The next three landform categories have in common an origin that involves running water. Drainage patterns established by rivers/streams show considerable variety. One common type is dendritic. The influence of stream erosion and associated slope wash can determine the shapes and heights of the intervening land, whose character ranges from mountains to plains. Streams form deltas and alluvial fans when they drop their particulate loads on reaching stiller water. Marine waters produce a variety of shorelines whose outlines are partly controlled by the nature and elevation of the land affected by coastal erosion.

### Fluvial/Deltaic/Coastal Landforms

Switching to fluvial landforms, of which there are many varieties, we show first an archetype of dendritic drainage and then below, a part of a Landsat mosaic, in which the dendritic drainage has highly dissected the topography in flat-lying rocks.

The South Yemen Landsat scene above extends over part of the Hadhramut Plateau, an uplifted section of Tertiary limestones and shales, which experienced folding into a broad syncline (center) and two anticlines (top and bottom of the image, but not visibly evident). Typical dendritic drainage develops as the Wadi al Masilah, which, along with its tributaries, is a sometimes ephemeral stream (its valley is sand-filled) that obtains flow mostly after large storms but maintains enough water to support local farming. Much of this drainage likely developed during a wet period before the regional climate shifted to its present arid state. Note the typical headwaters pattern for dendritic drainage in the system in the upper right. The drainage pattern near the coast has become trellis-like.

17-7: The term "dendritic" was introduced above, but not defined. Using the image as a clue, what do you think this word means, in a geomorphic sense? ANSWER

The above subscene (about 100 km [62 mi] wide) is in West Virginia, next to the Kentucky border. The Ohio River flows just to the north of the image top. The area is part of the Appalachian Plateau, an uplifted sector of the crust involved in the Appalachian orogeny that did not experience folding, so that the rocks remain horizontal. The drainage has reached a level referred to as mature, in which a distinctive high-density dissection has reduced divides to sharp ridges, with little of the earlier uplands remaining, and narrow valleys. Although this pattern leads to maximum relief, the differences in elevation are seldom more than about 200 meters (656 ft).

Another example of dendritic drainage, this time imaged by the SIR-A radar, is this scene in east-central Columbia. The region is one of tall grasslands and forests.

A fourth example is also in South America. This STRM topographic image shows rivers with conspicuous tributaries. The area covered lies in western Brazil where it meets parts of Bolivia and Peru. Note the strong expression of the Trans-Amazon Highway (I interpret it to be made of concrete). There is a small impact crater seen in the lower left.

Mature river systems, with wide, flat floodplains and not too high above sealevel, tend to develop laterally shifting river channels over extended time periods. This is known as meandering. One result is abandonment of channel segments forming cutoff segments called oxbow lakes. This is strikingly illustrated in the floodplain of the lower Mississippi River, seen below. The floodplain edge is marked by bluffs in the state of Mississippi, on the right, with forests covering the uplands. Many oxbow lakes, some crescent-shaped, are evident in the floodplain. The silt-rich (blue) river west of the Mississippi is the Arkansas River.

Many rivers show almost no significant changes in channel configuration (mainly by meandering) in a human lifetime but some undergo large shifts on a time scale ranging from a few years to several decades. This latter case is illustrated by the Mamore River in Bolivia northeast of the Andes. The top illustration is an astronaut photograph taken in June, 2003 from the International Space Station (ISS); the bottom is a Landsat-7 image of the same scene in 1990. The red line in the top photo marks the 1990 centerline of the river's flow superimposed on the present course. One can readily note the new meanders, several small oxbow lakes, and the disappearance of several lakes.

