but the city's is not) flows into the Bay of Fundy. But the Bay changes its height by over 10 metres due to the tides, so at high tide, the Bay's water level is higher than the river's, and (salt) water from the Bay is forced backwards up the (freshwater) river. In this picture, "downstream" (towards the Bay and the ocean) is up towards the right, and "upstream" (inland) is towards the left. But as you can see from the turbulence around the rock, the water is flowing upstream.
Here are some movies of the water flowing upstream, stopping and turning around, and flowing downstream.