The estuary's salinity will have decreased, and the saltwater boundary will have migrated seaward. Autumn arrives at the point of autumnal equinox, when the sun crosses the plane of the earth's equator for the second time of the year. That happens about September 22, when days and nights are of approximately equal length. Warm weather usually continues through September and early October, and might proceed on to November. As the hours of daylight decline, however, the bay begins to cool.
Foliage changes color and dies.
Alders lose their leaves. Those that fall into the rivers, creeks, swamps, and marshes darken the water as they decompose, reducing sunlight penetration. Marsh plants also begin to die back and decompose, an important process that recharges the estuary's nutrient supply. Roughly coinciding with this process is a seasonal upwelling offshore. Flood tides rush into the estuary bearing nutrients from the ocean bottom, triggering an autumn plankton bloom on the bay.