When we discuss fluvial landforms, we should remember that two aspects must be considered: first, as shown above, the drainage patterns. But, the nature and shape of the land between streams is governed (at least in part) by the combination of stream action and slope adjustments. We saw in Section 2 (thrust belts in Greece) that different tectonic units - in terms both of the rock types involved and the structural styles - will control the expression of the individual mountains or groups thereof in their gross landform character. Here is another example: in the High Cordillera of the Andes Mountains, in this Landsat image you should be able to differentiate four tectonic zones whose individuality owes much to the response of their rock types to the effects of stream erosion at high elevations:

The story is different where flat-lying sedimentary rocks are involved. In 1967, M.L. King of South Africa proposed multiple planation cycles, based on his studies in southwestern Africa. These denudation surfaces occur in steps at different elevations and represent remnants of stream-eroded landscapes (similar to the peneplain concept) formed at different times. Dr. King would have been excited to see this next Landsat image, which shows in a single image four of the planation levels he proposed (the one along the coast is cut into an ancient granite surface):

In mountainous, often semi-arid terrain, erosion cuts away the uplands and deposits the debris in the lowlands, as streams flow over pediments and fill the basin. This ASTER view of part of the Andes in Chile, a narrow chain of mountains made up of Cretaceous sedimentary rocks, shows the Altiplano that has received both sediment waste and pyroclastic fallout from volcanoes. Note how numerous streams seem to start at the contact between basin and mountain (this is a modern example of an unconformity) but actually are a continuation of uplands drainage that stands out especially in the white band facing left (a dissected pediplane).

Streams laden with sediment can produce distinctive deposits in their channels and onto adjacent floodplains when conditions force deposition of their load. The continuing flow of water may thus be broken up into a network of intersecting branches producting a pattern known as a braided stream. This is well illustrated by this Eosat image of a mountain valley in Tibet, east of Lhasa, through which flows the Brahmaputra River:

This river flows south from the Himalayas through Bangladesh into the Bay of Bengal. It retains this braided appearance over most of its length, when seen in the dry season (it floods during the monsoon season), as in this Landsat image:

One landform associated directly with the river that makes it is a waterfall. This is hard to detect from space owing to its verticality. Nevertheless, water in the stream may spread out the channel just before it pitches over the drop - Victoria Falls in Africa is a good example. At the U.S-Canada border, the Niagara River, coming from Lake Erie and emptying in Lake Ontario, behaves similarly as it plunges over the regional Niagara Escarpment. Here is an Aster image of this popular tourist attraction:

Another spectacular water cascade is the famed Victoria Falls in Zambia in central Africa. It extends for 1700 meters (more than a mile as water from the Zambezi River flows over a plateau in a broad channel (vegetation-covered) until it drops precipitously (113 m [373 feet] on average) into a savannah landscape below. Here are two views in a photo taken by an astronaut from the International Space Station.

Victoria Falls is one of the 7 Natural Wonders of the World. It is an impressive sight as seen from an airplane:

Most primary rivers end up entering large bodies of "standing" water. There, a delta forms at the mouth of a stream where the load of sediment carried by running water is dumped as the stream empties into less mobile water (lake or ocean). In Section 4, we showed one example of a river delta: the bird's foot delta of the Mississippi River as it builds into the Gulf of Mexico in Louisiana, south of New Orleans. This ASTER image shows the tip of the delta in which several distributaries have built up deposits above sea level, giving the bird's foot effect:

As seen by Landsat, this enhanced image brings out more details in the sediments:

Here are three more classic examples:

The above Landsat image shows about half (the western part) of the Ganges Delta in Bangladesh (the India border is near the left edge, and Calcutta is at the center of the left edge). This is the world's largest delta, being more than 200 km (124 mi) in straight distance along the Bay of Bengal. The delta results from deposition of heavily silt-laden waters of the Ganges and the Brahmaputra Rivers, transporting sediment from the Himalayas far to the north. This next scene covers a larger area. The image has been reprocessed to convert the RGB color co-ordinates to the Intensity, Hue, Saturation (IHS) system of color expression:

The present-day Ganges drains southward just off the image to the right. That segment is now the active delta region. In both scenes above, we see the so-called abandoned delta which formed in the past when the Ganges flowed in various positions and shifted gradually eastward. Other rivers still flow into the Bay adding somewhat to the delta, as seen in the light blue sediment flowing into marine waters. These quasi-distributaries become tidal channels that tidal currents highly influence. The dark red tones along the coast are mangrove forests and swamps. In the upper left quadrant, the area is part of the depauperate delta, where the clay soils now support sporadic agriculture. People seeking farmland removed much of the forest that was once there. The entire region, especially the low flat areas near the coast, is vulnerable to frequent cyclones (hurricanes) that cause widespread damage and loss of life, because of high winds and tidal surges.

