In previous chapters, a flowing fluid has usually been assumed to be bounded on all sides by solid surfaces. For liquids, however, flow may take place when the uppermost boundary is the free surface of the liquid itself. The cross-section of the flow is not then determined entirely by the solid boundaries, but is free to change. As a result, the conditions controlling the flow are different from those governing flow that is entirely enclosed. Indeed, the flow of a liquid with a free surface is, in general, much more complicated than flow in pipes and other closed conduits.