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ABSTRACT: The Effect of Sea Level Rise on Coastal Plain Estuaries, with Examples from Chesapeake Bay

Steven M. Colman

Estuaries are geologically transitory features whose evolution depends on a delicate balance among relative sea level, basin geometry, shoreline erosion, fluvial sediment discharge, littoral drift, and tidal exchange. Models of modern estuarine development require specific sea level scenarios; almost all assume a continuation of the decelerating sea level rise of the last few thousand years. However, under constant external conditions, estuaries are ephemeral because they rapidly fill with fluvial and marine sediment. The rate of filling changes with time, but only a few thousand years are required to fill most estuaries. The persistence of estuaries, therefore, requires that relative sea level rises at a rate sufficient to compensate for the inherent tendency of estuarie to fill with sediment.

Coastal plain estuaries, of which Chesapeake Bay is a prime example, are often referred to as drowned river valleys. Although this description is appropriate for the first-order morphology of Chesapeake Bay, the implied passivity can be misleading, especially in the high-tidal-energy area of the bay mouth where dramatic spit progradation and channel migration have occurred in the last few thousand years. Holocene sediment accumulation rates are more irregular along the length of the estuary than most models would predict; but in general, sediment accumulation has been greater at the mouth and at the head of the bay and less along the middle reaches. If relative sea level were to stabilize, the estuary would fill with sediment from both ends within a few thousand years. Evidence for tw previous generations of the bay is preserved as the estuarine fill of major fluvial valleys, demonstrating that estuarine episodes have been closely tied to cyclic sea level changes.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990