Switching to fluvial landforms, of which there are many varieties, we start by showing the MacKenzie River in Canada, which shows many typical characteristics of a river. The river itself is fairly straight and broad. Its tributaries display a common condition in which those rivers - in order to lengthen their path and thereby lower their gradient (rate of decrease in elevation per unit length of the stream) - develop curves called meanders.
In addition to meandering, rivers show a variety of drainage patterns. This sketch shows the principal types:
In many space images, the type(s) present are not that easy to discern and classify. Tracing the rivers present usually reveals what the pattern type is. By far the most common type is the dendritic pattern. This Landsat subscene of the Dirty Bend, Utah drainage illustrates dendritic drainage:
The appearance of this type of drainage, so distinctive, can be enhanced by spatial filtering. This Landsat image of a highlands area west of Riyahd, Saudi Arabia, illustrates the effect:
This Landsat scene of South Yemen shows that the dendritic drainage has highly dissected the topography in flat-lying rocks. The subscene below it further amplifies the drainage style:
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.
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.
Both parallel and trellis drainage patterns are usually structurally controlled. Here is an example of each; see caption.
Radial drainage tends to develop on structural domes. It is even more common on slopes of stratocone volcanoes, as seen on the Bromo Volcano of eastern Java.
Normally, the sculpturing of landforms by running water is an imperceptibly slow process, taking millions of years to bring about notable changes. There is a rarely occurring exception to that: if the water is released suddenly, as from a dam burst which empties the backfilled lake, the huge volume thus emptied can carve into the downstream landscape in hours to days. This happened on a grand scale during the ice age in western Idaho, much of Washington, and a part of Oregon. The result is a modified landscape known as the Channeled Scablands. Its location (shown in gray) and the inferred origin are indicated on this map:
The present day Columbia River runs across the top of the image; Spokane, WA is near the upper right corner. The farmlands in the right third are in the Palouse country, a region of mainly wheat crops. Most of the lowlands are underlain by Columbia River basalts, but these are covered by windblown loess (fine dust) derived from Pleistocene glacial deposits. Such material is easily eroded by the rush of waters. Most of the waters had formed large glacial lakes (Lake Missoula in Idaho; Lake Spokane in Washington) developed behind natural dams made of glacial ice. When these burst, they sent cataclysmic floodwaters out over wide areas, largely confined to pre-existing valleys, that cut into the loess and basalt (the latter is responsible for the darker grays that denote the channels). Distinctive landforms resulted; one example appears below. Other examples are displayed in this USGS web site.
Many rivers have distinctive floodplains - flat areas affected both by erosion and by deposition - which extend beyond the river channel, often to bluffs that mark the edges of higher land being dissected. A typical example is the floodplain of the Brahmaputra on the Indian subcontinent:
Floodplains have great importance in agriculture. The river provides water that supports farming, usually with the aid of irrigation canals. This is the case for the Amu Darya river south of the Caspian Sea; beyond its floodplain are sand-covered uplands:
This next SPOT image focuses on the Shebele River in Ethiopia. Its floodplain extends well beyond its immediate course. The bluffs in this case are steep cliffs; the uplands are true mesas (flat surfaces):
Mature river systems, with wide, flat floodplains and not too high above sealevel, over extended time periods tend to develop laterally shifting river channels - the so-called meanders mentioned at the beginning of this page. Especially during flooding, the swollen river is often able to move straight ahead abandoning the meandering course leaving behind a cutoff channel segment called an oxbow lake. Meandering is a natural process that allows a river to adjust its gradient by lengthening its course. The photo below shows a typical meandering river; note that on the left two meanders are nearly touching so that in a flood they might be joined:
Meandering is strikingly illustrated in the floodplain of the lower Mississippi River, seen below in two images. In the upper one, 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. The lower image zeroes in on the meandering and shows now detached cutoff scars.
This map shows a history of meandering over a small stretch of the Mississippi River
Another river displaying numerous meanders is the Songhua River in the Manchurian plains of northern China:
The Rio Negro in South America also shows extensive meandering within its floodplain:
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.
In Section 3 (page 3-7) we showed an image of the "Goosenecks" of the San Juan River in Utah. We repeat this image here with a further explanation. The San Juan is one of many rivers found now within the Colorado Plateau. Back 10 million years ago the region was much lower in elevation. Its rivers, with low gradients then, tended to meander. But the region has since experienced extensive uplift of a mile or more, at a geologically rapid rate. The streams continued to flow in their developed channels but the higher gradients caused them to increase their downcutting while maintaining their meandering. The result for some rivers was to form canyons that are curved, preserving the meandering path.
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, and in more detail from SPOT:
Braided conditions represent deposition from streams choked with sediments. The resulting "islands" within the channel produce multiple segments of individual channelets within the broader master channel. A similar multichannel condition forms where a great deal of flow water (but not laden with much sediment) traverses flat lands and splits into individual channels, again separated by "islands". This occurs along several rivers in the Amazon Basin. Here is an example: the Demini River in northwest Brazil.
A similar condition can develop in deltas (see below)
One landform associated directly with the river that makes it is a waterfall. This may be hard to detect from space owing to its verticality. Nevertheless, water in the stream may spread out the channel just before or just after it pitches over the drop - Victoria Falls in Africa (see below) 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:
Horseshoe Falls on the Canadian side (west; left) shows off its spectacular visage in this IKONOS image:
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:
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):
Streams cause a wide variety of rock-based landforms: ridges; plateaus; mesas/buttes, canyons, etc. These landforms are often both spectacular and picturesque when developed in semi-arid and desert landscapes, as we have seen in the Southwest U.S. (pages 2-2 and 6-7.
This next scene is a splendid example of a large plateau as depicted in a Landsat-1 image. Situated on the Bolivian-Brazilian border, the plateau is called Serrania de Huanchaca. The cover is dense vegetation. The plateau is almost 2000 m (6600 ft) higher than the meandering Guapore River to its east.
Below is another plateau example, this time in the Guiani Highlands of northeast Venezuela. Although not obvious in this black and white version, the region is heavily vegetated. The Rio Caroni is the major draining river. Bedrock of nearly flat sedimentary layers rests on Precambrian crystalline rocks. Over the eons the back and forth course shifts through meandering of the Rio Caroni and other rivers have led to the upper layers being eroded back, in a manner similar to the King planation surfaces, into a series of stepped plateaus and mesas (the dark ones are heavily vegetated). The highest, Auyan-Tepui (dark area near center), tops at 2950 meters (9739 ft). Over its rim spills Angel Falls, highest in the world (979 m; 3212 ft), seen looking up from its base.
One of the most unusual fluvial-caused landforms is the so-called natural bridge, made by stream undercutting and penetration through a rock unit. One of the most famous in the western U.S. is the Rainbow Bridge in the Glen Canyon National Park.
Most primary rivers end up entering large bodies of "standing" water, thereby losing velocity and hence load-carrying capability. 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:
A classic example of a "bird's foot" type of delta is seen in this astronaut photo taken from the ISS while it flew across Canada. This is the delta of the Saskatchewan River made as it empties into Cedar Lake in Manitoba. The delta is in fact much smaller than the Mississippi River delta. It is made up mostly of muds and clays.