Coarse Sediment Transport by Flood Flows on Knik River, Alaska

Abstract

Knik River of southern Alaska is a glacial river with a valley train which was, until recently, swept each summer by a brief large flood resulting from the sudden draining of an ice-dammed lake (Lake George). These flows were 10 to 20 times larger than the postflood summer discharge of approximately 20,000 cfs and were responsible for the morphological and alluvial characteristics of the valley train. Flooding stopped after the 1966 season. During its first 16 mi of travel, where alluvial contamination from side streams is unimportant, the coarse fraction of the valley train changes systematically in size and shape. Grain size decreases by 94 percent, although this figure pertains to two populations: oversized particles and coarse particles which are readily moved by Knik flood flows. If the latter population alone is considered, reduction is 87 percent. At the same time, specimens of quartz and graywacke become progressively more elongated, and foliated specimens show the same change initially, but then deviate to become progressively more platy. Downstream from mile 16, Knik River alluvium is contaminated by detritus from an old outwash fan of the nearby Matanuska River. Sixteen miles of travel in a Kuenen-type abrasion tank reduced the size of Knik River gravel by only 8 percent and failed to change its shape. We conclude that downvalley changes in size and shape of coarse gravel are caused chiefly by sorting processes, aided by frost action which splits foliated particles into platy fragments. During shape-sorting, platy specimens are the most transportable, elongated specimens are intermediate, and compact (equidimensional) specimens are the least transportable. Our data support Russell9s (1939) contention that sorting is favored by aggradation and extreme variations in discharge.