Natural Tracers of Coastal Sediments

Komar, Paul D.; McManus, James; ,

Natural Tracers of Coastal Sediments

by Paul D. Komar, College of Oceanic and Atmos. Sci., Oregon State Univ., Corvallis, OR, United States,
James McManus, College of Oceanic and Atmos. Sci., Oregon State Univ., Corvallis, OR, United States,

Document Type: Proceeding Paper

Part of: Sand Rights '99: Bringing Back the Beaches

Abstract:

The properties of natural sediments can be used to determine their sources and to trace their transport paths through coastal environments. These properties include the mineralogies of the grains, particle-size distributions, and the degrees of grain rounding. Investigations of these sediment attributes can answer questions important to the development of a littoral sand budget, and help to deal with issues involved in the management of sand resources. The techniques of utilizing natural sand tracers are illustrated by research on the Oregon coast. Analyses of beach sand mineralogies show that they are derived from three main sources; the Columbia River on the north, the numerous rivers that drain the Coast Range, and the Klamath Mountains of southern Oregon and northern California. The compositions of beach sands accurately record the proportions derived from these sources, with the percentages varying systematically along the length of the Oregon coast. Numerous headlands now prevent the longshore transport and mixing of sands from these sources, so it is concluded that the beach sand compositions reflect along-coast mixing during the lowered sea levels of the Ice Ages when blockage by headlands was absent. Much of the sand on Oregon beaches is therefore relict in the sense that it does not represent modern-day contributions. Potentially the most important sources of modern sand to the beaches are the numerous rivers that drain the Coast Range. But most of these rivers pass through large estuaries, and investigations of the mineralogy, grain sizes, and grain roundness of estuarine sands indicate that the river sand is trapped within estuaries and does not reach the ocean beaches. On-going investigations of sediments in Tillamook Bay are examining whether land-use practices within the watershed - principally deforestation by logging and forest fires - have led to increased rates of sediment accumulation.

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