If the velocities [of water in rivers] remained constant in each point of the traversed space, the surface of the liquid would look like a plate of ice and the herbs growing at the bottom would be equally motionless. Far from that, the stream presents incessant agitation and tumultuous, disordered movements, so that the velocities change in an abrupt and most diverse manner from one point to another and from one instant to the next. As noted by Leonardo da Vinci, Venturi, and especially Poncelet, one can perceive eddies, large and small, with a vertical mobile axis. One can also see, at the surface, bouillons, or eddies with a nearly horizontal axis, that constantly surge from the bottom and thus form genuine ruptures, with the intertwining and mixing motions that M. Boileau observed in his experiments.
Adhémar Barré de Saint-Venant, 1872.