AMD An acronym with three possible meanings: acid mine drainage; abandoned mine drainage; or, less often, alkaline mine drainage.

ammonia volatization The transformation of ammonia carried in water, into gaseous form that is released to the air. It occurs when water is pulled up into plant tissue in the process of transpiration.

anticline An arch shaped fold in rocks in which the oldest rocks are in the core

aquifer A porous body of rock (unconsolidated gravel, for instance) or cracks within bedrock fissures through which groundwater moves. An aquifer stores and can yield, if tapped, significant amounts of water.

bedrock General term applied to solid rock underneath soil layers.

benthic habitat The bottom of a streambed where aquatic species live. Also, refers to materials such habitat are made of. In the Allegheny Watershed, clean, coarse, gravel creates the preferred benthic habitat for many fish and macroinvertebrates.

benthic organism The collection of organisms attached to or resting on the bottom of sediments

Benwood limestone A sedimentary type of rock, composed mainly of calcite and/or dolostone, with which the calcite is often interbedded

bioindicator A living plant or animal that by its presence and numbers in a particular habitat indicates that ecosystem's health and vitality. For instance, the presence of freshwater mussels in a river indicates good to excellent water quality.

biomass Referred to here as living plant tissues within an ecosystem involved in water and nutrient cycling, and/or reproduction. Typically defined as the total mass of all living organisms within an ecosystem, usually expressed in dry weight.

biota Plants and animals that occupy the same place together; i.e. stream biota.

chemical weathering The chemical action of water as it dissolves, transforms, or breaks down rocks and minerals into smaller pieces and particles.

coal Acarbon rich mineral deposit formed from the remains of fossil plants. These are deposited initially as peat, but burial and increase in temperatures at depth bring about physical and chemical changes.

Combined Sewage Overflow (CSO) In urban and suburban areas, runoff from roads, parking lots, and rooftops is often channeled into storm sewer pipes that combine with human sewage, that together flow to the sewage treatment plant. During storm events, human waste and mainly, storm water, overload the treatment plant's capacity, resulting in the discharge of both directly into creeks and rivers without treatment. This is called Combined Sewage Overflow (CSO). It occurs most often in townships and municipalities with aging infrastructure.

contaminant Organic or inorganic chemicals that upon contact or mixing, pollute stream or river waters. Suspended solids, such as silt in excess, can also contaminate water.

dendritic A branching tree-like pattern. Stream patterns in western PA are primarily dendriticÐthe main stem of a river has many tributaries which in turn are fed by many headwater streams. Seen from the sky, or depicted on a map, regional drainage patterns resemble a tree's many branches flowing into its trunk.

denitrification Conversion of water-borne nitrate or nitrite to gaseous products, mainly nitrogen or nitrous oxide. Denitrification occurs in the presence of bacteria mainly under anaerobic (without oxygen) conditions. Denitrification occurs for instance, in saturated wetland soils.

deposition Solid matter which is laid down on a surface by a natural process. Refers to the transport and laying down of materials carried by high waters that build up a floodplain, or contribute to the development of an island in mid-stream.

detritus Disintegrated matter or debris. In this case, organic bits and pieces of leaves and insects that are floating or suspended in stream waters contributing to the aquatic food chain.

ecoregions A land area with relative homogeneity in ecosystems that, under nonimpaired conditions, contain habitats, which should support similar communities of animals.

escarpment A steep slope or long cliff resulting from erosion or faulting that separates two relatively level areas of differing elevations. The Erie escarpment forms the northwest ridgeline that separates the Allegheny Watershed from the town of Erie and Lake Erie. This escarpment has a subtle elevation change of 800 feet over 2-3 miles.

fauna All the animal life normally present in a given habitat at a given time.

fen An area of wet peat that is typically alki\aline in reaction, or sometimes neutral or only slightly acidic.

floodplain The part of a river valley that is made of unconsolidated, river-borne sediment and is periodically flooded.

