Channel morphology, gradient profiles and bed stresses during flood in a step-pool channel

The stability of a step-pool channel is investigated on the basis of measur ements during a median annual flood. The study reaches range in gradient fr om 5% to 10% and the flow was 3 m(3) s(-1) in an 8-m wide channel. The esti mated shear stress based on the channel gradient was > 10(2) Pa, which impl ies that the channel ought to move half-meter class boulders. Calculations of the shear stress using flow resistance equations yield values of about 3 0 Pa. These figures imply that the largest boulders in the channel could be moved. In fact, the channel is stable. The effective stress remains well b elow the threshold to move the large, structure-forming boulders. The steps are stable structures that result from interlocking of the larger stones. They are semi-randomly located, depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks. The high variation in step-pool wavelength, step height, residual pool depth, and pool length is indicative of the random nature of step location and str ucture along a reach. There is no convincing evidence that special conditio ns govern their formation. (C) 2001 Elsevier Science B.V. All rights reserv ed.