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Fresh Surface Water
Water Chapter
- Introduction
- Overview
- List of Questions
- Introduction
- Overview
- Discussion
- HD National Trend
- List of Indicators
- References
Fresh Surface Water
- Macara Lousberg
Office of Water
roe@epa.gov
Chapters
What are the trends in extent and condition of fresh surface waters and their effects on human health and the environment?
Though lakes, ponds, rivers, and streams hold less than one thousandth of a percent of the water on the planet, they serve many critical functions for the environment and for human life. These fresh surface waters sustain ecological systems and provide habitat for many plant and animal species. They also support a myriad of human uses, including drinking water, irrigation, wastewater treatment, livestock, industrial uses, hydropower, and recreation. Fresh surface waters also influence the extent and condition of other water resources, including ground water, wetlands, and coastal systems downstream.
The extent of fresh surface waters reflects the influence and interaction of many stressors. It can be affected by direct withdrawal for drinking, irrigation, industrial processes, and other human use, as well as by the withdrawal of ground water, which replenishes many surface waters. Hydromodifications such as dam construction can create new impoundments and fundamentally alter stream flow. Land cover can affect drainage patterns (e.g., impervious pavement may encourage runoff or flooding). Weather patterns—e.g., the amount of precipitation, the timing of precipitation and snowmelt, and the conditions that determine evaporation rates—also affect the extent of fresh surface waters. Changing climate could also affect the extent of fresh surface water that is available.
The condition of fresh surface waters reflects a range of characteristics. Physical characteristics include attributes such as temperature and clarity. Chemical characteristics include attributes such as salinity, nutrients, and chemical contaminants (including contaminants in sediments, which can impact water quality and potentially enter the aquatic food web). Biological characteristics include diseases, pathogens, and—in a broader sense—the status of plant and animal populations and the condition of their habitat. In addition to their effects on the environment, many of these characteristics can ultimately affect human health, mainly through drinking water, recreational activities (e.g., health effects in swimmers due to pathogens and harmful algal blooms), or consumption of fish and shellfish. Because these three topics are complex and encompass many types of water bodies, each is addressed in greater detail in its own section of this report (see Sections 3.6, 3.7, and 3.8, respectively).
Like extent, the condition of fresh surface waters can be influenced by a combination of natural and anthropogenic stressors, such as:
- Point source pollution, including contaminants discharged directly into water bodies by industrial operations, as well as nutrients and contaminants in sewage. Even treated sewage contains nutrients that affect the chemical composition of the water.
- Nonpoint source pollution, which largely reflects contaminants, nutrients, and excess sediment in runoff from urban and suburban areas (e.g., stormwater) and agricultural land. Other sources include recreational activities (e.g., boating and marinas) and acid mine drainage. Nonpoint source pollution can be influenced by land cover (e.g., impervious surfaces that encourage runoff) and land use (e.g., certain forestry techniques and agricultural practices that encourage runoff and erosion). Nonpoint sources tend to be more variable than point sources. For example, pesticide concentrations in streams reflect the location and timing of pesticide application.
- Air deposition. Acidic aerosols, heavy metals, and other airborne contaminants may be deposited directly on water or may wash into water bodies after deposition on land. For example, mercury emitted to the air from combustion at power plants can be transported and deposited in lakes and reservoirs.
- Invasive species. Invasives are non-indigenous plant and animal species that can harm the environment, human health, or the economy.1 Invasive species can crowd out native species and alter the physical and chemical condition of water bodies.
- Natural factors. Precipitation determines the timing and amount of runoff and erosion, while other aspects of weather and climate influence heating, cooling, and mixing in lakes—which affect the movement of contaminants and the cycling of nutrients. The mineral composition of bedrock and sediment helps determine whether a water body may be susceptible to acidification.
The condition of fresh surface waters also may be influenced by extent. Stream flow patterns influence contaminant and sediment loads, while changes in the shape of water bodies—e.g., eliminating deep pools or creating shallow impoundments—can change water temperature. The extent of surface waters also represents the extent of habitat—a key aspect of biological condition. Some plant and animal communities are sensitive to water level (e.g., riparian communities), while others may be adapted to particular seasonal fluctuations in flow. Stressors that affect extent may ultimately affect the condition of freshwater habitat—for example, hydromodifications that restrict the migration of certain fish species.