In geologic time, which is often measured on scales of hundreds of thousands to millions of years, estuaries are often fleeting features of the landscape. In fact, most estuaries are less than 10, years old. Drowned river valley estuaries are formed when rising sea levels flood existing river valleys.
Bar-built estuaries are characterized by barrier beaches or islands that form parallel to the coastline and separate the estuary from the ocean. Barrier beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves. Fjords are steep-walled river valleys created by advancing glaciers, which later became flooded with seawater as the glaciers retreated.
These, and most other coastal plain estuaries in North America, were formed at the end of the last ice age between 10,, years ago.
As glaciers receded and melted, sea levels rose and inundated low-lying river valleys. Coastal plain estuaries are also called drowned river valleys. In the animation below, rising waters flood a low-lying river valley, creating a coastal plain estuary. The Chesapeake Bay center and Delaware Bay upper right are both examples of drowned river valley estuaries. Credit: NASA. Bar-built or restricted-mouth, estuaries occur when sandbars or barrier islands are built up by ocean waves and currents along coastal areas fed by one or more rivers or streams.
Water continually circulate s into and out of an estuary. Tides create the largest flow of saltwater, while river mouth s create the largest flow of freshwater. When dense, salty seawater flows into an estuary, it has an estuarine current. High tides can create estuarine currents. Saltwater is heavier than freshwater, so estuarine currents sink and move near the bottom of the estuary. When less-dense freshwater from a river flows into the estuary, it has an anti-estuarine current.
Anti-estuarine currents are strongest near the surface of the water. Heated by the sun, anti-estuarine currents are much warmer than estuarine currents. In estuaries, water level and salinity rise and fall with the tides. These features also rise and fall with the seasons. During the rainy season, rivers may flood the estuary with freshwater.
During the dry season, the outflow from rivers may slow to a trickle. The estuary shrinks, and becomes much more saline. During a storm season, storm surge s and other ocean waves may flood the estuary with saltwater. Most estuaries, however, are protected from the ocean's full force.
Geographical features such as reef s, islands, mud, and sand act as barriers from ocean waves and wind. Types of Estuaries There are four different kinds of estuaries, each created a different way: 1 coastal plain estuaries; 2 tectonic estuaries; 3 bar-built estuaries; and 4 fjord estuaries. Coastal plain estuaries 1 are created when sea levels rise and fill in an existing river valley.
Chesapeake Bay was formed at the end of the last ice age. Massive glacier s retreated, leaving a carved-out landscape behind. The Atlantic Ocean rushed to fill in the wide coastal plain around the Susquehanna River, creating a large estuary known as a ria : a drowned river mouth.
Tectonic activity , the shifting together and rifting apart of the Earth's crust, creates tectonic estuaries 2. California's San Francisco Bay is a tectonic estuary. The complex tectonic activity in the area has created earthquake s for thousands of years. The San Andreas fault is on the coastal side of the bay, where it meets the Pacific Ocean at a strait known as the Golden Gate.
The interaction of the San Andreas and Hayward faults contributes to downwarping , the process of an area of the Earth sinking.
Like the Chesapeake, the San Francisco Bay was only filled with water during the last ice age. As glaciers retreated, land around the bay experienced post-glacial rebound —without the massive weight of the glacier on top of it, the land gained elevation. The Pacific Ocean rushed in through the Golden Gate to flood the downwarped valley. When a lagoon or bay is protected from the ocean by a sandbar or barrier island , it is called a bar-built estuary 3.
The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create sandy, bar-built estuaries. The Outer Banks protect the region's coast from waves and wind brought by Atlantic Ocean hurricane s.
The islands and sandbars also protect the delicate, brackish ecosystem s created by the outflow of many rivers, such as the Roanoke and Pamlico. For these reasons, engineer s monitor the shifting sandbars of the Outer Banks, and constantly work to maintain them. Fjord estuaries 4 are a type of estuary created by glaciers. Fjord estuaries occur when glaciers carve out a deep, steep valley. Glaciers retreat and the ocean rushes into fill the narrow, deep depression.
Puget Sound is a series of fjord estuaries in the U. Like fjords found in Alaska and Scandinavia, the fjord estuaries of Puget Sound are very deep, very cold, and very narrow. Unlike many of those fjords, Puget Sound's fjord estuaries also have inflows from local rivers and streams.
Many of these streams are seasonal, and fjord estuaries remain mostly salty. Freshwater Estuaries Some estuaries not located near oceans. These freshwater estuaries are created when a river flows into a freshwater lake. Although freshwater estuaries are not brackish, the chemical composition of lake and river water is distinct. River water is warmer and less dense than lake water. The mixing of the two freshwater systems contributes to lake turnover —the mixing of the waters of a lake. Freshwater estuaries are not affected by tides, but large bodies of water do experience predictable standing wave s called seiche s.
Seiches, sometimes nicknamed sloshes, rhythmically move back and forth across a lake. The Great Lakes, in the United States and Canada, experience seiches and have many freshwater estuaries.
Estuary Ecosystems Many plant and animal species thrive in estuaries. The calm waters provide a safe area for small fish, shellfish, migrating birds and shore animals. The waters are rich in nutrients such as plankton and bacteria. Decomposing plant matter, called detritus , provides food for many species.
The estuarine crocodile, for example, is an apex predator of tropical Australian and Southeast Asian estuaries. The estuarine crocodile is the largest reptile in the world. A specimen caught in the Philippines in measured 6. Like most apex predators, estuarine crocodiles eat almost anything. Moving down the Bay, the water composition changes as salinity increases.
Both fresh and salt water contain many natural dissolved materials from several sources. Bacteria and other microorganisms decompose dead organisms and release compounds into the water.
Live organisms also release compounds directly into the water. Begin typing your search term above and press enter to search. Press ESC to cancel. Popular articles. Esther Fleming April 26, Table of Contents.
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