Fluvial terrace

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. . In 1884 the stream ran at top of the terrace. 1939 photo by United States Geological Survey. ;Fill terraces: Fill terraces are the result of an existing valley being filled with alluvium. The valley may fill with alluvium for many different reasons including: an influx in bed load due to glaciation or change in stream power which causes the valley, that was down cut by either a stream or river, to be filled in with material. Once this occurs benches composed completely of alluvium form on the sides of the valley. The upper most benches are the fill terraces. As it continues to cut down through the alluvium the fill terraces are left above the river channel (sometimes 100 m or more). The fill terrace is only the very highest terrace resulting from the depositional episode; if there are multiple terraces below the fill terrace, these are called "cut terraces". , Park County, Wyoming, 1923. The river at left has encountered a formation of erosion-resistant volcanic breccia, causing it to downcut more rapidly on the right, leaving terraces of different elevations. ;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 flood, 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 incised, 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 h1/t1 can give the average rate of incision(ri), where hi = height of river terrace from river and ti = age of surface. Dating of the abandoned terrace surfaces (treads) is possible using a variety of geochronologic 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. 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. ==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 (≥106annum) 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 (103-105 a) can reveal much about the relatively shorter climatic cycles, 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. Each river system will respond to these climate variations on a regional scale. In addition, the regional environment will determine how change in sediment and precipitation will drive river incision and aggradation. Terraces along the river will record the cyclic changes, where glacial and interglacial time periods are associated with either incision or aggradation. Tectonic uplift In areas where there is tectonic uplift, it can increase the slope of a river, increasing its flow rate and erosive power. This can cause a river to abandon its floodplain and cut downward into its 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. One example of this feedback between tectonic and climatic effects may be preserved in the Himalayan front and in the development of the rain shadow effect and the Asian monsoon. The Asian monsoon then increases erosion on the southern steep slopes of the Himalaya. 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. Enhanced uplift will then create higher topography, drive increased precipitation which will concentrate erosion, and further uplift. The interaction between tectonics and climate leads to more complex formation of river terraces, especially in the Himalaya and Tibetan Plateau.{{cite journal|first1=Zhenhua|last1=Ma|first2=Tingjiang|last2=Peng|first3=Zhantao|last3=Feng|first4=Xiaomiao|last4=Li ==See also==