In this work we study the stability of horizontal shear flows of an ideal fluid in an open channel. Stability conditions are derived in terms of the theory of generalized hyperbolicity of motion equations. We show that flows with monotonic convex profile are always stable, whereas flows with an inflexion point in the velocity profile might become unstable. To illustrate the criteria we give simple examples for stable and unstable flows. Then we derive a multilayered model that is an approximation of the original model and features a continuous piecewise linear velocity profile. We also formulate sufficient hyperbolicity conditions for the multilayered model.