Here are six more classic examples of distributaries:
The first is the delta formed by the now combined Tigris and Euphrates Rivers as it empties in Iraq at the head of the Persian Gulf. The river is building a dominant finger into the sea, but other delta land is being created by tributaries. The river is lined with farmlands growing a variety of crops including wheat, rice, sorghum, cotton, and millet. The dark areas in the upper left are swamps. Part of Kuwait is near the bottom:
Great rivers drain from the Himalayas and flow to the Indian Ocean on either side of the Indian subcontinent. On the west the Indus River that flows through Pakistan forms a delta at the ocean, as seen in this astronaut photo:
The next 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. Below this image is a Landsat subscene that 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 (red in lower image) sediment flowing into marine waters. These quasi-distributaries become tidal channels that tidal currents highly influence. The dark red tones (brown in lower image) 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.
Similar in appearance and ecology is the Irawwady Delta in southern Myanmar. Here it is, first in a regional setting as shown on Google and then in a Landsat-7 subscene:
The distributary system of the largest river in Madagascar, the Betsipoka, an island nation off the east coast of the African continent shows an interesting characteristic. It drains from the Madagascar highlands which include red soils. During the rainy season, the muds in the river give it a distinctive brownish-red tone, as shown in the upper scene. In the dry season the stream is normally dark, but the land between retains the reddish mud deposits. The insets show the corresponding effects of the red muds in seawater off the coast.
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, forming a distinctive semi-circular delta.
This color composite reveals the extent of vegetation on the Yukon Delta.
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.
The limited MSS image resolution causes informative details about the geomorphology of a feature such as the Yukon Delta region to be missed. This ASTER image, covering a smaller area at finer resolution, shows the many sediment-choked distributaries within the delta proper as well as other features marking the influence of interactive glacial processes:
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.
Another rather exotic delta is that of the Parana River between Uraguay and Argentina:
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. Here is an individual fan, formed off a mountain range in southern Iran; note that, as is typical, the stream splits into several distributaries. These multiple streams have carried beyond the fan where the occasional water has allowed green vegetation and agriculture to flourish.
Here is an alluvial fan in Tibet, which is adjacent to a lake:
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 deposited as fans. Note how numerous streams seem to start at the contact between basin and mountain2 (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 pediplain, constructed from the fans).
One of the biggest alluvial fans in the world occurs in the Badain Jaran desert of east Asia, where the Ruo Shui River drains north from the Nan Shan mountains:
Rivaling that in size (and not too far away) is this fan in the southern Taklimakan Desert in Sinkiang Province, western China. This ASTER image, in near natural color, shows active (water-bearing) distributaries in blue and abandoned distributaries in black.
Alluvial fans usually extend into lowlands or basins, those commonly bounded by mountains on either side. This is typical of the Basin and Range topography of Nevada, shown on the previous page. A comparable situation occurs in the Zagros Mountains of southern Iran. In the Landsat scene below the E-W trending basin is more than 200 km in extent. Multiple fans approach the basin deposits on both the north and south sides.
We turn now to landforms that have a very limited expression, in that they are confined to narrow strips of land adjacent to water bodies such as the oceans or lakes. These are collectively known as coastal landforms. A coast is technically the strip of land (usually a beach) that is the interface between land and water; this interface's exact position varies with the tides. But in a broader sense geomorphologists include among the coastal landforms features behind this interface (the strand line) influenced by the water (e.g., lagoons, marshes; dunes, etc.); also involved because of its influence on waves and currents is the offshore shelf.
Classifications of coastlines can be tricky. One grouping is coastlines of submergence and emergence (although these terms are now held to be obsolete and misleading by some geomorphologists). This takes into account the long term effect of rising or falling sealevel (which has in the past been affected mostly by water being transferred from the oceans into masses of ice at the continental scale - lowering the general level of the oceans relative to continental heights - or melting of glacial ice - raising the general level. Another classification recognizes the interrelation between a subaerial continent's relation to the two chief components of an active plate in the plate tectonics model. On one side of a continent the ocean may extend to a spreading ridge. The continental margin in this case is called passive - the Atlantic coast of North America falls into this category. On the other side there is extensive tectonic activity associated with subduction or transform fault displacement. This is the active case: The Pacific coast, with its system of mountains adjacent to the coastline is an example.
