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How Tillamook Bay Formed

"The bay came into existence about 8,000 years ago when the rising level of the sea at the end of the glacial Ice Age flooded the lower valleys of the Miami, Kilchis, Wilson, Trask, and Tillamook Rivers. Bayocean Spit formed along the western side of the bay, separating it from the ocean except for an inlet at the bay’s northwest corner." - Komar and Styllas

Tillamook Bay's Five Rivers

Tillamook River

17 miles long, the Tillamook river shares its final mile with the Trask river as it drains into the southeastern corner of Tillamook Bay. It drains a 61 sq mile basin in the foothills of the Norhtern Oregon Coast Range.

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Trask River

Named after Eldbridge Trask, who moved to Tillamook bay in 1848, the Trask river is 18 miles long from where it's two main forks join, with 30 mile north fork and 10 mile southern fork. The river is known for its Steelhead and Chinook Salmon populations.

Wilson River

About 33 miles long, the Wilson River drains the Northern Oregon Coast Range. Oregon Route 6 runs parallel to the Wilson River on its route from the coast to the Portland Metro area.

Kilchis River

14 miles  long, the Kilchis River drains a basin of about 65 square miles from its origins in the Tillamook state Forest to its mouth in the Western side of Tillamook Bay.

Miami River

The Maimi River runs into the north end of Tillamook Bay near Garibaldi. It descends over 1,700 feet along its 13 mile course through the Tillamook State Forest and Northern Tillamook County.

Styllas, Michael & Komar, Paul. (2006). Natural processes and human impacts as the causes of sediment accumulation in Tillamook Bay, Oregon. Shore and Beach. 74. 3-15.