Table 1. Examples of situations when combinations of water quality variables and extreme discharge conditions can produce environmental problems. Water quality variables Low flows High flows Pollutants Concentrations can reach toxic levels Can be washed out from adjacent, otherwise unflooded uplands; dilution reduces but does not eliminate risk for toxicity Drugs PPCP:s (Pharmaceuticals and Personal Care Products ) can become toxic; natural estrogens can feminize fish Nutrients Can lead to eutrophication and acidification; N levels can become toxic Removed from watercourse by downstream transport, uptake by riparian vegetation and denitrification Salts Can lead to acidification, mobilization of toxic metals and invasion of salt-tolerant species Organic matter and sediments Considerable addition that increases turbidity, which reduces primary productivity and may increase acidity and threaten fish production Organic matter can reduce pH Sedimentation of transported inorganic matter restructures channel High temperature Lowers oxygen content, makes contaminants more toxic, lowers productivity Low temperature Surface ice cover leads to reduced oxygen Open water and low air temperatures can foster excessive formation of frazil ice and anchor ice that damage aquatic biota Open water and temperatures rising from below to above 0°C lead to melting anchor ice that can jam up and produce local floods and upland ice that damage riparian and upland biota If high flows occur during periods with low temperatures and surface ice, water can be forced on top of the ice, often leading to floods, or the ice cover may break up and run the risk of jamming