BEDLOAD TRANSPORT IN A POOL‐RIFFLE SEQUENCE OF A COASTAL ALASKA STREAM1

Abstract

ABSTRACT: A Helley‐Smith pressure differential bedload sampler was used to measure bedload transport at consecutive riffle sections of a riffle‐pool‐riffle sequence on Bambi Creek, a small (154 ha), second‐order stream on Chichagof Island, Alaska, during four storms over a 2‐year period. Maximum bedload transport rate measured was 4920 kg/h at a streamflow of 2.35 m3/s corresponding to a storm having a 5‐year return interval. Transport of larger sediment (> 8 mm) varied systematically with streamflow at the two sampling locations. At flows up to approximately bankfull, transport of large sediment was greatest at the upstream site; at flows above bankfull, transport of large sediment was greatest at the downstream site. The net import of large sediment to the pool during moderate stormflows and net export of large sediment from the pool during flows above bankfull may be related to a “convergence” or “reversal” of competence between the upstream riffle and subsequent pool at flows approximating bankfull stage. Cross‐sections monitored within the study reach indicate that stormflows resulted in net filling of the riffle sections and net scour of the pool; periods of low streamflow resulted in net scour of the riffles and net filling of the pooL