The Sixes River estuary, south coastal Oregon, sits above the locked portion of the Cascadia subduction zone, which intermittently releases in subduction-zone earthquakes. One such Cascadia earthquake ???300 years ago caused subsidence and a tsunami at the Sixes estuary. The subsidence raised the river's base level, resulting in an ???3 km upstream shift of the head of tide of the estuary. At the upper end of the expanded estuary, more than 4 m of overbank sediment was deposited in the first decades or century after subsidence. Subsequent incision through the over-bank deposits accompanied the gradual emergence of the estuary, and attendant downstream shift of the head of tide, as relative sea level fell in response to interseismic uplift.
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Response of a small Oregon estuary to coseismic subsidence and postseismic uplift in the past 300 years