I've looked at Capt. John Smith's original map of Chesapeake Bay a hundred times, searching for clues to what the Bay was like in 1608. I can imagine him, with the armful of notebooks he must have kept, bending over the work in progress with his engraver, William Hole, putting those clues in place for the map's future users. There are hundreds of such clues, although it is likely we will never decipher all of them.

At the head of the Bay, a sketch of his shallop sports a spritsail rig. It has been the logo of "Past is Prologue" since 1997. Ahead of the boat is the stippling used by Hole to indicate water changes density, as if to indicate a lump in the bottom, just where the Susquehanna and Northeast rivers bifurcate.

For a long time, I believed this feature represented shallow water and the wide Susquehanna Flats. There is similar shading, though, along much of the Chesapeake shoreline, mostly along the Eastern Shore, as well as both sides down to the mouth of the James River. These marks do not seem to solely represent shallows on the Bay's edges because they do not extend all the way to the shore. Could they be indicating grass beds? These areas would be plausible habitat in a vital Chesapeake Bay.

The shallop and her crew sailed or rowed back and forth several times over the shallows at the head of the Bay, but Smith was mostly concerned with the geopolitical questions of negotiating peace among Native American tribes. Could this be the reason he makes no mention of these flats in his writings?

He makes no mention of any of the Bay's supposedly vast underwater meadows of seagrasses. There is only the indirect reference to "reeds" in his stirring Rappahannock River stingray account. (See "Past is Prologue," November 1998.) But Smith's observations were made long before the ecological significance of the Bay's grasses were known.

We now look for these references to validate our assumption that abundant submerged aquatic vegetation is associated with a Bay we imagine as "pristine." We search in vain until 1832, when a character in James Hungerford's Patuxent novel, "The Old Plantation," emotes about the underwater grasses, visible through clear water with fish swimming amid them.

There are stirring accounts of waterfowl, which European settlers hunted and ate. Dutch Missionary / explorers Jasper Danckaert (or Dankers) and Peter Sluyters, around 1660, wrote: "I have nowhere seen so many ducks together as were in the creek in front of this house. The water was so black with them that it seemed, when you looked up from the land below upon the water, as if it were a mass of filth or turf, and when they flew up there was a rushing and vibration of the air like a great storm coming through the trees and even like the rumbling of distant thunder, while the sky over the whole creek was filled with them like a cloud…and it is not particular to this place alone, but it occurred on all the creeks and rivers we crossed."

There is also this flowery bit from George Alsop, who by 1666, had spent four years in Baltimore County: "The Swans, the Geese and Ducks (with other water fowl) derogate in this point of settled residence; for they arrive in millionous multitudes in Mary-Land about the middle of September, and take their winged farewell about the midst of March."

Such abundance must have drawn upon the food source of tremendous underwater grass beds, the abundant mass of vegetable material which had grown all spring and summer without the pressure of millions of grazing birds. These migrating hordes would have arrived to an incredible, nutritious feast of vast grass beds, dominated (it is believed) by wild celery and redhead grass.

The Susquehanna Flats were a stunning example of this fruitfulness. As Americans began to aggressively harvest natural resources to fuel growth, the Upper Bay received particular focus. Immense nets were deployed to capture tons of migratory shads and herrings, and both recreational and market hunters began shooting numbers of migrating birds.

In the 1870s, market hunting had escalated to unsustainable levels. It was a time of stunningly large harvests with barrels of birds leaving by rail for markets in New York and Philadelphia. Hunting, coupled with habitat loss in breeding and wintering regions, contributed to long-term declines in migratory waterfowl populations.

Around 100 years later, floodwaters from 1972's Tropical Storm Agnes severely damaged the large grass beds necessary for ducks and many other species. Sediment silted over young plants and blocked sunshine from reaching these light-dependent meadows.

Nowhere was this more dramatic than in the Upper Chesapeake, where the water was colored opaque cocoa for many weeks, exactly when its already struggling underwater grasses were in the midst of their highest growth rates and ready to produce seed for the next round of reproduction. On the flats, and Baywide, the grass meadows disappeared.

