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


Concentrations can reach toxic levels

Can be washed out from adjacent, otherwise unflooded uplands; dilution reduces but does not eliminate risk for toxicity


PPCP:s (Pharmaceuticals and Personal Care Products ) can become toxic; natural estrogens can feminize fish


Can lead to eutrophication and acidification; N levels can become toxic

Removed from watercourse by downstream transport, uptake by riparian vegetation and denitrification


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