『Abstract
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,
in 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.』
Introduction
The river watersheds
Tillamook Bay
Natural processes and human impacts
Surface sediments−Sources and transport paths
The down-core history of sediment accumulation
Conclusions and discussion
Acknowledgments
References cited