These two photos (the first from space) give a good idea of typical Atlantic coast landforms which include low, relatively flat coastal plains, drowned river estuaries (the river mouth), coastal sand bars, and lagoons; the offshore continental shelf can extend out to sea for more than 100 km (62 miles). These views of coastal North Carolina illustrate this:
Distinct land forms are associated with Atlantic coast types of landforms, such as those illustrated in this diagram:
However, under the right circumstances cliffs can be produced in otherwise flat coastlines not involved in mountain building. In this photo, the horizontal Cretaceous chalk beds that make up the White Cliffs of Dover show such a setting:
Pacific coast landforms are characterized by (often) irregular coastlines, embayments, narrow beaches, often rugged topography (such as mountains) on the land side, and rapid deepening of waters in the narrow continental shelf. These views are examples.
Where the mountains are actually encroached upon by the ocean, a rugged and irregular coastline is the norm, as shown here.
In between the mountains are narrow beaches:
Space images don't always do justice to the coastal landforms they may contain. This is due in part to the fact that the landforms tend to be concentrated along thin linear strips rather than spread over much of the images. Still, as the examples that follow illustrate, much of the larger scale features of this group of landforms can be expressed in images. We will start with more images along the western coast of North America.
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. Narrow patches of flat land almost at sealevel can develop midst the high ranges that meet the waters, as shown here:
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.
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 by drainage off 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.
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. The Chesapeake Bay (drowned Susquehanna River) in Maryland and Virginia and the Delaware Bay south of Philadelphia are classic example, as seen together in this MODIS image.
The Chesapeake and Delaware Bays are both examples of estuaries. When rivers meet the ocean or large lakes they either form estuaries or deltas. We saw examples of deltas earlier on this page. Let us now consider marine estuaries, which usually consist of brackish water (salty ocean water mixed with fresh river water). Several examples are shown next, with their special characteristics described in the captions:
Previously on this page we saw the region where the Tigris-Euphrates River(s) empty into the Persian Gulf, as an example of a delta. This area is shown again below to illustrate with labels the multiple coastal characteristics that include estuarine deposits:
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 extreme form of an atoll is the type that has just a single, narrow band of coral reef, enclosing a lagoon that comprises almost all the area of the island. The top of the submarine land mass (volcanic) is now completely submerged. The Arno Reef in the Marshall Islands and the Oeno atoll are two prime examples:
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. These submerged sand shoals are 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 principle of Isostasy (rebound of depressed land after load removal to maintain gravitational equilibrium). But the ocean levels have also risen and fallen depending on the amount of ice locked up in glaciers and ice caps. Thus, at earlier times during the last few million years of off-again/on-again glacial episodes, shorelines have been established at various higher levels as sealevel fluctuated. The shifts in water level and the depressions and rebounds have combined to produce stranded deposits and erosive features on the present land surface 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. In this area, the set of higher shorelines is probably more the effect of rebound than of water level rise..The resulting beach lines are quite evident in this Landsat image:
An aerial oblique photo defines the succession of ridge/beach lines in more detail:
One of the consequences of continental glaciation is that sea water becomes converted to great masses of ice on both continents and open seas. Sea levels fall when this water is withdrawn. They rise again during interglacial melting. As we saw around Hudson Bay, this can give rise to terraces and cliffs. Areas away from the glacial ice, such as near the equator, are susceptible to sea level fluctuations. This is nicely illustrated by the several terraces on Isla Blonquilla, offshore from Venezuela, in the Caribbean:
Traces of ancient strand (beach) lines owing primarily to sealevel changes, but also often influenced by tectonic or isostatic adjustments, are found throughout the world. This next image indicates a succession of shorelines masked partly by vegetation in the Mosquito Coast (so-named from the Moskito River rather than a surfeit of mosquitos) along the Gulf of Mexico where Honduras and Nicaragua come together at the Coco River:
Three of the next four landform groups are also tied in with water as a prime formative agent.