People expect drinkable, swimmable, boatable, and fishable freshwaters. However, rivers and lakes have many pressures. These pressures include land and water use for urbanization, agriculture, and aquaculture; hydrologic engineering structures such as dams, dykes, channelization, and construction of drainage and connecting canals; water extractions for industry, drinking water, irrigation, and power production; water pollution with toxics, excess nutrients, and suspended sediments; capture fisheries; and invasion of exotics. UV-B is an interactive pressure with climate change owing to the change in water clarity influenced by drought. Under these conditions, UV-B penetrates farther and causes more damage in clear waters than in murky waters.

Human demand for water in many areas will increase more rapidly with climate change and population increases and leave fewer waters unmodified by water projects (see Chapter 4). Water projects interact with many aspects of climate change as related to natural resource and environmental management. Additional dams will increase the difficulty of managing migratory fish populations in streams. Sedimentation that occurs above dams will reduce downstream transport of sand, sediments, and toxic substances. Lakes and reservoirs with increased water withdrawals will reduce the suitability of the littoral zone for fish spawning and nursery areas. Diversions of water by canals, ships, or pipes will transport exotics into new watersheds and confound biodiversity and exotics issues.

Many responses of lakes and streams to climate change were documented in the SAR by Arnell et al. (1996), and more have been added by Cushing (1997) and Domoto et al. (2000). Responses include warming of waters; reductions in ice cover; reduction in dissolved oxygen in deep waters; changes in the interaction between waters and their watersheds; changes in biogeochemical cycling; greater frequencies of extreme events, including flood and drought; changes in growth, reproduction, and distribution of organisms; and poleward movement of climate zones for organisms. Only a few are mentioned in the following subsections.

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