Functional Groups See River Continuum.

gradient Stream gradient is calculated by taking the elevation change of a streambed and dividing it by the distance traversed by the stream. Low gradient streams flow with less velocity, speed, and energy than higher-gradient streams. Stream gradient is also a determining factor of fish habitat.

groundwater Water occurring below the earth's surface, contained in pore spaces between soils, and cracks in bedrock. It either passes through or stands in the soil and underlying geologic strata, and moves freely under the influence of gravity.

headwaters, headwater stream A small creek or stream that begins in the highest elevations of a watershed. See stream order.

herbaceous Plants that are mostly leafy with little woody tissue.

humus The topmost layer found in forest soils. Humus is dark-colored and full of living and dead organic materials. It is a soil layer active with the decomposition of plants and animals. Its chemical composition is complex and variable. It holds water and improves the water-retaining properties of soil; humus enhances soil fertility and workability. See microbic soil activity

impervious (surface) Prevents water from entering soil directly. Impermeable: unable to penetrate. Impervious surface: a material, such as asphalt, that covers soil and does not allow water to penetrate into soil layers below; prevents infiltration.

infiltration The downward movement of water into soil. Infiltration capacity: the maximum rate at which soils and rocks can absorb rainfall and snow melt. Infiltration capacity tends to decrease as the soil moisture content of the surface layers increases. Factors influencing infiltration capacity: the size of soil, sand, gravel, or clay particles, the pore spaces between them, vegetation cover, and steepness of slope.

invertebrate An animal without a spine. Insects and freshwater mussels are invertebrates.

leachate The solution formed when water percolates through a permeable medium. When derived from solid waste, in some cases the leachate may be toxic or carry bacteria. In mining, leaching of waste tips can produce a mineral rich leachate which is collected for further processing.

macroinvertebrate A relatively large invertebrate (an animal without a spine), longer than 0.2-0.5mm; a dragonfly.

Marsh A more or less permanently wet area of mineral soil, as opposed to a peaty area, e.g. around the edges of a lake or on a floodplain of a river.

microbic soil activity Microbes are living microscopic organismsÐthere can be thousands to millions in a teaspoon of healthy forest soil. A microbe eats debrisÐleaves, dead insects, dead wood, pieces of mushroomsÐthen adds its corpse as organic residue, often in a matter of minutes. Aerobic (oxygen-using) microbes need air to breathe and for fermentation to take place. The materials they help to decompose biodegrade into a soft, dark, sweet-smelling humus.

Mississippian The penultimate period of the Paleozoic Era, preceded by the Devonian and followed by the Permian. It began about 355 Ma ago and ended about 286 Ma ago.

moraine A ridge of glacially deposited material, called till, laid down at the leading edge of a glacier, or during intermittent stages of advance and retreat. Moraines can range in height from 1-100 meters. Moraine State Park features a glacially deposited moraine.

morphology Refers to the form and dynamic structure of streams and rivers. Applies to the study of shapes and form of regional landscapes, or individual organisms.

non-point source pollution (NPS) Water pollution originating from a wide variety of common, everyday sources. Typical NPS pollution sources are contaminants on land that are carried off in surface runoff (during rainfall or snow melt) that make their way into nearby streams. Runoff can carry excess nutrients and chemicals from agriculture, golf courses, and lawns; salt from highways; oils from parking lots; and sediments from eroding streambanks.

Ordovician The second of six periods that constitute the Palaeozoic Era, named after an ancient Celtic tribe, the Ordovices. It lasted from about 505 to 438 Ma ago.

orogeny A mountain building episode. Refers to one episode in the formation of the Allegheny and Appalachian Mountains caused by intense deformation of rocks due to folding and faulting.

orographic event When the presence of a mountain or ridgeline forces moist air to rise, condense and form clouds, sometimes resulting in precipitation.

Pennsylvania sandstone A type of sedimentary rock, formed of a lithified sand and, and comprising grains between 63mm and 1000 mm in size and bound together with a mud matrix and a mineral cement

percolate The movement of water downward into soil layers. Displacement of air between soil particles by water. The rate of percolation is measured in inch(es) per hour.

permeate, permeable A material's ability to allow water to penetrate. Refers to water's ability to move down into soil layers.

photosynthesis The process by which plants create food for their growth by generating organic compounds from carbon dioxide (in air), water (in soil), in the presence of sunlight.

plateau Broad, comparatively flat areas of great extent and elevation usually 150-300 meters above the adjacent countryside. The Allegheny Plateau has a nearly smooth surface that is intensely dissected by deep valleys.

porous alluvium An alluvial deposit

precipitation Water that falls from clouds in the sky. Rain, snow, sleet, hail, even fog are different types of precipitation.

proto- The earliest form of something. Proto-continent refers to the earliest form of a continent.

point source pollution By-products emanating from end-of-pipe sources, such as industrial discharges, or smokestack emissions.

precipitation Water that falls from clouds in the sky. Rain, snow, sleet, hail, even fog are different types of precipitation.

ridgeline A topographic feature of connected highpoints. Ridgelines define and separate watersheds.

riffles The shallow zone within a sand or gravel streambed. Depending on stream height, riffles are usually noticeable by the small waves of an excited water surface.

riparian Riparian literally means "riverside". Riparian vegetation is vegetation that grows along streams and rivers. Riparian forests are well established stands of vegetation that include grasses, shrubs, and large, well-developed trees.

