Fluvial terraces are elongated terraces that flank the sides of and fluvial all over the world. They consist of a relatively level strip of land, called a "tread", separated from either an adjacent floodplain, other fluvial terraces, or uplands by distinctly steeper strips of land called "risers". These terraces lie parallel to and above the river channel and its floodplain. Because of the manner in which they form, fluvial terraces are underlain by fluvial of highly variable thickness.
Types
There are two basic types of fluvial terraces,
fill terraces and
strath terraces. Fill terraces sometimes are further subdivided into
nested fill terraces and
cut terraces. Both fill and strath terraces are, at times, described as being either
paired or
unpaired terraces based upon the relative elevations of the surface of these terraces.
- Cut terraces
- Cut terraces, also called "cut-in-fill" terraces, are similar to the fill terraces mentioned above, but they are in origin. Once the alluvium deposited in the valley has begun to erode and fill terraces form along the valley walls, cut terraces may also form below the fill terraces. As either a stream or river continues to incise into the material, multiple levels of terraces may form. The uppermost being the fill terraces and the remaining lower terraces are cut terraces.
- Nested fill terraces
- Nested fill terraces are the result of the valley filling with alluvium, the alluvium being incised, and the valley filling again with material but to a lower level than before. The terrace that results for the second filling is a nested terrace because it has been "nested" into the original alluvium and created a terrace. These terraces are depositional in origin and may be able to be identified by a sudden change in alluvium characteristics such as finer material.
- Paired and unpaired terraces
- Terraces of the same elevation on opposite sides of either a stream or river are called paired terraces. They occur when it downcuts evenly on both sides and terraces on one side of the river correspond in height with those on the other side. Paired terraces are caused by river rejuvenation. Unpaired terraces occur when either a stream or river encounters material on one side that resists erosion, leaving a single terrace with no corresponding terrace on the resistant side.
Formation
Terraces can be formed in many ways and in several geologic and environmental settings. By studying the size, shape, and age of terraces, one can determine the geologic processes that formed them.
Long-lived river (fluvial) systems can produce a series of terrace surfaces over the course of their geologic lifetime. When rivers Aggradation, sediment deposits in sheets across the floodplain and build up over time. Later, during a time of river erosion, this sediment is cut into, or River incision, by the river and flushed downstream. The previous floodplain is therefore abandoned and becomes a river terrace. A river terrace is composed of an abandoned surface, or tread, and the incised surface, or riser.
Age determination
Fluvial terraces can be used to measure the rate at which either a stream or river is downcutting its valley. A simple calculation of h
1/t
1 can give the average rate of incision(r
i), where h
i = height of river terrace from river and t
i = age of surface.
Dating of the abandoned terrace surfaces (treads) is possible using a variety of Geochronology techniques. The type of technique used, however, is dependent on the composition and age of the terraces. Currently used techniques are magnetostratigraphy, low temperature thermochronology, cosmogenic nuclides, radiocarbon, thermoluminescence, optically stimulated luminescence, and U-Th disequilibria.[Dating by electron spin resonance has been used to date terraces of the Têt valley, Pyrénées-Orientales, France. See: ] Additionally, if there is a succession of preserved fossils, biostratigraphy can be used.
Using the resulting date and the elevation above its current level, an approximate average rate of downcutting can be determined.
Incision versus aggradation
The ages of incision and flooding (
aggradation) can have different interpretations for each fluvial system, where each region may respond independently to external variation. Many variables control the behavior of the river and whether it erodes or floods. Changes in the steepness of the stream gradient, the
Sediment flux contained in the river, and the total amount of water flowing through the system, all influence how a river behaves. There is a delicate equilibrium that controls a river system, which, when disturbed, causes flooding and incising events to occur and produce terracing.
Climate and tectonics
When terraces have the same age and/or shape over a region, it is often indicative that a large-scale geologic or environmental mechanism is responsible.
Tectonic uplift and climate change are viewed as dominant mechanisms that can shape the earth's surface through
erosion. River terraces can be influenced by one or both of these forcing mechanisms and therefore can be used to study variation in tectonics, climate, and erosion, and how these processes interact.
Scale of observation
Scale of observation is always a factor when evaluating tectonic and climatic forcing. At a glimpse in geologic time, one of these forcing mechanisms may look to be the dominant process. Observations made on long geologic times scales (≥10
6annum) typically reveal much about slower, larger-magnitude geologic processes such as tectonism
from a regional to even global scale. Evaluation on geologically short time scales (10
3-10
5 annum) can reveal much about the relatively shorter climatic cycles,
local to regional erosion, and how they could drive terrace development. Regional periods of terrace formation likely mark a time of when stream erosion was much greater than sediment accumulation. River erosion can be driven by tectonic uplift, climate, or potentially both mechanisms. It is difficult in many areas, however, to decisively pinpoint whether tectonism or climate change can individually drive tectonic uplift, enhanced erosion, and therefore terrace formation. In many cases, tectonic-climate interactions occur together in a positive feedback cycle.
Climatic changes
Rivers in continental interiors that have not experienced tectonic activity in the geological recent history likely record climatic changes through terracing. Terraces record natural, periodic variations driven by cycles such as the Milankovitch cycle.
[ These cycles can describe how the Earth's orbit and rotational wobble vary over time. The Milankovitch cycles, along with solar forcing, have been determined to drive periodic environmental change on a global scale, namely between glacial and interglacial environments.]
Tectonic uplift
In areas where there is tectonic uplift, it can increase the slope of a river, increasing its flow rate and erosion. This can cause a river to abandon its floodplain and cut downward into its river bed. The abandoned floodplain then becomes a terrace above the new river level. If the tectonic uplift occurs episodically, the river may form multiple terraces.
Tectonic–climatic interactions
Tectonic uplift and climatic factors interact as a positive feedback system, where each forcing mechanism drives the other.
The Himalayas act as an orographic barrier that can impede atmospheric circulation and moving air masses. When these air masses try to move up and over the Himalaya, they are forced up against the barrier. The mass condenses as it rises, releasing moisture, which results in precipitation on that flank of the mountains. The condensation of the water vapor warms the air. The resulting drier air gets compressed and further warmed on the other side of the barrier, which can produce desert conditions downwind. This is known as the rain shadow effect. In the Himalaya, the rain shadow is an important environmental factor in developing the Asian monsoon.
Tectonic uplift during the creation of high mountainous regions can produce incredible surface elevations and therefore exposure of rocks to wind and water. High precipitation can drive enhanced erosion of the exposed rocks and lead to rapid denudation of sediment from the mountains. Buoyancy of the crust, or isostasy, will then drive further tectonic uplift, in order to achieve equilibrium, as sediment is continuously stripped from the top.
The interaction between tectonics and climate leads to more complex formation of river terraces, especially in the Himalaya and Tibetan Plateau.
See also
