This chapter examines the dynamics of flow in rivers and streams. Fundamental hydraulic concepts are introduced, including classification of flow in open channels as steady versus unsteady and uniform versus nonuniform; characteristics of one-, two-, and three-dimensional open-channel flows; mathematical treatment of open channel flow as fully three-dimensional (Navier–Stokes equation); simplifications of energy and force relations based on one-dimensional, steady, uniform treatments of open-channel flow; ratios of forces (Reynolds and Froude numbers); and resistance relations for reach-averaged one-dimensional flow. Boundary-layer theory is reviewed to relate the forces acting on the channel boundary to the velocity profile and to turbulent stresses, to derive the law of the wall for the turbulent part of the boundary layer, to show how the law of the wall can be used to determine boundary shear stress and roughness height, and to examine how boundary roughness, including the development of bedforms, affects boundary-layer structure and flow resistance.