The Holocene deltas of the Vancouver and Seattle-Tacoma areas underlie highly urbanized communities. On the right, the margin of the Fraser delta is home to extensive residential, commercial and port development. Historic sloughs, above, lace much of the modern delta. The Duwamish delta front is dominated by piers, industry, tourist attractions and a large marine container port.

A common element of the deltas is that they fill late Pleistocene fjords (below) formed by the advance and retreat of the Strait of Georgia and Puget lobes of the cordilleran ice sheet. On the right, the former and deep Duwamish and Puyallup embayments were filled by deltas prograding northward away from Mt. Rainier, and towards the Sound.

The diagram on the right depicts the convulsive and episodic advancement of the Puget Deltas during times of Holocene volcanic activity affecting the watershed. Averaged progradation rates are below.