Sedimentologic analyses of 21 sediment cores, ranging from 0.4 to 9.6 m in length, reveal that the shallow geologic framework of the Skagit River Delta, western Washington, United States, has changed significantly since 1850. The cores collected from elevations of 3.94 to -2.41 m (relative to mean lower low water) along four cross-shore transects between the emergent marsh and delta front show relatively similar environmental changes across an area spanning 75 km2. Offshore of the present North Fork Skagit River and South Fork Skagit River mouths where river discharge is focused by diked channels through the delta, the entire 5-7-km-wide tidal flats are covered with 1-2 m of cross-bedded medium-to-coarse sands.