Brackish Water: Where Fresh Water Rivers Meet A Salt Water Sea

Mucking up the system

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.

The “episodic” behavior of sediment deposition is good news and bad news. 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 6,000 years, the natural dredging by large storms has maintained nearly constant water depth in the Hudson estuary.

The bad news is that the sediment retains a “memory” of all of the contaminants that have passed through it over the years. 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 1970s because they were shown to be toxic to fish and wildlife, and to the humans who consume them. Yet we still have a contamination problem in the Hudson and other rivers because PCBs are slow to decay and each new flood remobilizes these “legacy” contaminants and prolongs our exposure.

Trickle-down effects

Billions of dollars are now being spent to clean up American estuaries contaminated by industrial pollution. In Boston, for instance, the new sewage system created to save Boston Harbor cost taxpayers about $5 billion. The Superfund program of the U.S. Environmental Protection Agency collects and spends billions of dollars more to remediate estuaries.

Often the remediation strategies are complex and controversial. In the case of Hudson River, there is a heated debate about whether PCB-contaminated sediments should be removed—dredged with high-tech methods that theoretically minimize environmental harm—or left undisturbed. That debate pivots on the episodic storm phenomenon: Are the contaminated sediments there to stay, or could they get stirred up when the next hurricane washes through the Hudson Valley?

Aside from cleanup initiatives, parts of the Hudson need to be dredged for navigational purposes. Dredging is not that costly or difficult, but finding a place to put contaminated sediments is a problem. The Port of New York has been filling up abandoned Pennsylvania coal mines with its contaminated mud, but that is not a long-term solution.

While the problems of American estuaries are complicated and expensive, they pale in comparison to Asian estuaries. The entire nation of Bangladesh lies within the estuary and lower floodplain of the Ganges-Brahmaputra River. Other Asian rivers such as the Mekong, Chiang Jiang (or Yangtze), and Huang Ho (or Yellow River) are crowded and strained by concentrated human settlements. Global sea-level rise is causing a loss of land, increased flooding, and increased salt intrusion in these estuaries.

The demand for water upstream for irrigation and domestic use significantly reduces freshwater flow through these systems. The Indus River and Huang Ho estuaries have suffered from drastic reductions of freshwater flow over the past several decades, and the impact of these human alterations is just now being recognized. New policies about land use, water diversion, and even global carbon dioxide production (which affects global warming and sea level rise) will be needed to protect these vulnerable estuarine environments and their human inhabitants.

Stirring up new ideas

One of the challenges of estuarine research is that most of the significant problems are interdisciplinary, involving physics, biology, chemistry, geology, and often public policy and economics. Estuaries are also incredibly diverse, coming in all shapes and sizes. Yet scientists are continually challenged by public policymakers to generalize our results from studies of one estuary and apply them to the rest of the world’s estuaries.

As scientists, one of our roles is to predict changes in the environment, given different natural and human-induced influences. To foresee the health of estuaries in the future, we have some fundamental questions to answer about the present and the past. How far will salt intrude if river flow is cut in half? Do changes in river flow increase or decrease the rate at which sediments shoal the estuary? What effect do such changes have on the fish that spawn in fresh water?

What we learn will be critical for a human population that increasingly values coastal waters. We need sound public policy to reduce vulnerability to coastal flooding and to protect drinking water, food supplies, and some of the world’s most important habitats. We will develop better policies only if we can ground them in better science.