Environmental Commentary by Tom Horton
Its common knowledge the healthy Chesapeake Bay described by John Smith in 1608 was greener, its forest extending across more than 90 percent of its six-state watershed.
Less appreciated is how much soggier, boggier, swampier, wetter was that good, green watershed. Beavers that likely numbered in the low millions controlled the hydrology of a hundred thousand streamsdamming, impounding, essaying upon the landscape a rich and damp mosaic .
The term Great Shellfish Bay, the American Indians name for the Chesapeake, might be amended to include Great Beaver Bay if we acknowledge the intimate connections between water and watershed.
The wetter, better-beavered landscape checked sediment; converted nitrogenous runoff that plagues todays water quality into harmless gas, damped the peaks of floods and recharged the bays rivers during droughts. It created a cleaner, clearer, stabler place for life.
Bevies of beavers might be the cheapest way to a better bay; but legions of tree-felling, land-flooding rodents would conflict too heartily with human schemes.
But beaver mimicry is another matter, restoring wetness every way and everywhere we can from rain gardens throughout suburbia, to strategically placed, engineered wetlands, to retention dams in farm and roadside drainage ditches.
Current efforts just scratch the surface of what is possible. Low-lying, poorly drained, heavily ditched farmland everywhere should be a prime target for purchase and reversion to the wild and wet. Retrofitting every paved surface to let stormwater soak in would advance the cause further.
We neednt achieve the watershed of John Smith to see results. The bay showed substantial resilience in the face of increasing human pressures for centuries, truly falling fatally apart only in the 1950s.
All these vital and hopeful links between the once and future bay are detailed wonderfully in a unique library in the bays bottom opened only recently by scientists like Grace S. Brush of Johns Hopkins University.
Her 2009 paper, Historical Land Use, Nitrogen and Coastal Eutrophication: A Paleoecological Perspective (Estuaries and Coasts), should be required reading for anyone at any level teaching about Chesapeake Bay (or about beavers).
Essentially, Brush figured how to read" the bays land use history going back millennia, and how changes in the watershed translated into changes in the water.
She did this by extracting yards-long cores of sediment from across the bays bottom. Then, bottom to top, ancient past to present day, she analyzed the seeds and pollens and other data deposited there.
At a level deep in the sediments washed from the watershed around A.D. 1000, for example, charcoal from more frequent fires corresponds to a centuries-long medieval warming period. She can tell when forest clearing for European agriculture began as pollen from the ragweed of open fields increases; and when the bays underwater grass pollens dived sharply in the 1970s after centuries of relative abundance.
This last evidence was critical in disabusing the false hope of some government officials that the seagrass decline was just a natural cycle.
Volumes from the library, summarized in Brushs paper, show how forests began to take a serious hit around the middle 1800s as a growing human population, the invention of the steel plow and markets for timber led to clearing of all but about 20 percent of the watershed by the early 1900s in many parts of the watershed.
The watershed since then actually re-greened to nearly 60 percent forest as people moved into towns and cities, and farming became able to produce more food on fewer acres. But it was a different sort of green, significantly less wet.
Trapping largely abolished the beavers and their dams by the mid-1750s. Indeed, the sediment cores show a small but significant decrease in nitrogen runoff, a trend that would not turn up until human sewage and commercial fertilizers came on strong in the 1900s. Brush suspects the decline may reflect a decline in beaver poop.
In addition, drainage ditching for farms, roads and development further dried out the landscape. Well-drained is not so good as it sounds. And increased paving made rain run off hurriedly, no time to soak in.
So it was a different kind of green that returned. Brushs sediment cores show a big decline in pollens from wetland vegetation and moisture-loving plants in general.
The sediment library documents a most telling shift occurred rapidly in mid-20th century, as human inputs of nitrogen and other pollutants soared. The dominant algae in the bay shifted dramatically from bottom-dwelling types, to surface-floating algae.
The estuary was beginning to flip, to lose its bottom its oysters, its plankton, and by the 1970s its vital seagrass meadows. The capsize of an incredible ecosystem was complete.
It wasnt just the loss of wetness. Overharvesting, oyster diseases, intensive agriculture, sewage and air pollution were all culprits.
Still, theres a future for the Chesapeake in its past, as remembered in its sediments, and its wet.
Tom Horton has written extensively about the Chesapeake. Distributed by Bay Journal News Service.