View Full Version : Marshland
08-16-2008, 06:34 AM
For a DnD 3.5 campaign I am hoping to run I need a variety of terrain features, with mountains / hills both sides of the continent, but also a very large marsh at one end.
I assume to have a marsh form you need a slow running river to split up and stagnate, what I am not sure is what natural effects differ to decide if you end up with a river delta, a marsh or floodplanes?
Also does anybody know of any real life marsh land I can check out for inspiration?
08-16-2008, 10:08 AM
How about the Florida Everglades?
08-16-2008, 11:16 AM
For rivers to form marshy regions the obvious ones are distance from river source to river mouth, change in elevation from source to mouth or degree of flatlands near mouth. Type of vegetation. And age of river. Younger rivers tend to have more direction and force, whereas older rivers begin to meander in the flatland regions.
Marsh tends to be grassy and reedy, less trees in region. Trees exist in swamp regions, which help determine swamps from marshes.
The age of the river is one of the most important details to marsh formation.
08-16-2008, 01:46 PM
Wikipedia to rescue:
In geography, a marsh, or morass, is a type of wetland which is subject to frequent or continuous inundation. Typically a marsh features grasses, rushes, reeds, typhas, sedges, and other herbaceous plants (possibly with low-growing woody plants) in a context of shallow water. A marsh is different from a swamp, which has a greater proportion of open water surface, and is generally deeper than a marsh. In North America, the term swamp is used for wetland dominated by trees rather than grasses and low herbs.
The water of a marsh can be fresh, brackish or saline. Coastal marshes may be associated with estuaries and along waterways between coastal barrier islands and the inner coast. The estuarine marsh, or tidal marsh, is often based on soils consisting of sandy bottoms or bay muds. An example is the Tantramar Marsh of eastern Canada.
Below water decomposition processes often produce marsh gas, which may through self-ignition manifest as Will o' the wisps (aka. Jack-a-lanterns or spirites).
NEXT River Deltas:
A delta is a landform where the mouth of a river flows into an ocean, sea, estuary, lake or another river. A delta is formed only when a channel deposits sediment into another body of water. It builds up sediment outwards into the flat area which the river's flow encounters (as a deltaic deposit) transported by the water and set down as the currents slow. Deltaic deposits of larger, heavily-laden rivers are characterized by the main channel dividing amongst often substantial land masses into multiple streams known as distributaries. These divide and come together again to form a maze of active and inactive channels. This hydrogeologic formation is known as a delta. A delta can sometimes be misinterpreted as an alluvial fan. The two terms, however, are not interchangeable. A delta is formed in water and an alluvial fan occurs on land.
The deposit at the mouth of a river is usually triangular in shape and size. The triangular shape and the increased width at the base are due to blocking of the river mouth, with resulting continual formation of distributaries at angles to the original course. These distributaries start out flowing fairly fast, but slow as more sediment is deposited and ultimately, the water flows elsewhere. Change in the depositional process is directly related to a river or streams competency (the size of particles it can transport) and its capacity ( the amount of material it can move). This change in flow affects the particle size in the suspended and bed loads, the size of the particles decrease as the flow slows and the larger particles are deposited. This deposition goes on continually in a cyclic fashion, creating alternating sediment beds of coarse and fine grain deposits. As a channel deposits sediment in one location, it becomes more difficult for the channel to reach the body of water. As this occurs, the channel will change course to go by the way of steepest gradient. This happens continuously as the channel moves back and forth from the mouth of the river. As sediment is laid down in this fashion, the predictable fan-shape of the delta is formed. Herodotus the great historian used this term for the Nile river delta because the sediment deposit at its mouth had the shape of upper-case Greek letter Delta: Δ.
Where delta formation is river-dominated and less subject to tidal or wave action, more deposition occurs and a delta may take on a multi-lobed shape which resembles a bird's foot. An example of a river dominated delta is the Mississippi River delta. Another type of delta is a wave dominated delta. Deposition is still occurring on this landform, however, waves erode the outer edge of the structure, giving it an even more identifiable delta shape. An example of this is the Nile River delta. The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomset.The bottomset beds are created from the suspended sediment that settles out of the water as the river flows into the body of water and loses energy. The suspended load is carried out the furthest into the body of water than all other types of sediment creating a turbidite. These beds are laid down in horizontal layers and consist of smaller grains. The foreset beds in turn build over the bottomset beds as the main delta form advances. The foreset beds consist of the bed load that the river is moving along which consists of larger sediments that roll along the main channel. When it reaches the edge of the form, the bed load rolls over the edge, and builds up in steeply angled layers over the top of the bottomset beds. The angle of the outermost edge of the delta is created by the sediments angle of repose. As the forsets build outward (which make up the majority of the delta) they pile up and miniature landslides occur. This slope is created in this fashion as the bedload continues to be deposited and the delta moves outward. In cross section, one would see the foresets lying in angled, parallel bands, showing each stage of the creation of the delta. The topset beds in turn overlay the foresets, and are horizontal layers of smaller sediment size that form as the main channel of the river shifts elsewhere and the larger particles of the bed load no longer are deposited. As the channels move across the top of the delta, the suspended load settles out in horizontal beds over the top. The most famous delta is that of the Nile River, and it is this delta from which the term is derived, because the Nile delta has a very characteristic triangular shape, like the (upper-case) Greek letter delta (Δ). Ganges/Brahmaputra combination (this delta spans most of Bangladesh and West Bengal) is the world's largest delta, and empties into the Bay of Bengal. Other rivers with notable deltas include the Niger River, the Sacramento-San Joaquin, the Mississippi, the Rhine, the Rhône, the Danube, the Ebro, the Volga, the Lena, the Tigris-Euphrates, the Indus, the Krishna-Godavari, the Truckee, the Carson River, the Kaveri, the Ayeyarwady, and the Mekong.
