Rivers provide less fresh water Heat, dryness help bring saltier flavor to the bay


Maryland's extraordinarily hot, dry summer has gotten the least attention in the one-fifth of the state that always is wet -- the Chesapeake Bay.

But the effects on the bay and its tidal rivers have not been trivial. Take Baltimore Gas and Electric Co., whose Calvert Cliffs nuclear plant depends on sucking 2.4 million gallons a minute from the bay for cooling.

For a few days in July and August, one of its two units had to run at half power, because even at the 40-foot depths from which it draws water, the temperatures rose above the 85 degree level needed to meet safety standards.

The main effects on the bay, though, have had to do with salt, as weak flows of fresh water from the rivers allowed ocean water to dominate the estuary.

It seems a paradox for a summer to get so dry that a farmer would fear to irrigate his crops, but if you were pumping water, as many do, from convenient tidal rivers, that is exactly what has happened in recent weeks.

Betsy Gallagher, agricultural extension agent for Dorchester County, recently sampled salinity for a farmer who normally uses the Transquaking River for irrigation.

Even for relatively salt-tolerant crops like sorghum, she says, irrigation must be stopped when the salt gets above one or two parts per thousand parts of water.

Corn is a bit less tolerant than that, and soybeans cannot take more than .3 ppt. (Fresh water has a salinity of zero, and open ocean water is around 35 ppt; the bay's highest salinities, at its mouth, run around 28 ppt).

So the sample she took stopped the agriculture agent short -- "it was way, way over the limit" -- around 14 ppt.

That is even saltier than scientists would expect in a summer like this; but estuaries like the bay, more than rivers, lakes or oceans, are capable of such surprises.

To start with, the estuary (its Latin root implies turmoil and commotion) constantly is flowing two ways at once.

As fresher, lighter water from tributaries flows down the bay along the surface, saltier, heavier ocean water wedges up the bay along the channel bottoms.

And because the bay is so shallow -- 22 feet on average -- this classic, two-layered parfait of sweet and salt can get squished around by even gentle pressure from winds.

Such a scenario a few years ago amazed scientists when they began sampling salinity not far below the mouth of the Susquehanna River.

The Susquehanna is the source of nearly half the fresh water in the bay, so they were expecting to find the salt near zero, or a few parts per thousand.

Instead, it ranged from 11 to 15 ppt. That was the same year a soft drink bottling plant in Havre de Grace had to shut down temporarily because of salt in its water intakes.

What apparently had happened, says William Boicourt, an oceanographer with the University of Maryland, is that a northeast wind had been blowing.

Northeasters force the bay's surface waters down and out of the estuary, and that triggers a corresponding up-the-bay surge from the salty bottom layer.

Such vagaries occur so often that it is impossible to understand the bay's circulation without taking the wind into account.

The inability to do so was a major, unforeseen flaw in the 7-acre concrete replica of the bay built by the Army Corps of Engineers to emulate the workings of the estuary. It is now a shrimp-rearing facility.

Dr. Boicourt says northeasterly winds, combined with a dry spring and summer, may explain some extraordinary incursions of salt we have been documenting in the Nanticoke River, near where I live.

Most everyone in Maryland knows part of the Nanticoke well, since they cross it on U.S. 50, bound for Ocean City, between Cambridge and Salisbury. The stretch of river there is a breakpoint of sorts, where the salinity normally begins to peter out and the water turns fresh.

This fresh-salt interface is precisely what spawning striped bass seek in April and May. Out on the river a few years ago with biologists who were electroshocking spawning stripers to the surface, we boated and released 15 within sight of U.S. 50. Their weights ranged from 45 to perhaps 100 pounds.

Currently, the salt there is an astounding 8.4 ppt; and salt is measurable in the Nanticoke for nearly another 20 miles upstream, to where it runs under U.S. 13 at Seaford, Del. Even in the drought years of the 1960s, the salt in the river was not so high.

This dance of salt with sweet up and down the bay, shifting with the seasons and the climate and the winds, is of more than passing interest, because its balance sets the parameters of life for so much in the estuary.

The location and extent of the tidal rivers' great spawning and nursery areas for shad and herring, rockfish and white perch, are determined by the levels of salt.

Salt sets the northern limit of hard clams in the bay (around Smith Island); fuels or suppresses outbreaks of the oyster disease, MSX (look out this year!); determines where jimmies and sooks congregate to mate and shed their shells, and determines the annual sea nettle crop (pray for wet springs on this account).

The bay's two-layered flows -- fresh-on-top and salt-on-the-bottom -- also are important to the feeble-swimming larvae of dozens of species who use it to move about.

The salt wedge thus helps disperse the spawn of crabs and fish throughout the bay. Many species have adapted to rise in the water column when they want to go south, catching the outflowing fresh surface water, and to sink into the salt layer to move northward.

Given the variability of river inflows to the bay from wet to dry years, you might expect salinity to vary even more than it does.

Historically, river volumes have swung 50-fold from the wettest to the driest months, and some whole years have contributed three times as much fresh water as others.

But while individual tributaries may swing capriciously in salinity, the bay as a whole remains somewhat more stable from year to year, Dr. Boicourt says.

Summertime readings taken from the bay's deep trench between Dorchester and Calvert counties show a range of only about 16 to 26 ppt during nearly 50 years of record.

The biggest exception was a reading of 10 ppt in 1972, when Tropical Storm Agnes dumped more fresh water into the bay during a few June days than it gets in some whole years.

How does this year compare? No record buster, Dr. Boicourt says, but no slouch either -- one of the four saltiest years on record going back to 1949.

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