Spontaneous formation of roll waves is reported during floods in small canalised rivers, located within the urban area of the city of La Paz. The waves, with velocities up to 10 m s-1, cause serious damages to canal structures and nearby streets and houses. The phenomenon, which is commonly associated with the concept of flow instability, was analysed using the Montuori method for clear water. Surprisingly, the Montuori method predicted no waves formation in the rivers of interest.