River terrace: what it is, how it is formed and examples

A fluvial or alluvial terrace is an ancient floodplain that is elevated on both sides of a valley that has been left relict by the natural processes of lateral migration, accumulation (deposition) or erosion of rivers.

Morphologically, it is composed of a relatively flat strip of land that is distinguished from the floodplain of other terraces or adjacent highlands by the presence of steeper slopes called ascenders. These terraces are arranged parallel to and above the current channel and its flood plain, constituting remnants of ancient river levels that allow us to reconstruct the evolutionary history and changes in the river regime.

A river terrace takes place in several steps:

1. Initial sediment deposition: the river fills the valley with alluvial materials, forming a wide floodplain. This can occur due to climate changes that increase sediment load or decrease flow, or due to base level elevation that reduces the river gradient.
2. Plain Rise: As deposition continues, the floodplain gradually rises.
3. Restart of erosion: when the balance is broken (due to variations in flow, base level or climate), the current begins to lock in and carve a new channel at a lower level, cutting through the previously accumulated sediments. Here you can read about River Erosion: what it is, types, consequences and examples.
4. Formation of the terrace: the old flood plain is then elevated on the margins of the valley, constituting the river terrace.

There are two types of river terraces:

Terraces on rock

Equivalent to the “strath terraces” in Anglo-Saxon literature. They are formed through three successive phases: vertical erosion, lateral erosion and again vertical erosion, acting on the rocky substrate. In general, they have only a thin cover of gravel, corresponding to the old river bed. In tectonic contexts, these terraces are valuable indicators of the continuous uplift of the land and have a high conservation potential.

Accumulation terraces

They result from a succession of phases: vertical erosion, followed by a stage of expansion or accumulation of sediments, and again vertical erosion. They reflect periods of stability in which deposition predominated over erosion. Accumulation may be due to an increase in sediment load, reduced flow, or changes in the river’s base level. Their formation is usually related to climatic and eustatic variations, although they can also be influenced by tectonic activity or drainage captures.

River terraces can be found in different regions, such as:

• Mountain river valleys: they appear in regions where fluvial incision, the process by which a river deepens its channel, is intense due to slope and tectonics. The terraces are arranged in steps along the valley, showing old flood levels. Examples: Mendoza River terraces and Colorado River terraces in the Argentine Andes, and Rhone River terraces in the Swiss Alps.
• Intermontane valleys and foothills: they are formed in transition areas between mountains and plains, where rivers alternate phases of sediment accumulation and sedimentation. Examples: terraces of the Lerma River in Mexico and terraces of the Limay River in Argentine Patagonia.
• Large alluvial plains: in low areas close to the mouth, the terraces are caused by changes in the base level, such as variations in sea or lake level. Examples: terraces of the Paraná River in Argentina and terraces of the Mississippi River in the United States.
• Tectonically active regions: in areas with land uplift, each episode of river incision leaves new stepped terraces, which are valuable indicators of tectonic dynamics. Examples: terraces of the Bío-Bío river in Chile and terraces of the Loire river in France.
• Glacial or periglacial environments: associated with pulses of sediment loading and unloading during glaciations and deglaciations, the terraces record phases of aggradation and erosion linked to the climate. Examples: terraces of the Baker River in Chilean Patagonia and terraces of the Rhone River in Switzerland.
• Ancient fossil river basins: Even in areas where present-day rivers have changed course or disappeared, relict terraces remain as geomorphological records of the past. Examples: relict terraces of the Desaguadero River in Argentina and fossil terraces of the Thames River in England.

River terraces are important since they allow the evolution of soils to be studied, incorporating the time factor with relative ease. Although absolute dates are not always available, it is possible to establish a fairly precise relative chronology using different criteria such as the presence of faunal remains, the geomorphological characteristics of the relief or cartographic interpretation. In this way, terraces offer a valuable opportunity to analyze pedogenetic processes over broad time scales.

Furthermore, they represent fundamental elements in human history. Archaeological sites have been found on many river terraces, including remains and artistic expressions, which makes these relief forms fundamental testimonies both for understanding the natural dynamics of landscapes and for reconstructing the cultural evolution of societies.

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