A Gilbert delta is a specific type of delta that only occurs in fresh water and with coarse sediments. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. Other rivers, particularly those located on coasts with significant tidal range, do not form a delta but enter into the sea in the form of an estuary. Notable examples include the Saint Lawrence River and the Tagus estuary.
In rare cases the river delta is located inside a large valley and is called an inverted river delta. Sometimes a river will divide into multiple branches in an inland area, only to rejoin and continue to the sea; such an area is known as an inland delta, and often occur on former lake beds. The Niger Inland Delta is the most notable example. The Amazon has also an inland delta before the island of Marajo.
08-16-2008, 01:46 PM
A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. It includes the floodway, which consists of the stream channel and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current.
Floodplains are formed in two ways: by erosion; and by aggradation. An erosional floodplain is created as a stream cuts deeper into its channel and laterally into its banks. A stream with a steep gradient will tend to downcut faster than it causes lateral erosion, resulting in a deep, narrow channel with little or no floodplain at all. This is the case of entrenched rivers such as the Virgin River in Zion National Park in the U.S. state of Utah and the Colorado River in the Grand Canyon in the U.S. state of Arizona. As the stream approaches its base level, lateral erosion increases, creating an extremely broad floodplain, as in the case of the Platte River flowing across the Great Plains of the United States. There, the boundary between river and floodplain is not clear. In unmodified drainage systems where the terrain is fairly flat and rainfall intermittent, a floodplain may take the place of a river entirely. Instead of a defined streambed, there is simply a broad flat area where water flows from time to time.
An aggradational floodplain is created when a stream lays down thick layers of sediment. This happens when the stream's gradient becomes very slight and its velocity decreases, forcing it to drop sediment brought from higher regions nearer its source. Consequently the lower portion of the river valley becomes filled with alluvium. In times of flood, the rush of water in the high regions tears off and carries down a greater quantity of sediment resulting in planation (creation of a flat terrain) as well as aggradation. Thus, a stream such as the Laramie River in the U.S. state of Wyoming, widens its valley by working in meanders from side to side and covers the widened valley with sediment. Glacial drainage may also form an aggradational floodplain simply by filling up its valley with alluvium.
Aggradational floodplains are more common than erosional ones. Any obstruction across a river's course, such as a band of hard rock, may form a floodplain behind it. Indeed, anything that checks a river's course and causes it to drop its load will tend to form a floodplain. Aggradational floodplains are most commonly found near the mouths of large rivers, such as the Rhine, the Nile, the Ganges and the Mississippi, where there are occasional floods and the river usually carries a large amount of sediment. Natural levees form inside which the river usually flows, gradually raising its bed above the surrounding plain. Occasional breaches during floods cause the overloaded stream to spread in a great lake over the surrounding country, where the silt covers the ground in consequence.
Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream. These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water being drawn from them being pre-filtered compared to the water in the stream.
Geologically ancient floodplains are often represented in the landscape by stream terraces. These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream.
Sections of the Missouri River floodplain taken by the United States Geological Survey show a great variety of material of varying coarseness, the stream bed being scoured at one place, and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand or of fine silt, and it is probable that any section of such an alluvial plain would show deposits of a similar character.
The floodplain during its formation is marked by meandering or anastomotic streams, ox-bow lakes and bayous, marshes or stagnant pools, and is occasionally completely covered with water. When the drainage system has ceased to act or is entirely diverted for any reason, the floodplain may become a level area of great fertility, similar in appearance to the floor of an old lake. The floodplain differs, however, because it is not altogether flat. It has a gentle slope down-stream, and often, for a distance, from the side towards the center.
08-16-2008, 02:02 PM
There's also quite the variation between northern and southern marshes. Most of the D&D marshes are the more stereotypical southern swamps with gators and outlandish critters.
One thing that is often true of southern swamps is that they are very fertile. You may find that the big difference you can refer to is jungle and irregular rivers cutting through wide silt fields.
AEG has a good d20 book on wilderness called "Wilds".
There is also a growing homebrew thread on swamps, again for D20, at Giant In the Playground.
08-16-2008, 03:51 PM
From reading those wiki articles it doesn't seem to be so much about the land type the feature is in, but more whats upsteam. If I have the rivers coming from lands less likely to give silt deposits (hills and grasslands rather than plains or deserts) it seems it should form a swamp/marsh
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