Some of these can be measured in dollars and cents, while others cannot. Estuaries provide places for recreational activities, scientific study and aesthetic enjoyment. Estuaries are an irreplaceable natural resource that must be managed carefully for the mutual benefit of all who enjoy and depend on them. Below are additional ways in which estuaries are important click to expand :. Thousands of species of birds, mammals, fish and other wildlife depend on estuarine habitats as places to live, feed and reproduce.
And many marine organisms, including most commercially-important species of fish, depend on estuaries at some point during their development. Because they are biologically productive, estuaries provide ideal areas for migratory birds to rest and refuel during their long journeys.
Because many species of fish and wildlife rely on the sheltered waters of estuaries as protected spawning places, estuaries are often called the "nurseries of the sea.
Estuaries have important commercial value and their resources provide economic benefits for tourism, fisheries and recreational activities. The protected coastal waters of estuaries also support important public infrastructure, serving as harbors and ports vital for shipping and transportation. The economy of many coastal areas is based primarily on the natural beauty and bounty of estuaries.
When those natural resources are imperiled, so too are the livelihoods of those who live and work in estuarine watersheds. Over half the U. Yet there is hope. Estuaries are the borderlands between salt- and freshwater environments, and they are incredibly diverse both biologically and physically.
The diversity and the high energy of the ecosystem make estuaries remarkably resilient. With a better understanding of these systems, we can reverse their decline and restore the ecological richness of these valuable, albeit muddy, environments.
When river water meets sea water, the lighter fresh water rises up and over the denser salt water. Sea water noses into the estuary beneath the outflowing river water, pushing its way upstream along the bottom. Often, as in the Fraser River, this occurs at an abrupt salt front.
Across such a front, the salt content salinity and density may change from oceanic to fresh in just a few tens of meters horizontally and as little as a meter vertically.
Accompanying these strong salinity and density gradients are large vertical changes in current direction and strength. Pliny the Elder, the noted Roman naturalist, senator, and commander of the Imperial Fleet in the 1st century A. The opposing fresh and saltwater streams sometimes flow smoothly, one above the other.
But when the velocity difference reaches a certain threshold, vigorous turbulence results, and the salt and fresh water are mixed. Tidal currents, which act independently of estuarine circulation, also add to the turbulence, mixing the salt and fresh waters to produce brackish water in the estuary.
In the Fraser River, this circulation is confined to a very short and energetic frontal zone near the mouth, sometimes only several hundred meters long. In other estuaries, such as San Francisco Bay, the Chesapeake Bay, or the Hudson River, the salt front and accompanying estuarine circulation extend inland for many miles.
The landward intrusion of salt is carefully monitored by engineers because of the potential consequences to water supplies if the salt intrusion extends too far. For instance, the city of Poughkeepsie, N. Roughly once per decade, drought conditions cause the salt intrusion to approach the Poughkeepsie freshwater intake. The last time this happened, in , extra water had to be spilled from dams upstream to keep the salt front from becoming a public health hazard.
Estuarine circulation serves a valuable, ecological function. The continual bottom flow provides an effective ventilation system, drawing in new oceanic water and expelling brackish water. This circulation system leads to incredible ecological productivity. Nutrients and dissolved oxygen are continually resupplied from the ocean, and wastes are expelled in the surface waters. This teeming population of plankton provides a base for diverse and valuable food webs, fueling the growth of some of our most prized fish, birds, and mammals—salmon, striped bass, great blue heron, bald eagles, seals, and otters, to name a few.
The vigor of the circulation depends in part on the supply of river water to push the salt water back. The San Francisco Bay area has become a center of controversy in recent years because there are many interests competing for the fresh water flowing into the Bay—principally agriculture and urban water supplies extending to Southern California.
Estuarine circulation is also affected by the tides; stronger tides generally enhance the exchange and improve the ecological function of the system. The Hudson estuary, for example, is tidal for miles inland to Troy, N.
Estuaries have their problems. Some are self-inflicted; some are caused by the abuses of human habitation. An estuary, with all of its dynamic stirrings, has one attribute that promotes its own destruction: It traps sediment. When suspended mud and solids from a river enter the estuary, they encounter the salt front. Unlike fresh water, which rides up and over the saline layer, the sediment falls out of the surface layer into the denser, saltier layer of water moving into the estuary.
As it drops, it gets trapped and accumulates on the bottom. Slowly, the estuary grows muddier and muddier, shallower and shallower. Occasionally a major flood will push the salt right out of the estuary, carrying the muddy sediment along with it.
Sediment cores in the Hudson River indicate that sediment may accumulate for 10, 20, or even 50 years, laying down layers every year like tree rings. But then a hurricane or big snowmelt floods the river, wipes out the layers of sediment, and sends the mud out to sea.
It is good because a big storm can keep an estuary from getting too shallow too fast. In fact, it appears that over the last 6, years, the natural dredging by large storms has maintained nearly constant water depth in the Hudson estuary.
Environmental regulations are far stricter now than they were 50 years ago, and we have stopped using many chemicals that play havoc with the environment. For instance, polychlorinated biphenyls PCBs were banned in the s because they were shown to be toxic to fish and wildlife, and to the humans who consume them. In slightly stratified or partially mixed estuaries, saltwater and freshwater mix at all depths; however, the lower layers of water typically remain saltier than the upper layers.
Salinity is greatest at the mouth of the estuary and decreases as one moves upstream. Even though Puget Sound is classified as a fjord in terms of its geology, it does not exhibit the characteristics of a fjord when classified by water circulation. In this animation, you can see the slight stratification of the blue-colored freshwater flowing from the river on the right hand side of the image, and the green- colored seawater from the ocean on the left hand side of the image flowing beneath it.
A vertically-mixed or well-mixed estuary occurs when river flow is low and tidally generated currents are moderate-to-strong. The salinity of water in a vertically-mixed estuaries is the same from waters surface to the bottom of the estuary.
Strong tidal currents eliminate the vertical layering of freshwater floating above denser seawater, and salinity is determined by the daily tidal stage. An estuary's salinity is highest nearest the ocean and decreases as it moves up the river. This type of water circulation might be found in large, shallow estuaries, such as Delaware Bay. We normally think of estuaries as places where rivers meet the sea, but this is not always the case.
Freshwater or Great Lakes-type estuaries do not fit the definition of a brackish water estuary where freshwater and seawater mix. Freshwater estuaries are semi-enclosed areas of the Great Lakes in which the waters become mixed with waters from rivers or streams.
Although these freshwater estuaries do not contain saltwater, they are unique combinations of river and lake water, which are chemically distinct.
Unlike brackish estuaries that are tidally driven, freshwater estuaries are storm-driven. In freshwater estuaries the composition of the water is often regulated by storm surges and subsequent seiches vertical oscillations, or sloshing, of lake water.
While the Great Lakes do exhibit tides, they are extremely small. Most changes in the water level are due to seiches, which act like tides, exchanging water between the river and the lake. Tidal changes in water level only average about 3 cm. As a storm-driven estuary system, during periods of low water flow, a barrier sand beach will often close the mouth of the estuary, isolating it from Lake Erie. Water movement through the sand barrier beach is generally very limited.
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