The Rhine Falls in Switzerland

The Rhine Falls (Rheinfall in German) are the largest plain waterfalls in Europe.The falls are located on the High Rhine between the municipalities of Neuhausen am Rheinfall and Laufen-Uhwiesen, near the town of Schaffhausen in northern Switzerland, between the cantons of Schaffhausen and Zürich. They are 150 m (450 ft) wide and 23 m (75 ft) high. In the winter months, the average water flow is 250 m³/s, while in the summer, the average water flow is 700 m³/s. The highest flow ever measured was 1,250 m³/s in 1965; and the lowest, 95 m³/s in 1921.

The falls cannot be climbed by fish, except by eels that are able to worm their way up over the rocks.
source : http://en.wikipedia.org/wiki/Rhine_Falls