Water stagnation or still water occurs when water stops flowing for a long period of time. Stagnant water can be a significant environmental hazard.

Malaria and dengue are among the main dangers of still water, which can become a breeding ground for the mosquitoes that transmit these diseases.

Stagnant water can be dangerous because it provides a better incubator than running water for many kinds of infectious pathogens. Stagnant water can be contaminated with human and animal feces, particularly in deserts or other areas of low rainfall. Water stagnation for as little as six days can completely change bacterial community composition and increase cell count.

Stagnant water may be classified into the following basic, although overlapping, types:

• Water body stagnation (stagnation in swamp, lake, lagoon, river, etc.)
• Surface and ground water stagnation
• Trapped water stagnation. The water may be trapped in human artifacts (discarded cans, plant pots, tires, dug-outs, roofs, etc.), as well as in natural containers, such as hollow tree trunks, leaf sheaths, etc.

To avoid ground and surface water stagnation, the drainage of surface and subsoil is advised. Areas with a shallow water table are more susceptible to ground water stagnation due to the lower availability of natural soil drainage.

Some plants prefer flowing water, while others, such as lotuses, prefer stagnant water.

Various anaerobic bacteria are commonly found in stagnant water. For this reason, pools of stagnant water have historically been used in processing hemp and some other fiber crops, as well as linden bark used for making bast shoes. Several weeks of soaking makes bast fibers easily separable due to bacterial and fermentative processes known as retting.

• Denitrifying bacteria
• Leptospira
• Purple bacteria (both sulfur and non-sulfur)
• Brain-eating amoeba

• Asian swamp eel
• Lepisosteidae (commonly known as the gar)
• Northern snakehead
• Pygmy gourami
• Spotted barb
• Walking catfish

Stagnant water is the favorite breeding ground for a number of insects.

• Dragonfly nymphs
• Fly maggots
• Mosquito larvae
• Nepidae (water scorpions)

• Algae
• Biofilm
• A number of species of frogs prefer stagnant water.
• Some species of turtles, such as the mata mata.

• Anoxic waters
• Eutrophication (excessive enrichment by nutrients and minerals)
• Slough (hydrology)
• Wetland
• Residence time distribution
• Water pollution