The influence of the reservoirs discharge variation on riverbed armoring in mountainous rivers was investigated. The flume experiments were carried out by using clear water (no upstream sediment supply) under two conditions: alternating discharges and increasing discharges. For each case, the bed load transport rate, flow depth and bed morphology were measured. The bed load transport rate in the period of riverbed armoring was simulated using an exponential decay function. Based on the analysis of bed shear stresses and the change of bed load transport rates before and after the formation of armoring layer, the process of riverbed scouring and armoring and the critical condition of the armored layer’s breakage were studied. The variation of armored bed topography can directly reflect the degree of riverbed scouring. The experimental results showed that the exponential decay law was capable of simulating bed-load transport rate during riverbed armoring, and the simulation results were close to the extremum outsourcing line of the measurements. For alternating discharges, the maximum discharge, which occurred at the first time during each period, was the control factor for the formation and destruction of armoring layer, because the bed-load transport rate produced by the extreme discharge at the first time was always greater than that produced by a smaller discharge or the extreme discharge at the second time. In each period, the condition (armoring versus scouring) of the riverbed surface was determined by the extreme discharge and the initial armoring discharge. Compared to the initial armoring discharge, a larger extreme discharge leads to more significant armored bed. For increasing discharges, the condition of armoring layer was determined by bed shear stressτunder the current discharge and the critical bed shear stressτ0of the armoring layer. Whenττ0, the change of armored bed was not obvious; whenτ≥1.1τ0, the armored bed became unstable and the sediment transport rate increased significantly.
