The physical laws which govern the flow of water in a stream are: (1) the principle of conservation of mass (continuity), and (2) the principle of conservation of momentum. These laws are expressed mathematically in the form of partial differential equations, which will hereafter be referred to as the continuity and momentum equations. The derivations of these equations are presented in this chapter based on a paper by James A. Liggett from the book Unsteady Flow in Open Channels (Mahmmod and Yevjevich, 1975).