The Sixes River in southwestern Oregon has a summer discharge of only about 2 m3/sec. During these low-discharge conditions, a flood-dominated system of bottom tidal currents develops in the estuary and a deltalike sill, as much as 1.5 m in height, builds across the mouth of the estuary by upstream progradation. Flood-tide currents move across this sill at velocities of as much as 90 cm/sec 15 cm above the bottom, but the velocity of ebb-tide currents usually does not exceed about 40 cm/sec.
Dispersal patterns of dyed sediment injected at the river mouth during low river discharge show that flood-tide currents transport sand across the sill and up the estuary as far as 0.8 km (about one-fourth the length of the estuary) in a single flood-tide phase. During ebb tide, the sill impedes movement of salt water along the estuary bottom, producing a sharply stratified two-layer water system. Although tracer experiments show that some fine sand is removed from the estuary during the ebb phase, primary sedimentary structures and the mineral composition of the sand indicate that flood-tide dominance of the bottom tidal currents causes a net gain of marine sediment in the estuary while the sill is in place.
River discharge after winter storms may increase to more than 400 m3/sec, and large quantities of detritus, including gravel, are transported downstream into and through the estuary. High river discharge also causes erosion of the sill, greatly reducing the sediment-trapping capacity of the estuary. The finer fluvial detritus, together with fine marine sediment deposited during the summer, is swept from the estuary, leaving it floored largely by gravel. Thus, the hydraulic sediment-trapping mechanisms observed in the estuary of the Sixes River appear to be effective only on a seasonal basis under present hydrologic conditions.