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Drowned Valley Coast

Drowned valley coasts are also known as estuaries. Estuaries are confined bodies of water that occupy the drowned valleys of rivers that are not currently building open-coast deltas. The estuarine environment can also be defined as the complex of lagoon-bay-inlet-tidal flat and marsh that make up 80 to 90 percent of the U.S. Atlantic and Gulf Coasts.

During periods of lowered sea level, rivers incised the lower reaches of their valleys and discharged increasing amounts of sediment out onto the shelf. Deltas accumulated and fluvial channels were cut, dissecting parts of the delta plain. At the lowest stands of sea level, estuaries almost disappeared and were confined to river valleys. When sea level rose again, the valleys were flooded and the estuaries reappeared.

The largest drowned valley coasts in the U.S. are the Chesapeake and Delaware Bays on the Atlantic coast. Barriers and drowned valleys continue south to Miami, Florida, along the Atlantic coast.

 

Drowned Valley Types

Estuaries can be wave-dominated or tide-dominated. Wave-dominated estuaries are characterized by high wave energy compared to tidal influence. Waves cause sediment to move alongshore and onshore into the mouth of the estuary, forming sandbars or subaerial barriers and spits. The barrier prevents most of the wave energy from entering the central basin. At the head of the estuary, the river deposits sand and gravel, forming a bay-head delta.

Tide-current energy is greater than wave energy at the mouth of tide-dominated estuaries, resulting in the development of elongate sandbars. The bars dissipate wave energy, helping to protect the inner portions of the estuary. A bay-head delta is usually not present in the river-dominated portion of tidally dominated estuaries. Instead, the river channel merges directly into a single or a series of tidal channels that eventually reach the sea.

The shape of the flooded valley and the pre-existing geology also control the size of the estuary and the nature of sediment deposition. For example, tidal-wave amplification is less likely to occur in irregular valleys. The resulting estuaries are more likely to become wave-dominated. The Chesapeake Bay, with its extensive system of tributary valleys, is an example of this type.

 

Drowned Valley Sediments

Because estuaries occupy drowned river valleys, they function as sinks for enormous volumes of sediment. Estuarine sediments are derived from various sources including rivers, the continental shelf, local erosion and biological activity. Sedimentation is controlled by tides, river flows, waves and meteorology.

The low-energy conditions of estuaries, as opposed to those found on open coasts, allow for the deposition of fine-grained silts, muds, clays and biogenic materials. Estuarine sediments are typically soft and tend to be deposited on smooth surfaces that limit turbulence of the moving water. When allowed to accumulate, these materials consolidate and undergo various chemical and organic changes, eventually forming cohesive sediments.

Aerial photo of a drowned valley coast

Drowned Valley Coasts

Pritchard (1967) defined an estuary, a drowned valley coast, as a body of water where "...seawater is measurably diluted with fresh water derived from land drainage."

Estuary Volume

Coastal plain gradient, part of the overall plate tectonic setting, is one factor that determines estuary volume. Sea level rise over a flat coastal plain on a passive margin like the Gulf of Mexico creates long estuaries with large volume.

Drowned Valley Sediments

The Chesapeake and Delaware Bays feature shores with smooth, low-sloping profiles, turbid water along the shores, and vegetated backshores and mud flats that are exposed at low tide.