USGS -- Water Resources Data

Streamflow

Water Quality

What are temperature, pH, dissolved oxygen, and turbidity, and why are these important?

To assess the water quality of a river or stream, certain biological, chemical, and physical characteristics are measured. Some of these measurements-such as temperature, acidity (pH), dissolved oxygen, and turbidity-are made in the field. They are routinely done to provide an immediate and preliminary indication of the sanitary quality and health of streams. They often serve as an early warning "screen" of potential pollution problems. Such measurements are analogous to the vital signs of a physical exam-a person's height, weight, temperature, blood pressure, etc. -that may signal more serious health issues.
       Temperature--a measure of the warmth or coldness of the water-is important because it affects food supply, photosynthesis, metabolism, and the solubility of gasses and minerals. For example, colder water can hold more dissolved oxygen, which is critical for fish and other aquatic organisms.
        pH-a measure of how acidic or basic the water is-can directly affect the survival of aquatic organisms. For example, waters with a pH less than 4 have virtually no vertebrate life forms.
        Dissolved oxygen--a measure of the molecular oxygen dissolved in water-is an important determinant of whether the water body is suitable for aerobic (oxygen-requiring) organisms, such as fish and zooplankton. More dissolved oxygen is better; values greater than 5 or 6 parts per million generally will support diverse forms of aquatic life.
         Turbidity--a measure of the clarity of water or the amount of plant debris, sand, silt, and clay in the water-affects the amount of sunlight reaching aquatic plants. Excess turbidity can reduce reproduction rates of aquatic life when spawning areas and eggs are covered with soil.

Are the field measurements enough?

Often not, because while these field measurements are still very important, over the last 25 years additional, complex issues also have emerged. Hundreds of synthetic organic compounds, like pesticides and volatile organic compounds in solvents and gasoline, have been introduced into the environment. Over the last five years, improved sampling and analytical techniques have led to the "discovery" of microbial and viral contaminants, pharmaceuticals, and hormones. Measurement of these and many other chemical compounds, such as nutrients, radon, and trace metals, are done in a laboratory-—analogous to the blood work associated with a physical exam that helps to specify and explain the possible reasons for, and extent of, a person's physical health and ailments. The more complete analysis of stream chemistry, along with biological measurements of aquatic organisms and stream habitat, is optimal to achieve a similar picture of water-quality conditions. More and more, professional water-quality specialists and volunteer program coordinators alike are moving towards the combined and more complete approaches, as possible, because they generally help to identify the possible sources of contamination-such as pollutants introduced from agricultural runoff, stormwater, industrial discharges, or natural sources-and help to prioritize strategies towards stream protection and restoration. Access water-quality and biological data collected by USGS or contact your State environmental agency for more information on water-quality data.

Detailed explanations of the importance of these parameters and sampling and equipment considerations are provided by the U.S. Environmental Protection Agency's Office of Water.

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