Fresh Surface Water
What are the trends in the extent and condition of fresh surface waters and their effects on human health and the environment?
Importance of Fresh Surface Waters
Lakes, ponds, rivers, and streams hold less than one thousandth of a percent of the water on the planet, but 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.
Extent and Condition
The state of fresh surface waters can be monitored using two broad categories of indicators: extent indicators and condition indicators.
- Extent: The extent of fresh surface waters roughly equates to the quantity of water and reflects a variety of human and environmental 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.
- Condition: The condition of fresh surface water roughly equates to its quality, and reflects a range of physical, chemical, and biological 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 the ROE (see Drinking Water, Recreational Waters, and Consumable Fish and Shellfish).
Like extent, the condition of fresh surface waters can be influenced by a combination of natural and anthropogenic stressors, such as:
- Point source pollution. Point source pollution includes contaminants discharged directly into water bodies by industrial operations, as well as nutrients and contaminants in sewage.
- Nonpoint source pollution. Nonpoint source pollution 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 non-permitted 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 in 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.
- Extent. 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.
Extent as Habitat
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 fresh surface water habitat—for example, hydromodifications that restrict the migration of certain fish species.
The ROE includes nine indicators that relate to fresh surface waters.
- One of these indicators (Stream Flows) presents information about stream flow patterns, which is an aspect of surface water extent.
- Another (Freshwater Withdrawals) looks at the amount of water that the nation withdraws from surface water bodies every year.
- The other seven indicators characterize various aspects of condition, including the physical condition of sediments (Streambed Stability), the condition of biological communities (Benthic Macroinvertebrates in Wadeable Streams), and the chemical condition of the water itself (Acidity in Lakes and Streams, Nitrogen and Phosphorus in Agricultural Streams, Nitrogen and Phosphorus in Wadeable Streams, Nitrogen and Phosphorus in Large Rivers, and Pesticides in Agricultural Streams).
Among these ROE indicators, the issue of nutrients has the strongest coverage. The ROE fresh water indicators collectively show the presence of elevated nutrients in waters ranging from small streams to large rivers, and the coastal indicators illustrate how these excess nutrients ultimately lead to problems offshore. The ROE lacks indicators on some other aspects of condition.
EPA is exploring the inclusion of new indicators, both for additional aspects of condition and for additional freshwater resources. Under the National Aquatic Resource Surveys, EPA and states implement a series of studies using consistent field and lab protocols to generate nationally consistent, unbiased estimates of the condition of the nation's waters. The National Lakes Assessment includes trophic condition, biological condition, and key stressors such as nitrogen, phosphorus, and lake shore habitat. It also includes human health-related indicators of blue-green algae and microcystin, a toxic form of algae.
 National Invasive Species Council. 2005. Five year review of Executive Order 13112 on invasive species. Washington, DC: U.S. Department of the Interior.