4.6  Estuaries

The first type is a coastal plain estuary, or drowned river valley.  These estuaries are formed as sea level rises and floods an existing river valley, mixing salt and fresh water to create the brackish conditions where the river meets the sea.  These types of estuaries are common along the east coast of North America, including major bodies such as Chesapeake Bay and Delaware Bay (Figure 13.6.1). Coastal plain estuaries are usually shallow, and since there is a lot of sediment input from the rivers, there are often a number of depositional features associated with them such as spits  and barrier islands.

The presence of sand bars, spits, and barrier islands can lead to bar-built estuaries, where a barrier is created between the mainland and the ocean.  The water that remains inside the sand bar is cut off from complete mixing with the ocean, and receives freshwater input from the mainland, creating estuarine conditions (Figure 13.6.2).

Fjords are estuaries formed in deep, U-shaped basins that were carved out by advancing glaciers.  When the glaciers melted and retreated, sea level rose and filled these troughs, creating deep, steep-walled fjords (Figure 13.6.3).  Fjords are common in Norway, Alaska, Canada, and New Zealand, where there are mountainous coastlines once covered by glaciers.

Tectonic estuaries are the result of tectonic movements, where faulting causes some sections of the crust  to subside, and those lower elevation sections then get flooded with seawater.  San Francisco Bay is an example of a tectonic estuary (Figure 13.6.4).

In a vertically mixed, or well-mixed estuary there is complete mixing of fresh and salt water from the surface to the bottom.  In a particular location the salinity is constant at all depths, but across the estuary the salinity is lowest at the head where the fresh water enters, and is highest at the mouth, where the seawater comes in.  This type of salinity profile usually occurs in shallower estuaries, where the shallow depths allow  complete mixing from the surface to the bottom.

A salt wedge estuary occurs where the outflow of fresh water is strong enough to prevent the denser ocean water to enter through the surface, and where the estuary is deep enough that surface waves and turbulence have little mixing effect on the deeper water.  Fresh water flows out along surface, salt water flows in at depth, creating a wedge shaped lens of seawater moving along the bottom.  The surface water may remain mostly fresh throughout the estuary if there is no mixing, or it can become brackish depending on the level of mixing that occurs.