We have recently weathered a number of very wet years: 1993, 1994 and 1996 — the wettest year on record. In the first months of this year, we again experienced record rains, half again above the long-term average. But suddenly, precipitation ended, and save for a few thunderstorms, many parts of the basin have gone a couple of months without significant rainfall.
The Maryland Department of the Environment says this is not a drought, and if you look at river flows and at groundwater levels, we’ll benefit for quite a while from the recent abundant moisture stored in the earth that is slowly being released.
Time is against us, though, and we are looking at a serious drought in the not too distant future! History shows that droughts occur again and again, at intervals that vary, yet are frequent enough that almost everyone lives through a couple of bad ones. The way we’ve treated the Chesapeake watershed over the last few centuries means that bad droughts will be harder to manage as time goes on.
Aside from their effects on crops, severe droughts are especially hard on trees, and the stress is reflected in reduced growth — narrower annual rings visible in a cross section of the tree’s trunk. When a tree dies, or is felled, it has preserved in those rings an amazingly detailed and accurate history of its own life. Drought and prosperity, crowding by neighbor trees and the marks of fire and disease all make dendrochronology part art as well as science.
By looking at old, living “sentinel trees” and piecing together sequences of years from tree rings in the beams of buildings that can be accurately dated, dendrochronologists (tree-ring scientists) like Jack Heikkenen in Blacksburg, VA have reconstructed climate back to the 1500s. David Stahle, and colleagues (Science 280:564-567) have taken the record knowledge of Mid-Atlantic climate, pretty much on a year-to-year basis, back about 800 years and conclude that the Bay, even in its “pristine” and early colonial state, encountered some very severe droughts. These events indicate a run of years that were very dry, more so than in any years since modern hydrologists have been keeping records.
They found evidence of droughts in the rings of some ancient North Carolina cypress trees dating to the 1580s and 1600s that they conclude were severe enough to have caused the failure of the Roanoke Colony in North Carolina, and the near failure of the Jamestown Colony. Their evidence for drought is compelling, but in my reading of the early accounts, I fail to find evidence that drought was as important as politics, improvidence and Indians!
But this may not be so surprising, because with the great and nearly pristine basin forests intact, great quantities of water could be stored in nontidal wetlands, the woodland, forest soils and behind beaver dams. As a result, runoff in stormy times was less rapid, and water was stored in the ecosystem. In dry times, base flow out from the groundwater was sustained longer because of that storage.
Furthermore, the taproots of old growth forests run much deeper into that reservoir. Their evapotranspiration brings moisture at the rate of 40 or 50 gallons a night into upper soil layers. Once there, it’s available for shallow-rooted understory trees and herbaceous plants. That’s less likely with younger forests, and not at all possible with cleared land, suburban lawns or agricultural fields with shallow root zone crops like corn and soybean.
During long, dry periods, pollen records in the sediments of lakes and estuaries preserve a less detailed record of shifts in tree species and their relative successes, as the occurrence and relative ratios of pollen change over centuries or millennia. Palaeo-ecologists like Grace Brush at Johns Hopkins and Tom Cronin at the U.S. Geological Survey are exploring the Bay’s deep past using cores taken from the yards of silts and muds that have been accumulating in the estuary since its earliest years.
Brush, in particular, has looked thousands of years into the Bay’s past and found long periods when the climate was uncommonly dry. And, evidence from charcoal layering on the Bay floor indicates that forest fires swept large areas of the Eastern Shore.
The appearance of droughts and wet periods is also detectable in the preserved remains of many aquatic Bay species. At Osborn Cove off the Patuxent, some years ago, a breakdown on the cliff uncovered one edge of an old Native American habitation. They had fed upon oysters from adjacent St. Leonard Creek and left the shells in a “midden,” or layered deposit, which we carefully excavated under the guidance of the Maryland Historical Trust. Some of the oysters had growth marks on their hinges suggesting 42 years of age.
