Flood routing in natural channels and many other applications in hydraulic engineering based on the solution of the equations of unsteady flow require fast and accurate numerical methods. Numerical methods which are successful in other applications prove to be inefficient when used for flood routing. An implicit numerical method which is both fast and accurate can be established on the basis of: (1) a centered difference scheme to represent the primary differential equations in finite difference form; and (2) the simultaneous solution of the finite difference equations for each time step. The difference equations constitute a system of nonlinear algebraic equations which can be solved on a digital computer by Newton iteration method. The computational scheme becomes very efficient when advantage is taken of the sparseness of the matrix of coefficients of the linear systems employed in the iteration. Applications of the implicit method show that it can be conveniently used for highly irregular channels.