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Geological Observations:

  • Tillamook Head, OR and Point Grenville, WA have confined the beach sand discharged from the Columbia River for the last several thousand years.
  • North Head and Cape Disappointment are prominent geologic features that aligned littoral currents to form the Long Beach Peninsula and Willapa Bay.
  • The coastal barriers have prograded for some 4,000 to 5,000 years along the Long Beach Peninsula and Clatsop Plains, and only about 1,500 years along the northern end of the North Beach sub-cell. Barrier progradation rates are on the order of 0.3 to 0.6 m/yr over the last 4,000 years.
  • The beaches have experienced severe shoreline retreat caused by sudden 1 to 2 m drops in land elevation along the coast associated with large subduction-zone earthquakes that occur about every 500 years.
  • The shelf is the largest sink of the Columbia River sediment, followed by the deep sea slope, canyons and fans, the bays, and the barriers and beaches.

Historical Observations:

  • Following construction of the jetties at the Columbia River and Grays Harbor in the early 1900s, the beaches have grown seaward by many meters per year for several decades.
  • The jetties have influenced accretion and possibly erosion patterns on the beaches over alongshore distances of 20 km or more.
  • The carrying capacity of beach sands of Columbia River to the estuary has been reduced by approximately two-thirds over the last century.
  • Accretion rates along the coast have slowed dramatically over the past few decades.
  • High rates of erosion are occurring along sections of beach that had previously accreted most rapidly.
  • Local erosion sites appear to have either increasing erosion rates or an expanding spatial scale of erosion along the shoreline.

Processes Observations:

  • There are large regional gradients of change in shoreline progradation and recession patterns, as well as in sediment size, beach slope, and elevation change.
  • The shoreline position can migrate landward by as much as 100 m during a winter season. On average, the shoreline migrates landward on the order of 33 m during a winter season.
  • Seasonal beach elevation change is on the order of 0.5 m and rip currents scour as much as 2 m depth of sand from the upper beach face, causing local embayments.

Ecology - SEA Program | USGS - Coastal & Marine Geology

This is http://www.ecy.wa.gov/programs/sea/swces/overview/findings.htm
Maintained by CMAP, Washington Department of Ecology
Address questions and comments to George Kaminsky
Modified 22 Mar 2012