At the head of Turnagain Arm near Anchorage, the major lasting geologic consequence of the Alaska earthquake of 1964 was deposition of the Placer River Silt, an intertidal deposit covering an area of more than 18 km2 and containing more than 20X106 m3 of sediment. This formation, which was derived from erosion of intertidal bars in Turnagain Arm, averages 1.5 m thick seaward of the Seward Highway and 0.9 m thick landward. Its distribution is controlled both, by the arrangement of the major streams that enter tidewater and by manmade features such as the highway and railroad embankments. The Placer River Silt is still being deposited. The contemporary lowland sedimentation system of the Portage area includes a number of depositional settings: (1) In the Placer River Silt upper tidal flats, levees, and channelway fans, (2) elsewhere in the intertidal zone tidal stream channels, lower tidal flats, and saltwater marsh, and (3) in the supratidal zone gravel levees, freshwater marsh, bogs, and lakes. Since 1964, the critical environmental results of the earthquake-caused deposition have been abandonment of the settlement of Portage, formation of an unsightly blanket of silt, destruction of natural plant communities, localized erosion, and creation of quicksand hazards.
