High-resolution seismic profiling (300 km) and drill coring (17 sites to depths of 60 m below present sea level) in Grays Harbor, Washington, record the Holocene marine transgression and associated filling of the drowned-river basin (present surface area 290 km²) in the convergent Cascadia margin. Seismic-reflection records are used to map the morphology of the ancestral Chehalis-Humptulips river valley (axial channel depth 60-70 m below present sea level) and to produce an isopach map of the basin fill (total volume 8.7 km³). Drill coring establishes general textural sequences of sand and mud coarsening upward to sand and grave) in lower-bay reaches, and gravelly sand and sandy gravel fining upward to sand and mud in upper-bay reaches. Radiocarbon dating of core-sample wood, carbonate shells, and peaty muds yields a deposit depth-age curve in Grays Harbor beginning at 10,760±90 yrs (un-calibrated age in radiocarbon years before present, RCYBP) from 57 m depth below present sea level. Average basin sedimentation rates decrease from 1.2 cm/yr (10.5-7.5 ka) to 0.1 cm/yr (5.5-0 ka), following declining rates of eustatic sea-level rise in middle Holocene time. This trend corresponds to a four-fold decrease in average rates of basin-fill accumulation from 2.0 x 10⁶ m³/yr (8.5-7.75 ka) to 0.5 x 10⁶ m′/yr (5.5-0 ka), implying substantial river-sediment bypassing and/or diminished marine-sediment influx in the Grays Harbor basin during the latter half of the Holocene marine transgression.