Suspended sediment dynamics in Blue Fjord, western Prince William Sound, Alaska

Abstract

Glacier meltwater and suspended sediment discharge in Blue Fjord occurs over a brief 5-month period in summer. Suspended sediment concentrations in the meltwater stream reach 300 mg l −1, and this sediment forms a surface turbid plume at the fjord head. Suspended sediment concentrations in the surface plume range from 200 mg l −1 at the head to a few mg l −1 5 km away at the mouth. Turbidity does not seem to be related to density structure of the water column. Suspended sediment sinks through the water column with most sediment settling at slack low water. Sediment trap measurements show an April sediment flux of 1.5 mg dry sediment cm −2 day −1 at the head and 0.75 mg cm −2 day −1 at the mouth (mostly diatom frustules). September trap measurements yield a sediment flux of 53 mg cm −2 day −1 at the head and 2 mg cm −2 day −1 at the mouth (mostly detrital inorganic silicates in the mud size range). Bottom sediment in the fjord basin is mostly mud, with an admixture of sand at the fjord head. Grainsize modes decrease from an average of 46 μm at the head to 8 μm 2 km away; no trend is discernable for sediments in the outermost 4 km of the fjord basin. Mud accumulates in the fjord at the rate of about 100 mm/meltwater year at the head, 10 mm year −1 in mid-fjord, and 4 mm year −1 in the 190 m basin inside the sill at the fjord mouth.