Because there is sufficient water available from a river entering the ocean to support vegetation even in an arid climate, tropical forests can develop in such areas. This is the case where the Gambia river has built a delta that does not extend much into the Atlantic in southern Senegal (African West Coast), as imaged in near natural color by the MERIS sensor on ESA's Envisat.

This next Landsat MSS Band 7 (IR) image covers the central west coast of Alaska. Here the Yukon River flows into Norton Sound on the northeastern Bering Sea.

The main branch now carries sediment to the south end of a large semi-circular delta. It is actively extending the delta but an offshoot tributary is doing much of the deposition in the central part. This present delta is young (perhaps only a few thousand years since its start) that began with a major shift of the Yukon from a location not in this scene. Within and inland from the delta are numerous small lakes of ice origin. Along the coast at the bottom of the image are linear bands, which are beach ridges, developed when sea level was higher.

17-8: What is/are the main difference(s) between the Ganges and Yukon deltas? ANSWER

Similar to the Yukon Delta is the Lena Delta in eastern Siberia formed where the Lena River empties into the Laptav Sea north of the Arctic Circle.

Rivers can produce land "deltas", widespread deposits that build up as streams carrying heavy loads down steep gradients then encounter flatlands, with low gradients that cause the load to drop and spread out as the system meanders. These are called alluvial fans. One of the biggest in the world occurs in the Badain Jaran desert of east Asia, where the Ruo Shui River drains north from the Nan Shan mountains:

The next 3 coastal-landform examples display special types that also fall into two general categories: coastlines of submergence and emergence (although these terms are held to be obsolete and misleading by some geomorphologists).

The above scene lies in the Coastal Ranges along the Pacific Ocean in the region where the Alaskan Panhandle extends along Canada (near the top of the image). Juneau, Alaska's capital, is near the center. The region is tectonically active, with major faults separating individual crustal units known as terranes (see below). These faults and other structural features served as lines of weakness for erosional attack by streams and glaciers, which together carved out deep valleys. Some present day glaciers are visible in the Glacier Bay National Park area northeast of Juneau and elsewhere. After the close of the last major glaciation, melting glaciers are now in retreat to the extent that, as sea level has been rising, the ocean flowed into some large valleys cut earlier to below sea level by the ice, effectively drowning them. The resulting landform is a fjord–a Norwegian name assigned to submerged coastal valleys once occupied by ice.

17-9: How do you think the scenery shown in the above image has strongly influenced the economy of the region? ANSWER

Fjords abound along the coastline of Iceland. Here is a peninsula in the northwest that shows glacially-sculpted valleys now submerged as sealevel rose following the last Pleistocene glaciation.

Another example of a rugged, embayed coastline, shaped in part of drainage of higher land, and now influenced by the rise of sea level, is that of the West Falkland Island in the South Atlantic. A bit of the East Falkland Island is at the right edge of this Landsat subscene.

