Several aspects of climate change may lead to impacts on water quality. There is a consensus that the broad-scale hydrological cycle will intensify as the climate warms, with water quality adversely affected by the impacts of warmer temperatures, increased frequency of low-flow conditions, and possible increases in the intensity of episodic high-precipitation events. These two extremes of the hydrologic cycle, flooding and drought, pose potential threats to water quality.

At one extreme, heavy precipitation events may result in increased sediment and non-point source pollutant loadings to watercourses. This may make water treatment more difficult. Floods, in particular, increase the risk of water source contamination from sewage overflows, and runoff from agricultural land and urban areas. The location of water infrastructure, including both intakes and pipe distribution networks, could be increasingly vulnerable to precipitation extremes. Physical damage to dams and water operations and treatment facilities is a possible consequence of severe floods.  Regions with combined sewage and storm runoff systems could have more frequent sanitary control problems due to flooding.

At the other extreme, where streamflows and lake levels decline, water quality deterioration is likely as nutrients and contaminants become more concentrated in reduced volumes with longer water residence times. Warmer water temperatures may have further direct impacts on water quality, such as reducing dissolved oxygen concentrations. Cold-water species, such as most salmon and trout, are particularly susceptible to warm water temperatures, and increasingly frequent warm water conditions could bring new challenges to the way managed river systems are controlled. In addition, evaporative water losses could increase the salinity of surface waters, especially in lakes and reservoirs with long residence times. These stresses on water quality will increase if climate change leads to longer dry spells. Contaminants tend to accumulate on land surfaces during prolonged droughts. Pulses of contaminated runoff can occur when precipitation returns.  Water quality impacts are, therefore, likely to be rather complex and will vary with the physical, geographical and biological details of each water supply.