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Climate Change Impacts:
Water Supply and Pollution
Ontario's quarter-million lakes and countless rivers and streams hold about one-third of the world's available fresh water. The province's 11 million people rely on these waters, as well as on ground water and rainfall for drinking, agriculture, and industrial uses. Forty-five percent of Quebec's residents take their water from the St. Lawrence River, which flows from the Great Lakes.

As climate change takes effect, projected changes in rainfall, evaporation, and groundwater recharge rates will affect all freshwater users. Among the potential impacts of climate change with implications for water supply and pollution levels in Ontario are:

• A Decrease in Lake Levels
  Changes in the precipitation cycle and seasonal temperatures will affect lake levels. Overall, increasing temperatures will lead to a decline in ice cover and a greater rate of evaporation, causing lake levels to decline in both inland lakes and Ontario's four Great Lakes. As lake levels drop, costs to shipping on the Great Lakes are likely to increase, along with costs of dredging harbors and channels and of adjusting docks, water intake pipes, and other infrastructure.
  
  
• An Increase in Extraction Pressure
  With growing populations and projected drops in lake and ground water levels, climate change is expected to exacerbate an already contentious debate on water withdrawals from the Great Lakes. Conflicts may intensify as water shortages develop and pressure increases for more water for irrigation, drinking, and other human uses.
  
  
• A Reduction in Groundwater Recharge
  Reduced summer water levels are likely to diminsh the recharge of groundwater, which will cause small streams to dry up. This, in turn, will reduce the area of wetlands and result in poorer water quality and less habitat for wildlife.
  
  
• A Degradation of Wetlands' Flood-Absorbing Capacity
  Development and climate change will degrade the natural flood-absorbing capacities of wetlands and floodplains. More run-off in winter and spring from rain-on-snow events and in summer from intense downpours, cause increased flooding and erosion. These events in turn could increase pollution in lakes, rivers, and streams from municipal and farm run-off, which often enters surface waters untreated. Periodic droughts also concentrate sulfates and acids, and, when flushed into lakes during intense rains, increase acid stress in aquatic systems.