Small streams, called tributaries, join larger streams. The drainage divide separates water that flows into one stream from all other streams. Eventually the river empties into a final destination. Usually it's the sea, sometimes a lake, rarely (in deserts) a dry basin. The final destination is called the base level and dictates how deep the river can cut.
Once the river cuts to a certain level, the current can only just carry the sediment entering the stream. From then on, the river is only moving sediment, not cutting. Meanwhile other processes continue to erode the uplands.
Rejuvenation of an old-age landscape.
Rejuvenation of an early mature landscape.
Major rivers cut right across ridges. The ridge summits are all at the same elevation They are the relics of an ancient land surface that was uplifted. The soft rocks were eroded and the resistant rocks remain as ridges. Large rivers were able to cut down through the resistant rocks but small streams could not and were gradually diverted. Streams entirely in soft rocks had an advantage over those cutting across hard rocks, and gradually expanded their drainage basins.
Some rivers have their sources in Tibet and cut right through the highest ranges of the Himalaya.
When rivers cannot cut through an obstacle, or something blocks or cuts into their channel, they can be diverted.
|The Huang He has been called "China's Sorrow" because of its frequent and
catastrophic floods. The diagram above shows its drainage diversions during recorded
history. In U.S. terms, the diversions above, spanning almost 400 miles, would be roughly
equivalent to the Mississippi changing its outlet from Houston, Texas to Pensacola,
The river has filled in a former strait between the mainland and the Shandong Peninsula, which was once an island. The river exits the highlands laden with silt derived from China's loess deposits and has constructed a vast flood plain which is actually a gigantic alluvial fan. The surface of the fan is a gentle cone with a nearly uniform slope, which is maintained by frequent changes in the course of the river as old channels are abandoned.
With almost no topographic relief in the region, there is no place to seek refuge when the river floods. The diversions of the Huang He have produced some of history's greatest disasters:
The Volga and the Don almost meet (red circle). Between the two rivers is Volgograd, formerly Stalingrad, site of one of the fiercest and most pivotal battles of World War II (see the film Enemy at the Gates). Both rivers have a sharply barbed drainage, but which captured which? The Don flows to the Black Sea and can only erode to sea level, but the Volga flows to the Caspian, which is 92 feet below sea level. The dark shading shows the region below sea level. (Trivia question: name a major river whose delta is below sea level) Clearly streams flowing to the Caspian have a competitive advantage in capturing drainage, and it seems likely that a former small river cut back into the former Don river, diverting most of its drainage and creating the present Volga.
The Ural River also has a sharp bend and possibly might also have diverted a former tributary of the Volga. The possible former connection is shown with question marks.
In what has been called one of history's greatest environmental catastrophes, water from the Amu Darya, main water source for the Aral Sea, has been diverted for irrigation. The area of the lake is half of what it was in 1960 and the lake is now too salty to support fish.(The former outlines are shown)
The plain of the Amu Darya, like that of the Huang He, is a giant alluvial fan. Excess runoff from irrigation runs not to the Aral Sea but to Lake Sarikamysh. Not far away is the head of an intermittent stream, the Uzboy, that flows to the Caspian. It seems certain that the lower Amu Darya, like the Huang He, has seen frequent diversions and that at least some of the time it has flowed via Lake Sairkamysh and the Uzboy to the Caspian.
Very little is known of the history of the Aral Sea. A number of test borings in the lake were described by William Last and others at the 1998 meeting of the Geological Society of America in Toronto. These showed heavy concentrations of evaporites at several levels in the bottom sediments, including magnesium and potassium salts, evidence of extreme dessication. Thus, the lake has experienced several purely natural episodes of river diversion and drying during the last few thousand years.
You'd be blue, too, if you'd been robbed. One reason a cruise on the Rhine is so scenic is that Western Europe underwent gentle uplift in the last few million years. The Rhine rejuvenated and carved a deep gorge, complete with entrenched meanders. The nearby headwaters of the Danube were also uplifted, but the Danube flows across half the length of Europe to the Black Sea. Clealy, tributaries of the Rhine have a competitive advantage over the Danube in carving valleys.
|One small tributary of the Rhine, the Wutach, flows along a belt of soft Triassic
rocks. It rapidly eroded its valley headward and captured a former tributary of the upper
Danube. In the map, Switzerland is shown in purple. This is one of the world's more
peculiar international borders; there really are two little isolated pockets of Germany
completely surrounded by Switzerland.