River Continuum Concept A holistic and integrated approach to complex stream dynamics that ties together stream order, temperature, sunlight, nutrient cycling, and instream habitats. Macroinvertebrates, like insects and freshwater mussels, are categorized into Functional Groups based on feeding adaptation and food preferences.

river terrace A flat, terrace-like fragment of a former river valley that stands well above the present-day floodplain and river channel. Troy Hill, near Pittsburgh, is a river terrace.

runoff Rain or snowmelt not absorbed into top soil layers because they're saturated with water. Precipitation not absorbed by impervious surfaces (i.e. parking lots) or saturated soils that runs over land to lower elevations. Factors affecting runoff: vegetation cover (slows it down), slope (the steeper the faster it moves), and land cover (may or may not allow for infiltration).

scour Erosion resulting from glacial action, where surface material is removed, and rock fragments carried on the underside of a moving glacier create friction that abrades, scratches, and polishes the bedrock. Scouring also occurs in streambeds where the constant movement and erosive action of fast waters remove the build-up of silt and mud.

seep, groundwater seep Water flowing above ground level where the water table emerges from the ground surface. Unlike a spring with obvious trickles, a seep is a dispersed delivery of groundwater.

Silviculture The management of forests or woodlands for the production of timber or other wood products. Sometimes the term is used interchangeably with ‘forestry’, but is less comprehensive.

spp. Abbreviation for species.

stream order Classification of stream sizes. Headwaters are the smallest, identified as 1, or 1st order. When two headwater (1) streams merge together, their combined volume and size create a larger stream, assigned a 2, 2nd order. This continues, until the Mississippi, which drains into the Gulf of Mexico, is considered a 10th order stream by some, a 12th by others. (Stream orders are hard to pin down and have not been used in the Watershed Atlas to describe stream types).

substrate An underlying layer. Soil substrate: deeper layers of soil that get closer to bedrock with depth. Geologic substrate: underlying layers of geologic rock. Stream substrate: materials making up the bottom of a streambed.

swamps A wet area that is normally covered by water all year and is not subject to drying out during the summer

till Till starts out as bedrock and boulders that are plucked away by the scouring action of a glacier moving across a landscape. Rocks are crushed by a glacier's weight and eroded over the distance traveled. The eroded rocks becomes till, which is typically made up of cobbles, gravel, sand, and silt.

transpiration Transpiration is the process of evaporation of internal fluids from a plant's leaves or needles which effectively lowers the pressure in a plant's outer reaches so that a flow of moisture from the roots is drawn upward to replace what escapes to the air. Trees are fantastic pumps, lifting water up their stems for 100 feet or more. A growing plant transpires 5-10 times as much water as it can hold at once.

tributary A smaller channel of water that drains into a larger body of running water: a creek that empties into a stream; a stream that empties into a river.

turbidity Solids that are suspended or stirred up in water; usually creates a cloudy, or muddy condition.

understory plants Young trees that occupy the lower canopy of a forest; shrubs, herbaceous plants (such as ferns), and seedlings that occupy the forest floor.

water gap A gap in a mountain ridgeline where a river has maintained a downward cutting path as the mountain rises; usually through a vertical zone of weakness within the geologic layers of the mountain. The Conemaugh Gorge is a seven-mile long water gap through Laurel Ridge.

water table The upper surface of groundwater, or the level below which soil or other material is permanently saturated with water. Stream height is an indication of the height of the water table.

watershed The landmass over which water travels from its highest elevations down to its main stream, or river. Also, considered a drainage basin.

woody debris Trees and/or limbs that fall into a streambed. Woody debris tends to alter stream morphology and create habitat niches.

Glossary definitions drawn from:
The Concise Oxford Dictionary of Ecology, Michael Allaby, editor. Oxford University Press, 1994
The American Heritage Dictionary of the English Language, William Morris, editor. American Heritage Publishing Co.