Sediment Accumulation in Tillamook Bay, Oregon: Natural Processes versus Human Impacts

Tillamook Bay on the northern Oregon coast has experienced significant sediment accumulation and shoaling. Analyses show that part of the increased sedimentation was a result of substantial human impacts in the watersheds of the five rivers that drain into the bay. River discharges were enhanced by approximately 13% during the period 1931–1954, when commercial logging and a series of devastating forest fires occurred, compared with discharges in the years after reforestation. Potential annual sediment yields calculated from daily discharges were enhanced by 29% during 1931–1954, but actual yields would have been substantially greater as a result of increased erosion rates because of deforestation. Sand transported by the rivers consists primarily of rock fragments, in contrast to the quartz and feldspar sand carried into the bay from the ocean beach. Surface sediments collected throughout the bay consist, on average, of about 40% sand from the rivers and 60% from the ocean beach. Cores show increasing percentages of beach sand beneath the surface, with evidence for major episodic inputs rather than the higher percentages of river‐derived rock fragments that human impacts would have produced. Subduction earthquakes have struck the Oregon coast repeatedly during the past several thousand years; the most recent was in January 1700. The down‐core increase in beach‐derived sand in Tillamook Bay is from sand transport by the tsunami that accompanied the 1700 earthquake and the deepening of the bay from land subsidence at the time of the earthquake, which permitted more frequent and extensive spit overwash events during storms.