The Danube drainage basin is shown above in green, the drainage captured by the Wutach in yellow. The Wutach itself was pirated by yet another small tributary of the Rhine, and that captured drainage is shown in light blue. Drainage divides are red.
|Humid Climates||Arid Climates|
|Rain||Frequent||Rare, May Be Seasonal, Often Violent|
|Soil Cover||Thick||Thin or Absent|
|Vegetation||Thick||Sparse-no Continuous Cover|
|Overall Landscape Evolution||Mostly Uniform Processes||Episodic Processes|
Some geologists think this happens in all landscapes but is modified by soil creep and mass wasting in humid climates.
|Changes of the Mississippi River delta since 3000 B.C. Note that there are intervals of overlap for some deltas. It seems obvious the Mississippi is long overdue for a shift to the west, and in fact a huge spillway complex allows flood water to flow down the Atchafalaya, where a substantial delta has already begun to form. We don't simply let the river change its course because it would isolate New Orleans and Baton Rouge economically, it might possibly isolate the entire river if the new channel can't be navigated, and it would disrupt the wetland ecosystem along the Atchafalaya. Sooner or later, however, it will happen. Lake Ponchartrain (the big lake north of New Orleans) is actually a former bay cut off by one of the Mississippi's delta lobes.|
Where are the earlier deltas? Pre-Pleisocene deltas fill in most of Louisiana but are mostly buried and not known in detail. Pleistocene deltas formed when sea level was lower than at present and are somewhere under the present Gulf of Mexico. About 7,000 years ago sea level began to stabilize following the melting of the Pleistocene glaciers. That time coincides with the onset in many places of intensive settled agriculture and early civilization. Not only did it become possible to farm river deltas without having to resettle frequently as sea level rose, but the infilling of river valleys slowed, making it possible to farm flood plains as well.
|The Imperial Valley of California is below sea level because it is a former part of the Gulf of California cut off by the Colorado River delta. The valley has a year-round growing season and is one of the most important agricultural areas in the U.S. At one time it was almost entirely dry, but about 1910 an irrigation canal from the Colorado River overflowed and flooded a large part of the center of the valley. By the time the flood was stopped, residents decided that a lake might not be a bad idea, so it is still there. It's called the Salton Sea and is maintained artificially.|
|Some deltas can actually be inland. The Okavango River of Africa flows into a closed basin, where it soaks into the ground or evaporates. The Okavango Delta is one of the world's great wildlife habitats.||Other inland deltas include the Niger where it first enters the Sahara Desert, and the Sacramento-San Joaquin delta in California.|
|Lake Kioga in Uganda is a particularly interesting case of a tectonically dammed lake. The block east of the East African Rift tilted eastward. Prior to tilting the rivers probably joined the Congo and flowed west to the Atlantic. One watershed was tilted so far that the course of the river was cut off and the river filled its valleys. The lake now drains out through a former tributary valley and carries the upper Nile. Different colors denote different watersheds. Boundaries are in red and the horizontal black line is the equator. The largest lake is Lake Victoria; the lake north of it is Lake Kioga. See the map below for a detailed view.|
|A detailed map of Lake Kioga. Lakes are in blue and marshes in green. The Nile is in purple with red arrows showing flow directions.|
|Clear Lake is the largest and most misnamed natural lake entirely within California (Lake Tahoe is shared with Nevada). To the west is a steep and short drop to the Russian River, but the former valley there was dammed about a million years ago by lava flows. The valley flooded and found an outlet to the east, where more volcanic rocks slow deepening and erosion of the outlet.|
|As already noted earlier, Lake Ponchartrain (the big lake north of New Orleans) is actually a former bay cut off by one of the Mississippi's delta lobes.|
Created 12 Jan 1997; Last Update 3 November 1999
Not an official UW-Green Bay site