ABSTRACT
Pool-riffle sequences are one of the most common geomorphological features in many streams and provide important habitat diversity both in terms of flow and substrate. The conditions for their formation and self-maintenance are still the subject of active research, but it has become clear in later years that a combination of three mechanisms: 1) stage-dependent flow conditions, 2) three-dimensional flow patterns and 3 ) selective sediment transport over a mobile bed, can explain the resilience and ubiquity of pool-riffle sequences observed in the field. In this paper, we analyze the importance of these three mechanisms using different combinations of stage-dependent three-dimensional flow patterns in pool-riffle sequences and sediment size distributions obtained in both pools and riffles. Self-maintenance mechanisms are identified by evaluating erosional or depositional tendencies in pools and riffles for different flow conditions using local values of bed shear stress and their corresponding fractional sediment transport volumes. Self-maintenance is directly linked to episodes of pool erosion and riffle deposition and we use the term sediment transport reversal rates to indicate this situation, rather than velocity reversal or shear stress reversal that only consider flow variables. This approach allows us to compare, for the first time, the relative importance of each of the mechanisms and their role in the self-maintenance of these bedforms, which can vary from site to site. Computations are performed using existing field, laboratory and numerical simulation data from several study sites. We discuss the limitations of our approach and the extensions to more complex field cases.
