AN EXAMINATION OF FROUDE-SUPERCRITICAL FLOWS AND CYCLIC STEPS ON A SUBAQUEOUS LACUSTRINE DELTA, LAKE CHELAN, WASHINGTON, USA

Density-driven submarine flows, including turbidity currents, play an important role in the transfer of sediment into deep water. These bottom-hugging flows often produce flow-transverse bedforms along their path. A sedimentological and geophysical survey of the Stehekin River delta in Lake Chelan, Washington, reveals a downslope-elongate field of bedforms on the delta foreset associated with hyperpycnal discharges of the Stehekin River. An analysis of the bedform morphologies, delta geometry, and density contrast between lake and river water suggests that these hyperpycnal flows are Froude-supercritical. The bedforms are likely cyclic steps, flow-transverse bedforms that are bounded by stable hydraulic jumps between alternating subcritical and supercritical flow regimes. The ability to examine the three-dimensional bed configuration produced by natural density-driven flows adds valuable information to the body of experimental work focused on the behavior of cyclic steps in flumes.