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

A river or alluvial terrace is an ancient flood plain that is high on both sides of a valley that has been relicted by the natural processes of lateral migration, accumulation (deposit) or erosion of the rivers.

Morphologically, it is composed of a strip of relatively flat terrain that is distinguished from the flood plain of other terraces or the highlands adjacent to the presence of more pronounced slopes called ascending. These terraces are arranged in parallel and above the current channel and its flood plain, constituting remnants of old river levels that allow reconstructing evolutionary history and changes in the river regime.

A river terrace takes place in several steps:

1. Initial sediment deposit: The river fills the valley with alluvial materials, forming a wide flood plain. This can occur due to climatic changes that increase the load of sediments or decrease the flow, or by the raising of the base level that reduces the gradient of the river.
2. Planning elevation: As the deposit continues, the flood plain gradually rises.
3. Erosion restart: When the balance is broken (due to variations in the flow, the base level or the climate), the current begins to fit and work a new channel at a lower level, cutting the sediments previously accumulated. Here you can read about river erosion: what is, types, consequences and examples.
4. Terrace formation: The old flood plain is then raised in the margins of the Valley, constituting the river terrace.

There are two types of river terraces:

Roca terraces

Equivalent to “Strath Terraces” in Anglo -Saxon literature. They are formed by three successive phases: vertical erosion, lateral erosion and again vertical erosion, acting on the rock substrate. In general, they have only a thin coverage of gravels, corresponding to the old river bed. In tectonic contexts, these terraces are valuable indicators of the continuous lifting of the land and have a high conservation potential.

Accumulation terraces

They result from a phase succession: vertical erosion, followed by a stage of expansion or accumulation of sediments, and again vertical erosion. They reflect stability periods in which deposition on erosion prevailed. The accumulation may be due to an increase in sediment load, flow reduction or changes in the base level of the river. Its training is usually related to climatic and eustatic variations, although they can also be influenced by tectonic activity or drainage catches.

River terraces can be found in different regions such as: for example:

• Mountain rivers: They appear in regions where the river incision, the process by which a river deepens its channel, is intense by the slope and tectonics. The terraces are staggered throughout the valley, showing old flood levels. Examples: Terrazas of the Mendoza River and Terrazas del Río Colorado in the Argentine Andes, and Rhone River terraces in the Swiss Alps.
• Intermontanos and foothills: They are formed in areas of transition between mountains and plains, where rivers alternate sediment accumulation and fitness phases. Examples: terraces of the Lerma River in Mexico and Terrazas of the Limay River in Argentine Patagonia.
• Large alluvial plains: in low and close to the mouth sectors, the terraces originate from changes in the base level, such as sea level or lake variations. Examples: Terraces of the Paraná River in Argentina and Terrazas of the Mississippi River in the United States.
• Tectonically active regions: in areas with terrain surveys, each episode of incision of the river leaves new staggered terraces, which are valuable indicators of tectonic dynamics. Examples: Bío-Bío River terraces in Chile and Terrazas del Río Loira in France.
• Glaciers or periglaciar environments: associated with loading and discharge of sediments during glaciations and swallows, the terraces record phases of amance and erosion linked to the climate. Examples: Terrazas del Río Baker in the Chilean Patagonia and terraces of the Rhone River in Switzerland.
• Old fossil river basins: even in areas where current rivers have changed course or missing, relict terraces remain as geomorphological records of the past. Examples: Relicious terraces of the Desaguadero River in Argentina and fossil terraces of the Thames River in England.

The river terraces are important since they allow studying the evolution of soils incorporating the time factor with relative ease. Although there is not always absolute dates, it is possible to establish a fairly precise relative chronology by resorting to different criteria such as the presence of faunistic remains, the geomorphological characteristics of the relief or cartographic interpretation. In this way, the terraces offer a valuable opportunity to analyze pedogenetic processes over wide time scales.

In addition, they represent fundamental elements in human history. In many river terraces, archaeological deposits have been found, including artistic remains and expressions, which makes these relief forms into fundamental testimonies both to understand the natural dynamics of landscapes and to reconstruct the cultural evolution of societies.

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