where H is the critical Shields parameter for the initiation of sediment motion calculated taking into account local slope, where H = 0.15S0.25 and the grain size heterogeneity at each location,. qs is the density of sediment, D is the diameter of the sediment grains, and m is a constant. Critical shear stress was calculated for the grain size at each location and for the typical 1 m diameter boulders that protect channel walls.