The creek’s salinity today varies widely from year to year, pretty much tracking the overall annual rainfall pattern. It has ranged, in the 24 years I’ve taken data, from extremes of perhaps 2 parts per thousand after periods of deluge, to about 19 parts per thousand (almost 2/3 the salinity of ocean water) in the driest years. Some oyster shells from the midden, which Native American pottery scraps dated between A.D. 750 and 1650, are deeply honeycombed in their early growth years by a boring sponge (one of two Cliona species). These creatures establish colonies, growing on any sedentary, calcium-bearing shell and eroding “pits” to “mine” this mineral to meet their own needs. (The oyster, in this case, is rarely killed.) Because this sponge requires higher salinity than is usually present in the creek, its presence indicates a period of drought sufficient to have moved salinity miles upriver those centuries ago. When the drought ended, these same individual shells ceased to have any evidence of the sponge for the balance of the years of life that remained before they were eaten.
These periodic, broad-scale movements of the salt front are borne out by this same sponge species in the mainstem Chesapeake Bay as well.
Among the small marsh snails accidentally carried up from the creek with the oysters by Native American harvesters is a second gastropod species called Nassarius vibex. It, too, corroborates a period of higher salinity at the time surrounding the Native American occupation of this site. On a still longer time scale, Cronin is working out salinity dynamics for the remains of tiny organisms, preserved like pollen, in cores from the Bay floor.
Monitoring in the modern Chesapeake around BGE’s Calvert Cliffs Nuclear Power Plant revealed a similar wide-ranging change in the abundance and distribution of a small, soft-bodied worm inhabiting the Bay floor, a change unrelated to the plant, but accompanying a salinity change that occurred on a decadal timescale in the 1970s and 1980s.
The great drought of the 1930s created “dust bowl” conditions in middle America and was exacerbated by agricultural practices and the Great Depression. After that hard time, the drought of record in our lifetimes occurred in the Chesapeake and the Northeast in the mid-1960s, and was prolonged over several very painful years. This was a basin from which about 40 percent of the original forest cover had been removed and across which almost all of the remaining forest was relatively young.
The Potomac River flow fell to 609 cubic feet a second, just drops against the hundreds of thousands of cubic feet per second we’ve seen at wetter times. The District of Columbia and its suburbs were in dire straits for drinking water. The Potomac below what was then a very primitive Blue Plains wastewater plant was pea green with thick algae blooms.
And sea nettles, which benefit from dry years with low sediment input and high salinity, were also discouragingly abundant. Watch for sea nettles in any dry year: more abundant, usually bigger, and much farther up the Bay and her rivers.
This drought, though more than 30 years ago and in a time with a much smaller population, created real water shortages. Planners, including the Interstate Commission on the Potomac River Basin, worked the next two decades to put in place water supply — and water allocation — agreements to tide over users during a future dry time. They also negotiated minimum flows in the river that were designed to assure a safe water supply for fish and invertebrates. The same scenario under a broad program by the U.S. Army Corps of Engineers, was repeated in other Bay tributaries.
Today, the watershed is adding about a million people a decade, and covering thousands of square miles with highways, subdivisions and mall parking lots. At the same time, the lessons of history — and in this case relatively recent history — are clear. We have destroyed at least half of the basin’s original wetlands, and despite regulations, we are still chipping away at the remainder. We are reducing forest cover and fragmenting surviving stands across the entire watershed. We’re paving over land areas, so the wet weather recharge of the underlying aquifers is blocked and the water is instead shunted off to exacerbate flooding in local streams. More and more wells penetrate down and suck unrelentingly at the still considerable underground reserves every year, in wet weather or dry. We have found that damming the Bay’s rivers damages the chances of success for river-spawning fish like shad and herrings. Old dams are being breached to let fish upstream rather than new ones being built.
It stands to reason that the next severe “millennium scale” drought to arrive will be harder to manage than its predecessors. We’re getting close to being overdue for one just about now.