Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences

The stability of the pool–rife sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–rife sequence bars during high ows, offsetting gradual scour of rifes and deposition in pools during low ows. Despite several attempts, reversal has rarely been recorded in eld measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase‐shifted with respect to the pool–rife sequence bedform prole, so that maximum shear stress occurs upstream of rife crests at high ow, and downstream at low ow. Such phase‐shifts produce gradients of shear stress that explain rife deposition, and pool scour, at high ow, in accord with sediment continuity. The proposal is supported by results of a one‐dimensional hydraulic model applied to the surveyed bathymetry of a pool–rife sequence in a straight reach of a gravel‐bed river. In the sequence studied, the upstream phase‐shift in shear stress at high ow was associated with variations in channel width, with width minima occurring upstream of rife crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of rife crests. Assuming that the width variation is itself the result of ow deection by rife crests at low ow, and associated bank‐toe scour downstream, low and high ow can be seen to have complementary roles in maintaining alluvial pool–rife sequences. Copyright © 2004 John Wiley & Sons, Ltd.