Depression
Depression
—a wetland or aquatic ecosystem with closed (or near-closed) elevation contours, which increases in depth from the perimeter to a central area of greatest depth and within which water typically accumulates.
Although they may at times have a river flowing into or out of them, depressions are especially characterised by their closed (or at least near-closed) contour shape, which makes them relatively easy to identify on topographic maps.
Depressions may be flat-bottomed (in which case they are often referred to as pans) or round-bottomed and may have any combination of inlets and outlets or lack them completely. Natural depressions may have no inlets or outlets (i.e. Drakensberg tarn and Sirkelsvlei), or an outlet channel (i.e. Bass Lake), or may have an inlet channel but no outlet channels (i.e. Burgerspan).
Some depressions are so extensive that, at first glance, they may appear to be large flat areas that could be classified as ‘wetland flats’ or ‘floodplain flats’, whereas over a long distance they are actually pan-shaped or basin-shaped features, meaning that they are actually ‘depressions’. An extreme example of this is the famous Makgadikgadi Pans in Botswana. To confirm whether an apparently flat area has a depressional shape over a wide distance, you should consult a topographical map with contour lines.
Most depressions occur either where the water table intercepts the land surface (such as on coastal plains along the South African coastline), or in semi-arid settings where a lack of sufficient water inputs prevents areas where water accumulates from forming a connection with the open drainage network. Lakes are a type of depression that typically forms in a valley floor, where some sort of obstruction leads to the ‘drowning’ of the valley through the accumulation of water behind a barrier (in the case of a dam, which can be considered an artificial lake, the barrier has been created by human intervention).
The dominant water inputs and outputs of a depression are dictated primarily by the outflow and inflow drainage characteristics, as classified at Levels 4B and 4C, respectively. The hydrodynamics of a depression are, however, typically dominated by vertical water level fluctuations.
Comments
@tonyrebelo thanks for all these explanations. What about the whole of southern Africa ? Are all depressions, pans and all pans, depressions ? Just wondering how I should call this 'depression' project from Gaborone Botswana. There is an inflow but no outflow. https://www.inaturalist.org/projects/biodiversity-of-ledumadumane-pan
depressions with flat bottoms are called pans, so all pans are depressions, but not all depressions are pans - then can have for instance a river flowing through. Typically inflow without outflow would be a depression.
Thanks. Ill stick to pan !
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