taxa - Categories in the biological classification system for all living organisms. They are used to help organize information about the natural world.

tertiary consumer - A carnivore that feeds on other carnivores.

threatened species - A species that is close to being endangered.

tolerance - The ability to endure; resistance to toxic substances or other pollutants.

topsoil - The surface layer of soil, usually rich in humus.

total dissolved solids (TDS) - Dissolved and suspended solids in water; material left behind after a water sample is filtered and evaporated. Rainwater will have TDS less than 10ppm; municipal water systems will have TDS less than 500 ppm.

total phosphorus - A test that measures all the forms of phosphorus in a sample.

toxic - A substance that is harmful or, in some cases, poisonous, if ingested or touched. A substance that damages the pristine state of the environment.

transect - A long, narrow sample study area.

trash - Dry waste material, such as boxes and cans. image

tree of heaven - Ailanthus altissima; an exotic invasive plant, originally from China, that grows at the wood's edge. image

tributary - A smaller river or stream that flows into a larger river or stream. Usually, a number of smaller tributaries merge to form a river.

turbidity - The amount of solid particles suspended in water that cause light rays shining through the water to scatter. Turbidity is measured in nephelometric turbidity units (NTUs) or Jackson turbidity units (JTUs).

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unbiased - Impartial; without prejudice.

urban - Pertaining to or constituting a city.

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variable - A condition in a scientific experiment or observation that is subject to change. A variable factor in which change occurs naturally or is made to occur by the investigator is called the independent variable; a variable that changes as a result of change in the independent variable factor is called the dependent variable.

vegetation - Plants or plant life, in general. The condition of the vegetation around a stream is a good indication of the health of the aquatic environment.

verge vegetation - Vegetation that starts at the top of the stream bank and extends from the bank to the next major vegetation or land use change. image

velocity of a stream - The speed and direction of the water flowing in a stream, an important factor in determining what organisms can live in the stream. Measured in units such as feet/sec or meters/sec.

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water penny - Aquatic macroinvertebrate of the family Psepheridae; larvae are very flat oval or round shapes and are tan, brown, or black in color; have six small legs and cling to the undersides of rocks; sensitive to pollution. image

water quality index (WQI) - A method for measuring water quality in rivers. Nine parameters are measured and weighted to develop the index: dissolved oxygen, fecal coliform, pH, biochemical oxygen demand, temperature change, total phosphates, nitrates, turbidity, and total solids.

water snipe - Aquatic macroinvertebrate; family Athericidae; pale green tapered body with many caterpillar-like legs, conical head, feathery "horns" and back end; somewhat sensitive to pollution.

waterfowl - Birds whose primary habitat is aquatic.

watershed - A land area that drains water to a particular stream, river, or lake. Its boundary can be identified by locating the highest points of lands around the waterway.

weed - A plant considered unattractive, undesirable, or troublesome.

wetland - A lowland habitat, such as a marsh, swamp, or bog that has periodically waterlogged soils or is covered with a shallow layer of water resulting in reduced soil conditions yet it still permits standing vegetation.

wild celery - A native SAV, Vallisneria americana. It is found in freshwater rivers and tributaries of the Chesapeake Bay. It has linear ribbon-like leaves, 1.5 m long and 1 cm wide, emerging from the base of the plant.

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zooplankton - A diverse group of small protists and animals, such as tiny crustaceans, that serve as food for larger freshwater and marine invertebrates.

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