Where Rivers Join
Contents:
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.
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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.
Where the Rivers Meet the Sea
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: 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.
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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.
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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.
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Billions of dollars are now being spent to clean up American estuaries contaminated by industrial pollution. The Superfund program of the U. Environmental Protection Agency collects and spends billions of dollars more to remediate estuaries. Often the remediation strategies are complex and controversial. To see what your friends thought of this book, please sign up.
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To ask other readers questions about Where the Rivers Join , please sign up. Be the first to ask a question about Where the Rivers Join. Lists with This Book. Feb 19, Carol Patrick rated it did not like it. Rachel rated it really liked it Apr 16, Billie Rain rated it liked it Apr 19, Jane Eaton Hamilton rated it really liked it Jan 15, Megan Mitchell rated it it was amazing Nov 30, Dec 10, George Ilsley rated it it was ok Shelves: An unfocused memoir in the form of diary entries, gleanings from research and others bits of ephemera. The emotional pain was real but the storyline was hard to grasp.
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The vigor of the circulation depends in part on the supply of river water to push the salt water back. Taxes, fees not included for deals content. For instance, the city of Poughkeepsie, N. What is a suitable word to describe a place where two rivers meet? 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. Daniel rated it really liked it Dec 02,