A stream is the central component of a landscape where water drains downslope to the lowest point via a network of drainage channels. The dynamic and complex drainage system may have taken centuries to become naturally established, and its ongoing processes can be interrupted or modified by human intervention. An assessment of stream health therefore must consider the stream's natural setting as well as how human activities have changed it. Also important to an assessment are evaluations of activities that can directly affect water quantity or quality. This suite of information helps managers make appropriate decisions to protect the existing good quality of a stream, or to restore a stream that is not healthy.
The geology , climate, and vegetation of a drainage basin provide the natural setting in which a stream develops. The shape of the land and the type of soil play a large role in determining a stream's characteristics. A fast flowing stream in a steep mountain setting takes on different characteristics than one that slowly meanders through a broad, flat valley. Climatic factors such as precipitation patterns help determine when and how much water is delivered to a stream. Vegetation along a stream corridor and on its banks influences its stability.
These natural features can be modified by human activities. Building dams, removing streambank vegetation, and pumping water from a stream are potentially harmful to stream health. Hence, historical information helps identify what the stream and its watershed may have looked like prior to human disturbance, where impacts may have occurred, and how human influence has changed the stream's characteristics. Quantifying stream flow indicates how a stream responds to precipitation events, identifies times of the year when stream flow is at a critical low, and, combined with water use data, determines whether water is being over-utilized. When too much water is removed or if flows are restricted, fish and other organisms living in the stream can be negatively affected.
Measuring water quality is extremely important in determining a stream's health. Water-quality experts have determined what levels of chemicals and microbes in stream water are healthy for a particular use, such as drinking, recreating, irrigating, or supporting fish. These safe levels have been adopted by state and federal governments as water-quality standards. If water quality in a stream is measured and compared to those standards, the health of the stream as it relates to that particular use can be determined.
For example, drinking water must be free of pathogens . Concentrations of harmful chemicals must be below the standards. Temperature, dissolved oxygen , metals, and chemical contaminants must be at levels that do not harm fish and other stream-dwelling organisms.
The water-quality component of a stream health assessment addresses six general categories: sediment , nutrients, bacteria, temperature, dissolved oxygen, and toxic chemicals.
Sediment is delivered to a stream through the erosion of upland areas or from the stream's banks. Too much sediment may cover gravel on a streambed, smothering eggs previously deposited in the gravel by spawning fish, or burying the gravelly substrates needed by aquatic insects. Measuring the amount and size of sediment in the stream water and on the streambed helps determine whether there is an excess and where it may be coming from.
Nutrients are chemicals that are commonly at low levels in natural fresh water. Too many nutrients can lead to excessive growth of aquatic plants, perhaps so excessive that the living and decaying vegetation robs a stream of its dissolved oxygen, which fish need to survive. Excessive plant growth also can interfere with human uses by plugging pumps that withdraw water, or by choking waterways used for recreation. Phosphorous and nitrogen are the major nutrients that cause this plant growth in water, and therefore must be measured to determine whether they are at levels high enough to stimulate the overabundant growth.
Coliform bacteria are used as an indicator of the sanitary quality of a stream's water for drinking and swimming. Fecal coliform bacteria are good indicators of human-caused pollution because they come from the gut of warm-blooded animals, including humans. Fecal coliform bacteria in streams can come from a variety of sources, including sewage, animal feedlots, pastureland, and cities. The discovery of fecal coliform bacteria in streams is an indication that disease-causing organisms may be present.
Cool water temperatures and high dissolved oxygen levels are critically important to fish. If the water is too warm or the oxygen level is too low, most fish will not be able to survive. Hence, measurements of temperature and dissolved oxygen are important indicators of stream health.
Chemicals are used for manufacturing, pest control, normal household use, and agriculture, forest, and city operations. They are intended to be safely handled and controlled, but occasionally find their way into streams. The result can be catastrophic to the point of killing everything living in a stream. Lower levels of toxic chemicals can have less catastrophic effects but still can be considered dangerous to humans who drink the water and eat the fish. It is important during a stream health assessment to determine whether these toxic chemicals are likely to be in the water by looking at potential sources. Once it is determined that a source of this toxic chemical is likely to exist, a stream assessment must focus on testing for the presence and level of that particular toxic chemical.
Because populations of fish and aquatic insects will respond to changes in stream quality and water quantity, determinations of their diversity and
Because vegetation near a stream greatly influences its health, measuring the type and size of this riparian vegetation is one of the most important components of a stream health assessment. Riparian vegetation influences fish habitat and stream quality by acting as a filter to help keep sediment and other pollutants out of streams. Shade from nearby trees and bushes keeps stream waters cool.
The phrase "Everyone lives downstream" has become a common slogan in water management. Water does not stay in one place, but instead moves as a result of natural forces. As streams flow downgradient from their headwaters , they pass through forests, farms, and communities on their way to larger streams and ultimately to the ocean. Along that path, any use of the water or its drainage basin in a manner that degrades water quality or changes how the stream acts will be passed on to other users downstream.
Downstream users inherit the impacts caused by upstream users. A few examples of watershed activities and potential downstream impacts follow.
In maintaining stream health and the human uses that a stream supports, it is critical to remember that what happens at a particular place within a watershed has the potential of influencing the characteristics of the watershed downstream. Hence, a good way to protect water quality is to undertake wise land-use planning that recognizes sensitive areas in watersheds. Perhaps the most important factors are for humans to use chemicals properly, dispose of wastes properly, and minimize pathways by which household chemicals reach streams.
SEE ALSO Ecology, Fresh-Water ; Erosion and Sedimentation ; Forest Hydrology ; Fresh Water, Natural Composition of ; Fresh Water, Physics and Chemistry of ; Land Use and Water Quality ; Microbes in Lakes and Streams ; Modeling Streamflow ; Nutrients in Lakes and Streams ; Pollution of Lakes and Streams ; Pollution Sources: Point and Nonpoint ; Stream Ecology: Temperature Impacts on ; Stream Hydrology.
Richard J. Pedersen
Governor's Watershed Enhancement Board. Oregon Watershed Assessment Manual, July 1999.
U.S. Environmental Protection Agency. Nonpoint Source Monitoring and Evaluation Guide, November 1996.
U.S. States Environmental Protection Agency. A Watershed Assessment Primer, December 1994.