Laminar is a term used by hydrologists, river scientists, rafters, and other experienced river watchers to describe the smooth and even ﬂow of a river unimpeded by rocks, trees, or other obstructions. This condition is as close as a river can come to the laminar ﬂow used by physicists in their description of all ﬂuids: a state in which all particles move in the same direction, perfectly in parallel. Some textbooks liken the ﬂow to layers of playing cards sliding over each other. In a river, a rock, hole, or sudden narrowing of channel width can cause more complicated ﬂow structures, such as vortices and eddies, to form. Particles begin to move in different directions. When this occurs, laminar ﬂow is no longer present, and the ﬂow begins to be described as turbulent.