The great, shallow and grass-covered flats figured in Chesapeake history. They lay near the head of navigation for all Chesapeake Bay. The Susquehanna went on hundreds of miles into modern New York state and thus served as a colonial and early U.S. highway into thousands of square miles of forested and agriculturally productive land. Four other navigable rivers of modest size reached into the landscape surrounding the head of the Chesapeake.

At this nexus was the small community of Harmer's Town, which saw significant military traffic and the requisitioning of supplies during the American Revolution. Reflecting comments attributed to the Marquis de Lafayette, town fathers incorporated as a city in 1785 under the name Havre de Grace, or Harbor of Grace. With the trade potential and fairly deep channels ringing the shorelines to accommodate shipping, Havre de Grace had pretensions to rival Baltimore.

The head of the Chesapeake was surveyed and in 1799 an excellent bathymetric and terrestrial map drawn by C.P. Hauducoeur was published. This served as a pilot chart for the merchant vessels coming to take cargos from this rich basin. But the map also highlighted the immense shoals or flats lying amidst all these peripheral channels, so the potential for a real harbor, where vessels could anchor while awaiting cargo, in queue for wharfage or seek shelter from weather simply wasn't there.

Hauducoeur's chart opens another comparison with modern, well-surveyed and accurate charts by the National Oceanographic and Atmospheric Agency, and its predecessor agencies.

By inspection, I am convinced that the flats have increased in extent. While the Chesapeake and Delaware Canal approaches up the neighboring Elk River are periodically "maintenance dredged" for shipping to and from the Delaware, the peripheral channels around the flats have generally decreased in depth. There is a dearth of soundings across the flats on the early charts, though, so a rigorous comparison is not easy.

While our knowledge of the flats is extremely dim until Hauducoeur's work, there is one more facet in their history that can be plumbed:

The location of modern Havre de Grace was nearly 200 miles from any hint of the sea during Earth's most recent glacial epoch, 22,000 to 19,000 years ago. As the world's continental ice sheets melted, the sea level rose. Geologist Peter Vogt wrote that "tidewater, fresh at first, reached the fall line of rivers like the Susquehanna about 8,000 years ago."

Vogt says that the sediment delta at the river mouth began to accumulate very early. Vogt, along with Tom Cronin of the U.S. Geological Survey and Jeff Halka of Maryland's Department of Natural Resources, examined data from old sediment cores (taken perhaps for bridge construction) and estimated that the pile of sediment on the flats is now about 131 feet thick above the glacial period's channel.Â

Vogt says that the rate of sediment accumulation is sure to have accelerated during the period after the European settlement. In fact parts of Pennsylvania reported the loss of all of their topsoil within 25 years of commencing agriculture, and the research of Dorothy Merritt and Bob Walters at Franklin and Marshall College in Lancaster, PA, documents vast sediment translocation from plow-based grain agriculture in Lancaster and York counties in the 18th and 19th centuries.

Vogt explains that "river plumes debouching onto tidal flats have to grow in two dimensions (They remain shallow because constant wind and waves keep sediment in suspension and moving toward the edges.) until deeper waters are reached on the distal prograding slope (outer edges) of the flats."

This is consistent with changes I have noted in the size of the flats from the 17th to 20th centuries, as well as sediments slumping off into the once deeper navigation channels north of and to either side of the flats.

It remains for someone better than I to quantify this and perhaps estimate the millions of tons of sediment that have accumulated here post-colonization, or even post-Tropical Storm Agnes.

Agnes and the constant flux of nutrients and sediment from human activity wreaked havoc with the Bay's submerged grass beds, and consequently interrupted or stopped many of the nursery and ecosystem functions they provided.

But the Bay is resilient and with help from state and federal restoration efforts, modest spring freshets and some dry years, grasses returned to the flats.

Next month: A recent visit to the Susquehanna Flats.