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Heavy rain can cause sediments to become suspended in water, reducing its quality, as seen in the brown swath above in New York City's Ashokan reservoir following Hurricane Floyd in September 1999. but increased water demands by society in regions that already rely on groundwater will clearly stress this resource, which is often drawn down faster than it can be recharged.164 In many locations, groundwater is closely connected to surface water and thus trends in surface water supplies over time affect groundwater. Changes in the water cycle that reduce precipitation or increase evaporation and runoff would reduce the amount of water avail- able for recharge. Changes in vegetation and soils that occur as temperature changes or due to fire or pest outbreaks are also likely to affect recharge by altering evaporation and infiltration rates. More frequent and larger floods are likely to increase groundwater recharge in semi -arid and and areas, Water II'' tesouir °ceem where most recharge occurs through dry stream - beds after heavy rainfalls and floods 141 Sea -level rise is expected to increase saltwater intrusion into coastal freshwater aquifers, making some unusable without desalination 146 Increased evaporation or reduced recharge into coastal aquifers exacerbates saltwater intrusion. Shallow groundwater aquifers that exchange water with streams are likely to be the most sensitive part of the groundwater system to climate change. Small reductions in groundwater levels can lead to large reductions in streamflow and increases in ground- water levels can increase streamflow.165 Further, the interface between streams and groundwater is an important site for pollution removal by microor- ganisms. Their activity will change in response to increased temperature and increased or decreased streamflow as climate changes, and this will affect water quality. Like water quality, research on the impacts of climate change on groundwater has been minimal 149 t° ° :Illiiiinaut:(r (:Iii aiinqg iii III III IIpIiac(r a udkliiit;ioiriiaulll uuurd1k::Nru� oiru� au�Illur(� au�(l t:ur(� ss( :NJ1 aut:(rrr t:(rins. In many places, the nation's water systems are al- ready taxed due to aging infrastructure, population increases, and competition among water needs for Lake Superior Summer Air and Water Temperatures 1979 to 2006 I 918O 1985 19910 11199 5 201010 2005 Year Austin and C:o1rnan166 The recent large jump in summer water temperature is related to the recent large reduction in ice cover (see Midwest region). farming, municipalities, hydropower, recre- ation, and ecosystems .167-169 Climate change will add another factor to existing water management challenges, thus increasing vulnerability.170 The U.S. Bureau of Recla- mation has identified many areas in the West that are already at risk for serious conflict over water, even in the absence of climate change 171 (see figure next page). Adapting to gradual changes, such as changes in average amounts of precipitation, is less difficult than adapting to changes in extremes. Where extreme events, such as droughts or floods, become more intense or more frequent with climate change, the eco- nomic and social costs of these events will increase. 172 Water systems have life spans of many years and are designed with spare 47