Detention and retention basins are natural depressions or excavations, which can be used for temporarily storing flood water to reduce peak floods downstream. Detention basins are similar to retention basins except for the fact that the latter do not have controlled outlets. Detention basins hold the water temporarily and then slowly release it through a natural or man-made drainage channel, while water collected within retention basins slowly percolates into the ground or evaporates. According to the topography, the type and size of detention and retention basins can be different. They can be brought into operation at the desired stage of a flood wave, enabling reduction in flood peaks downstream. Often, natural depressions are also used for agricultural purposes.

Temporarily stored flood waters flow back into a natural drainage/river within a few days depending on the size of the basin, drainage capacity of the outflow and the need to keep space for subsequent flood waves. However, in the case of retention basins, flood waters may remain stored for days or months, as they can only be released through infiltration or evaporation.

These basins generally do not alter the sediment and organic matter balance of the river. Their impacts on the natural flow regime of the river depend on their inflow characteristics and operation policy. When flood flows are detained for a few days only, there can be no important changes in water quality. However, if water is stored for long periods, the changes in water quality could be similar to those occurring in ponds, such as increased temperature, decreased dissolved oxygen (DO), eutrophication, etc. These basins can be built to function as artificial wetlands or as permanent ponds, and thus help to create habitats for aquatic and semi-aquatic species. A useful option for environmentally sound flood management is to utilize gravel pits in or near flood plains, especially around large towns, as wetlands and ponds, thereby providing wildlife habitats, as well as serving recreational purposes. They can also play a valuable role in building awareness of the local population about flood risks as well as biodiversity, if they are designed and managed according to these aims. (WMO 2006a)