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Coastal Plain
The Delmarva Peninsula is a good example of a Coastal Plain province, being an extensive, low-relief area that is bounded by the Atlantic Ocean to the east and by the Piedmont, a relatively high-relief province to the west (the Refuge is entirely in the Coastal Plain). The geologic province of the Coastal Plain actually extends beyond the shoreline across the Continental Shelf. It is only during times of glacial melting and high sea level that much of the Coastal Plain is drowned. The Coastal Plain along the Atlantic and Gulf coasts of the United States is among the largest, being hundreds of miles wide, and oldest, at 100 million years, in the world. Coastal plain strata have been a source of considerable oil and gas as well as various economic minerals. Although the coastal plain province is typically stable tectonically, there may be numerous normal faults and salt dome intrusions. (Image courtesy of University of DE: USGS) |
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Piedmont
Only the northernmost portion of New Castle County encompasses a section of the Piedmont province, an area near the foot of a mountain or plateau (the Piedmont) that extends from New York to Alabama, east of the Appalachian Mountains and west of the Atlantic Coastal Plain. In Delaware, the Piedmont begins somewhat north of the I-95 corridor. The plateau is cut by numerous small rivers, whose fall line is along the eastern edge of the plateau as can be seen on the Brandywine Creek in Wilmington. “Piedmont” is French for “foothills”. The region is characterized by a temperate climate, even rainfall year round, and a topography of low rolling, unglaciated hills. (Image courtesy of the University of DE: USG)
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Intertidal Zone
The intertidal or littoral zone is the area of the marsh or river that is exposed to the air at low tide and is underwater at high tide (area between tide marks). On the Christina River the tidal range averages 5 feet and the intertidal zone is characterized by unvegetated mudflat just above the low water mark, transitioning to emergent plants like spatterdock, arrow arum, wild rice, cattail, bulrush, rice cutgrass, smartweed, Phragmites, and finally purple loosestrife near the high tide mark. Organisms in the intertidal zone are adapted to an environment of harsh extremes. Wave action can dislodge the organisms and sun exposure can create a climate ranging from very hot to well below freezing in addition to the stress of desiccation. These edge habitats are often significant ecologies that support a diverse range of organisms. (Image courtesy of Bob Meadows, DNREC) |
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Freshwater Marsh
The Christina River is located 65 miles up Delaware Bay; far enough away from the ocean to receive no salt and function totally as a freshwater tidal system. The twice daily tide brings nutrients and sediment from the estuary and river into these marshes, making them productive places for plants to grow. In the absence of salt, a more diverse assemblage of plants species can thrive, more than is seen in salt marshes found further down bay. These rich plant communities also support a diverse range of bird species that use the marshes for feeding, nesting, and breeding, more than any other wetland type. Estuarine and marine fish species also utilize these wetlands as critical nursery habitat during their early life stages. (Image courtesy of Bob Meadows, DNREC) |
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Uplands
In a topographic sense, “uplands” are generally considered to be lands that are at a higher elevation than the adjacent land, such as a stream or floodplain. Uplands rarely flood for long and typically are the place in the landscape where we build our homes and businesses and where agricultural crops are grown. The water table (sub-surface water level) in upland areas is usually located a couple feet below the surface and is an area where upland adapted plants like chicory, black-eyed Susan, evening primrose, and sugar maple are commonly seen growing. Undisturbed uplands are among our most threatened habitats in Delaware due to limited regulatory protection or preservation programs. (Image courtesy of Bob Meadows, DNREC) |
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Ditch
Ditching in combination with diking was a common practice used in the Old World to drain wet areas and make them suitable for farming as pasture or crop lands. These practices were brought to the Delaware Estuary by the early Dutch and English settlers and initiated the wide spread loss or degradation of many of our pristine wetland habitats. By the mid-1800’s, all the tidal marshes along the Christina River had been diked, ditched, and drained for agriculture. These marshes had been some of the richest and most diverse habitats in the region for plants, waterfowl, anadromous fish, among many other wildlife species. Ditching was also used to channelize or shorten the length of many rivers in the state. The largest example of a ditch in Delaware is the C & D Canal. (Image courtesy of Bob Meadows, DNREC) |
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Channel
If you ever look at an aerial photograph of a natural, unaltered stream course that flows through relatively flat country like we have on the Delmarva Peninsula, you rarely see a straight line of any appreciable length. All streams naturally cut a very sinuous channel that moves around on the landscape over time. Flowing water naturally cuts or erodes material from the outside edge of the channel, where the current is strongest, and redeposits this material on the inside portion of the bend, where the water velocity is lowest. This continual redistribution of sediments along the channel also moves where the marsh plants and animals occur and tends to rejuvenate the habitat. (Image courtesy of Bob Meadows, DNREC) |
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Sediment and sedimentation
Sediment, the small organic or inorganic particles that are transported by the tide throughout the marsh is the foundation of this habitat. In addition to its role in the movement of the stream bed as discussed in the “Channel” profile, sedimentation plays a crucial role in building the marsh up in times of sea level rise. On each high tide, tidal water overtops and floods the marsh plain, depositing a minute quantity of sediment on the marsh surface. Next time you’re on the boardwalk in front of the Center, you will see evidence of this sediment as revealed by the grayish white film on the leaves of the marsh plants. Over the course of 10 years, this process will raise the marsh elevation by more than 1 inch, over 100 years, nearly a foot. Until recently, this rate has kept pace with sea level rise and our marshes have prospered. That may not be the case by the end of this century. (Image courtesy of Bob Meadows, DNREC) |
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Meander Channel
Stream channels naturally move or meander across the marsh landscape as described in the “Channel” profile, elsewhere on this display. As the channel continues this naturally curving and bending process over time, the folding becomes so severe that it eventually pinches off a meander bend when the neck at the base of the bend narrows to the point the current breaks through. The stream, following the path of least resistance, establishes a new channel through this created breach that totally bypasses the old bend. This abandoned bend is referred to as an oxbow (a reference to the shape of an ox yoke) and if it is large enough, it may persist for many years as an isolated oxbow lake, but that is not typical of the smaller stream systems in Delaware. (Image courtesy of Bob Meadows, DNREC) |
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Fall Line
In Delaware, the fall line marks the transition from the eastern Atlantic Coastal Plain and the western Appalachian foothills or Piedmont. This line also marks the limit of tidal influence on a stream which is best seen on Brandywine Creek in Wilmington, just downstream of the Market Street Bridge. On the Christina River, this point today is reached at Smalley’s Dam, just upstream from the town of Christiana, which historically was the “head of tide” and last navigable port above Wilmington. The term fall line was derived from the tendency of actual water falls to form at this point along the stream due to the transition from the hard rock substrate of the Piedmont to the more erodible strata of the Coastal Plain. (Image courtesy of Bob Meadows, DNREC) |
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Confluence
From the air, the network of water channels in a tidal marsh resembles a tree with its many branches. Starting at the top with the smallest twig, they steadily merge as you move downward, forming larger and larger branches and limbs until the robust trunk is reached. In the farthest reaches of a marsh, small trickles of water convey the estuary’s life-sustaining cargo of sediment, plankton, oxygen, and water to the limited tide at the upland edge. As these small stream channels merge into larger channels, their depth and width increase accordingly to accommodate the added flow, all the time increasing in velocity and strength. This increasing power suspends and carries more sediment that allows the channel to meander across the marsh plain. Many estuarine organisms ride the tide in and out of the marsh to spawn, feed, and seek refuge. (Image courtesy of Bob Meadows, DNREC) |
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