An estuary's mouth is where the river joins the ocean.
Estuaries are known by many names--coves, sounds, bays, inlets, lagoons--but all these terms describe the section of a river where it flows into the sea and interacts with tidal currents. Preexisting landforms influence the type of estuary that forms. Biologically rich, shallow estuaries are very different from deep, cold, fjord-like estuaries. Most estuarine environments contain five zones that continue to create and shape distinctive landforms today. Storms and tides produce new landforms within estuaries are products of.
Inactive River Valley Landforms
The Hudson River Valley is an ancient estuary.
Inactive river valleys are old landforms associated with estuaries that have been modified by rising sea levels. In the past when sea levels were much lower, rivers traveled farther to the ocean, sometimes cutting deep river valleys along the way. Over the last 10,000 years ocean waters backed up, "drowning" these river valleys and creating estuaries. New York's Hudson River estuary, which was also widened by glacial ice, is one example.
Barrier Island Landforms
Point Pelee, Ontario, is an example of a sand spit.
Barrier islands influence the creation of new estuaries by partially isolating an area of the ocean. They are also the principal new landforms of active estuaries. An existing sand spit or island can create an estuary if it grows, gradually enclosing a bay. Many estuaries along the Atlantic and Gulf Coasts of the United States were formed this way. Sediment carried by the river creates new mudflats and sand bars within the estuary. Storms shape and rework these landforms, which alter river flow and sediment loads. Tidal currents and surges scour away or wash in additional material.
Fjord-type Estuary Landforms
Fjord-type estuaries are long, narrow and deep.
Fjord-type estuaries exist wherever rivers flow into fjords and mix with seawater. Fjords are mountain valleys which were carved out and widened by glaciers in the past, and have subsequently flooded with rising sea levels. As an estuary landform, a fjord creates an estuary environment that is relatively narrow and deep. The mouth is shallowest part of the estuary due to deposits of glacial debris on the bottom.
Estuary Landforms by Zone
This river in Devon, England includes landforms in different estuarine zones.
Five zones may be identified in most estuaries. Each zone influences the creation of distinct landforms. The head of an estuary is typically an area of mud deposits where the river follows its channel and has not yet widened out. The upper reaches of an estuary, where the river first begins to widen, contains mudflats. The middle reaches include a mix of mud and sand, where sandbars may be exposed at low tide. The lower reaches are sandy, where landforms such as sandbars may be exposed at all times. The mouth is defined by the edge of the coastline. Its landforms include the beach and rocks on out to sea. These zones are less identifiable in deep fjord-type estuaries.
Estuary Landforms and Ocean Tides
Surfers ride the strong tidal bore in the Severn Estuary.
Weak or strong tides in relation to river flow help determine the overall shape of an estuary and its landforms. Small tidal ranges permit the formation of barrier islands and often result in shallow salt wedge estuaries. Stronger tides of a limited range produce wide estuaries that don't extend very far upstream. Strong tides influence landforms far inland, creating long linear sandbars and estuaries with a trumpet-shaped flare, such as the Severn Estuary in Great Britain.
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