FIGURE 6.7 Developed sand spit at Stinson Beach in Marin County, California, where a revetment has been constructed in an effort to protect the homes. This spit cannot migrate with sea-level rise. SOURCE: Copyright 2002–2012 Kenneth & Gabrielle Adelman, California Coastal Records Project, <www.Californiacoastline.org>.
Washington coast (Komar, 1997). Many of the dune areas exposed along and inland from the west coast shoreline today formed during the lower sea levels of the past. At the end of the last ice age, when sea levels were about 120 m lower than today, the entire continental shelf was exposed. Sand from rivers and streams was deposited across this extensive plain, and onshore winds produced large dune fields, such as those in the Coos Bay area of central Oregon, which extend along the coast for nearly 240 km and are encroaching into some developed areas (Figure 6.8; Komar, 1997). As sea level rose, many of the dunes were cut off from their vast reservoir of offshore sand. Dunes still form and are active today along the shorelines of all three states, but they have a lower supply of sediment and are much less extensive than those that formed in the past.
Decades of observations of coastal dunes around the world have shown that the frontal dune, which is closest to the beach, is an ephemeral and unstable feature (e.g., McHarg, 1969). Sand dunes typically accrete or expand under the force of onshore winds and an ample supply of sand, but they can erode quickly under severe wave attack at times of high tide or elevated sea level. The hazards of building on the frontal dune have been known for centuries (McHarg, 1969). Nevertheless, many housing developments in California, Oregon, and Washington have been constructed on dunes and are periodically threatened or damaged (Figure 6.9). Dunes, whether modern or Pleistocene, can be expected to retreat quickly under rising sea levels and larger waves.
Coastlines have been retreating globally since sea level began rising at the end of the last ice age, approximately