Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Tributary valleys almost always join the main valley at exactly the same elevation, even though the valleys may begin many miles apart. This is very unlikely unless the rivers have cut the valleys.
The river only erodes a slot. Other processes carry material from the valley walls to the river to be carried awav. So rivers only deepen their valleys; other processes widen them.
Rejuvenation of an old-age landscape.
Rejuvenation of an early mature landscape.
|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.|
|In the diagram at left, the percents show river flow in each channel when there is more than one. In about 1000 years the falls will pass Goat Island, the American Falls will be abandoned, and there will only be a single fall. In about 6000 years the falls will retreat to the tip of Grand Island and there will be a triple falls.
As the falls retreat, the hard rock of the lip of the falls will decrease in elevation and the thickness of soft rock above will increase. By about 15,000 years (blue) the present falls will be much lower and their retreat will slow to near zero. The river will cut a gorge upstream from the falls in the soft rock, and when the gorge reaches Buffalo in around 30,000 years, it will encounter another layer of resistant rock and a second falls will form.
Source: Philbrick, S. S., 1974; What Future for Niagara Falls? Geological Society of America Bulletin. Vol. 85; no. 1, p. 91-98.
|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 own former valleys. The lake now drains out through a former tributary valley and carries the upper Nile. Different colors denote different watersheds. National 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 Updated 24 May 2020
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