Episode 32
Copyright © John Page Williams, Jr. all rights reserved.
This is John Page Williams with another reading from Chesapeake Almanac. The month is October and this chapter is entitled "River Meanders."
Route 50 east from Annapolis carries hundreds of thousands of people each year across the Bay Bridge and down the Eastern Shore to Salisbury, Ocean City, Assateague, and Chincoteague islands. For years, heavy traffic has been the rule, and even though the highways have been streamlined, there are still bad backups.
One of the backups that has disappeared, however, was 15 miles south of Cambridge, where Route 50 constricted from four to two lanes for an old steel bridge over a narrow river with a small town beside it. Many people sitting in the traffic jams had to consult their maps to see what the town was--Vienna, Maryland--and to look at the signs on the bridge to discover what river they were crossing--that would be the Nanticoke.
The backups are mostly gone now, in summer as well as at other seasons, because the root to the ocean has been streamlined over a new bridge. New interchanges and bridges speed travelers on their way and most of them cross the divided four-lane bridge high over the Nanticoke without a thought. In August, a few might notice the white-and-crimson wildflowers--that would be marsh hibiscus--in bloom in the big marsh on the other side of the bridge. And in October, they might admire the warm colors from tickseed sunflowers in the marsh and the red maple, ash, sweet gum, and black gum trees at the back edges of the marsh.
But not many look at the pattern of the town and the marsh, or see what it indicates about the way the river works. They have places to go, so it's hard for most of them to get excited about the physics of river flow, even if it has profound effects on both human and natural history. Nevertheless, the river does have a good story to tell.
Vienna is an old town built on the outside of a sweeping curve called a meander. This one is part of a long series on the Nanticoke, stretching 20 river miles from the Delaware line down to Sandy Hill. Every long river entering the Bay has meanders. Obviously there's a good physical reason for their existence.
Think of the mass of water flowing in a river, even a relatively small one like the Nanticoke in Vienna. The bed is about 200 feet across and the channel is just over 40 feet deep at maximum. The bend is a little over a mile long. Water weighs 62.5 pounds per cubic foot, plus the weight of anything dissolved (like salt, of which there isn't much of Vienna) or suspended (like sediment, usually a good bit at Vienna). The weight of that section of river is considerable, and it is, of course, connected to the rest of the water in the river.
This great mass of fluid moves back and forth in response to downstream current, driven by rainfall and by tides, but it has tremendous inertia, and therefore tremendous power. Strange as it sounds, a river "works" as it flows downhill, losing energy to friction between its water and its banks. It will make any changes in elevation as gradual as possible, to make the energy loss as uniform as possible. And it will remove or at least wear down obstructions that retard its flow.
The tidal portions of the Bay's rivers flow through the unconsolidated, erodible gravel, sand, clay, and mud of the coastal plain. In these media, it's relatively easy for our river to carve out a shape that fits its needs. For these rivers, that shape is a series of meanders. Like a skier taking a curving path down the face of a slope, the meanders allow these rivers to smooth out abrupt changes in elevation. These achievements are especially important as each river gathers more mass from downstream tributaries and from the water the tides bring up from the Bay. The region of heavy meandering tends to be not at the head of navigation, but in the middle tidal reaches.
But curves cause friction as a river collides with its banks. The inertia of the great water masses will resist turning. To minimize the friction, the meanders tend to assume specific curve shapes that cause the least total change in direction through the whole process of bending. The curves with these geometric properties are called sine-generated curves and are similar to half-circles.
So what do these remarkable curves have to do with live creatures of the Chesapeake--people and fish and birds and marsh plants? Look at a map or chart of a river with meanders. The insides of the curves will frequently be marked to signify marshes. Often there will be a town like Vienna on the outside of a curve.
Here's the reason for the pattern. As the river flows around the curve--in either direction, depending on the tides--the water on the outside and much of the surface will accelerate, both sideways and ahead. The water on the inside will slow down, just like runners on a track. The result is a slow but powerful spiral motion that will erode the outer bank and deposit sediment on the inside. Thus the outsides of the curves are usually deep, with firm banks of heavy material like sand and gravel. The insides build shallow, fertile mud banks.
In selecting town sites, both Native Americans and European settlers looked for the outsides of curves, with firm ground for building and deep water for docking. The latter attribute doesn't mean much today, but for most of the past three and a half centuries, it was critical. Look at a map of the old steamboat ports: Walkerton on the Mattoponi, or Aylett on the Mattoponi, Dixie on the Piankatank, Tappahannock and Leedstown on the Rappahannock, Nottingham on the Patuxent, Deep Landing on the Chester, Ganey's Wharf on the Choptank, Rehoboth on the Pocomoke, and, of course, Vienna on the Nanticoke. All are on the outsides of meanders, and there are at least as many more. Vienna, by the way, was also an important shipbuilding town. Several Bay schooners were built there, on ways that launched the ships right down into the deep water on the curve.
The insides of the meanders, meanwhile, provide superb wildlife habitat in the form of fresh and brackish tidal marshes. The big marsh opposite Vienna is full of muskrats, juvenile and forage fish, herons, grass shrimp, and a host of other creatures. The river there has been an important nursery area for young rockfish, and it has its share of big rock as well. Eagles fish it, and a shrewd angler can find a good meal of white perch and catfish. Detritus from the marsh fuels much of the lower river's food web. At this season of the year, its harvest of seed-producing plants like wild rice and smartweed is measured in tons, and its value to waterfowl arriving for the winter would be difficult to calculate.
Shipways, striped bass, marsh hibiscus, and wild rice have all owed their existence in Vienna to that big meander. Slow down and ponder that fact the next time you cross the new bridge.
For more happenings around the Bay in October see our other Chesapeake Almanac podcasts and read our blog post "Leaf Peeping from the Water in October."