The Atlantic seaboard, seen below, is generally now a coast of emergence associated with regional uplift. Over the past 50 million years or so, seas have lapped well onto the eastern North American continent, laying down thick, subhorizontal sedimentary layers, but the ocean has been gradually retreating eastward. In the last few thousand years, a rise in sea level, resulting from glacial ice melt, has reversed this trend as marine waters drown coastal valleys (e.g., Chesapeake Bay) and push shorelines inland. Along much of the Atlantic coast from New Jersey to Florida, thin narrow lines of sand deposits, built up above sea level by deposits from ocean waters encroaching on shallow bottom slopes, form barrier islands. The image is a photo taken by an Apollo 9 astronaut of the famed Outer Banks of North Carolina. The point farthest east is Cape Hatteras, and the southern point is Cape Lookout. The wide stretch of water towards the mainland is Pamlico Sound, which, with Albemarle Sound inland to the north, we term a lagoon. Offshore, submerged sandbars–incipient islands–form hazards to shipping. Because of the irregular, cuspate coastline west of the barrier, geomorphologists argue that the island had already formed prior to current onlap by ocean waters, thus protecting the inner shores from wave erosion.

17-10: Why is it risky to live on the Outer Banks? ANSWER

Much of the Eastern United States, as well as other parts of the world, where topography is low and flat, are becoming coastlines of submergence, as sea level slowly rises with the current melting of glaciers, sea ice, and continental ice sheets. One consequence of this is that river valleys may be invaded by sea water forming estuaries (with brackish water). The Chesapeake Bay (drowned Susquehanna River) in Maryland and Virginia is a classic example. Another estuary is the Bristol Channel, between southern Wales and Devonshire in southwestern England, which marks the valley of the Severn River (a river of the same name flows into Cheasapeake Bay). More than 138 km (85 miles) in length, it is the largest such indentation in the British Isles. It is noted for its large variations in water height due to tides - an average of 8.8 m (29 ft) over the year and almost 14 m (47 ft) locally during the Spring; only the Bay of Fundy between Nova Scotia and New Brunswick/Maine has higher maximum tides (10.2 m or 34 ft). Both estuaries are also famed for their tidal bore effect: the narrowing of the channel owing to convergence of the shoreline causes a focusing of the water at high tides which is partly responsible for the heights reached. This Landsat image shows the Bristol Estuary (heavily laden with mud and silt); Cardiff, the capital of Wales, is on the north shore.

Courtesy Dr. Hilary S.C. Thomas, University of Wales

One of the more striking features found off coastlines are tropical islands or atolls built around a central emergent landmass, and fringed by, reefs. One of the classic "Paradise" islands in the Pacific Ocean is Bora Bora, 240 km 150 miles) northwest of Tahiti, in the Society Islands of French Polynesia. This is a narrow reef built out from a central island of volcanic origin, separated by a sparkling lagoon (the blue-green portion is very shallow water covering a white coral limestone floor). More than 4500 people live on this island, which is one of the most popular resort destinations in the South Pacific. First look at this Quickbird image of the whole island:

Because of the special beauty and romance of this type of oceanic island, we show two aerial oblique views of Bora Bora:

Close to the U.S, the best known of these are the Grand Bahamas, imaged here by SeaWIFS. Both Florida and Cuba are included. The light blue-green color is close to true, owing to the presence of stirred up calcium carbonate muds derived from the breakup of corals. No wonder the Bahamas are so popular - this looks like Paradise!

The white beaches that make the Bahamas so attractive to winter visitors are made up of carbonate sands derived mainly from precipitation of the CaCO3 by organisms. The ocean on the leeward side of the Bahamas is quite shallow, with clear water. Thus submerged sand shoals is easily visible from above, as shown in this Landsat-7 ETM+ image:

Probably the best known reef complex in the world is the Great Barrier Reef off the northeast coast of Australia. It is a very popular destination for "snorkelers" and scientists studying the habitats of underseas life. Here is a view made by Terra's MISR of much of this chain of coral islands.

Since the melting of the last glacial icecap that covered northern North America, parts of the Canadian Shield have been gradually rising, owing to the principal of isostasy (rebound of depressed land to maintain gravitational equilibrium). In the Hudson Bay-James Bay region this is marked by successive shore line beaches and ridges, each indicating the position for a time of the water's edge, long enough to establish deposits. This is quite evident in this Landsat image:

An aerial oblique photo defines the succession of ridge/beach lines in more detail:

Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net