Many drop structures and permanent bank protections are installed in a waterway to prevent lowering and enlargement of the channel cross-section. But the design rules of such structures are mainly based on the experiences and it is not rare to suffer from unexpected phenomena. Recentely, the authors have derived the equation to predict the critical condition of initiation of motion of bed materials on very steep mountain streams. Moreover, the authors have made clear the flow resistance characteristics and regime criteria in such streams. This paper discusses the hydraulic function and design rules of waterway stabilisation structures based on the above mentioned results on sediment hydraulics and hitherto results on the hydraulics of channel controls. The discussion suggests some design rules so far are unreasonable and under some conditions construction of many stepwise drop structures brings no benefit.