The sea lions stop bellowing and slip, one by one, off the jetty into the mocha-brown water of the Fraser River, near Vancouver, British Columbia. The surface of the water is smooth, except for a line of ripples moving slowly upriver. The sea lions seem to know that the calm surface belies turmoil beneath.
The tide has just turned, and a tongue of salt water is first creeping, then galloping, back into the Fraser just a few hours after being expelled by a strong outflow during the previous ebb. Although the surface appears calm, the underwater intersection of fresh and salt water roils with turbulent eddies as strong as any in the ocean. The confusion of swirling water and suspended sediments disorients homeward-bound salmon, providing an easy feast for the sea lions.
Not all rivers end as dramatically as the Fraser. But the mixing of freshwater streams and rivers with salty ocean tides in a partly enclosed body of water—natural scientists call it an estuary—fuels some of the most productive ecosystems on Earth, and also some of the most vulnerable.
Long before the advent of civilization, early humans recognized the bounty of the estuary and made these regions a focal point for human habitation. Unfortunately, overdevelopment, poor land use, and centuries of industrial contamination have taken a toll on most estuaries. Boston Harbor, San Francisco Bay, and the Hudson River are poster children for environmental degradation.
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.
How does an estuary work?
From a physicist’s point of view, the density difference between fresh and salt water makes estuaries interesting. 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. You can’t see these swirling waters from the surface, but a fisherman may find that his net takes on a life of its own when he lowers it into seemingly placid water.
Pliny the Elder, the noted Roman naturalist, senator, and commander of the Imperial Fleet in the 1st century A.D., observed this peculiar behavior of fishermens’ nets in the Strait of Bosphorus, near Istanbul. Pliny deduced that surface and bottom currents were flowing in opposite directions, and he provided the first written documentation of what we now call the “estuarine circulation.”
Saltwater intrusion
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.Y., 60 miles north of the mouth of the Hudson River, depends on the river for its drinking water. Roughly once per decade, drought conditions cause the salt intrusion to approach the Poughkeepsie freshwater intake. The last time this happened, in 1995, extra water had to be spilled from dams upstream to keep the salt front from becoming a public health hazard.
The lifeblood of estuaries
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. If it weren’t for this natural “flushing” process, the waters of the estuary would become stagnant, pollution would accumulate, and oxygen would be depleted.
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 pumping action leads to some of the highest growth rates of microscopic plants (researchers call it “primary production”) in any marine environment. 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. Environmentalists are determined that San Francisco Bay should get “its share” of the fresh water coming from the Sacramento-San Joachim delta because the vast freshwater habitats in the region are particularly vulnerable to salt intrusion.
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 153 miles inland to Troy, N.Y. The Algonquin Indians called the river Mohicanituk, “the river that flows both ways.”