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E. Coli (Escherichia Coli) — A bacterial species which inhabits the intestinal tract of man and other warm-blooded animals. Although it poses no threat to human health, its presence in drinking water does indicate the presence of other, more dangerous bacteria.

Ecohydraulics — A multi-disciplinary research focus among biologists and engineers aimed at improving sampling and modeling techniques, which integrates physical, chemical and ecological processes in aquatic ecosystems. The principal focus involves aquatic model development, verification and validation studies, multi-disciplinary assessment frameworks, river restoration and monitoring strategies, and applications of multi-disciplinary integrated assessment methodologies utilizing geographic information systems (GIS), remote sensing and decision support systems.

Ecological studies — Studies of biological communities and habitat characteristics to evaluate the effects of physical and chemical characteristics of water and hydrologic conditions on aquatic biota and to determine how biological and habitat characteristics differ among environmental settings in NAWQA Study Units.

Ecological Succession — An orderly, directional and therefore predictable process of development that involves changes in species structure and community processes over time. It results from a modification of the physical environment by the community and culminates in a stabilized ecosystem in which maximum biomass and symbiotic functions are maintained.

Ecoregion — An area of similar climate, landform, soil, potential natural vegetation, hydrology, or other ecologically relevant variables.

Ecosystem — The interacting populations of plants, animals, and microorganisms occupying an area, plus their physical environment.

Ecosystem Structure —
(1) The physical elements and spatial arrangement of the living and nonliving elements within an Ecosystem.
(2) Attributes related to instantaneous physical state of an ecosystem; examples include species population density, species richness or evenness, and standing crop Biomass.

Ecosystem Sustainability — The capacity of an Ecosystem for long-term maintenance of ecological processes and functions, biological diversity, and productivity.

Ecotone —
(1) A habitat created by the juxtaposition of distinctly different habitats; an edge habitat; or an ecological zone or boundary where two or more ecosystems meet.
(2) A transition line or strip of vegetation between two communities, having characteristics of both kinds of neighboring vegetation as well as characteristics of its own.

Ecotype — A locally adopted population of a species which has a distinctive limit of tolerance to environmental factors.

Effluent —
(1) Something that flows out or forth, especially a stream flowing out of a body of water.
(2) (Water Quality) Discharged wastewater such as the treated wastes from municipal sewage plants, brine wastewater from desalting operations, and coolant waters from a nuclear power plant.

Effluent Limitation — An amount or concentration of a water pollutant that can be legally discharged into a water body by a Point Source (PS), expressed as the maximum daily discharge, the maximum discharge per amount of product, and/or the concentration limit in the wastewater stream, as a 24–hour or 30–day average. The applicable technology-based standard is set by the U.S. Environmental Protection Agency (EPA) by Standard Industrial Classification (SIC) Code, but differs between new and existing sources and by broad types of water pollutants: conventional pollutants, toxic pollutants, nonconventional, nontoxic pollutants; dredge and fill wastes; and heat discharges.

Effluent Standard — The maximum amounts of specific pollutants allowable in wastewater discharged by an industrial facility or wastewater treatment plant. The standards are set for individual pollutants and apply across all industrial categories. This term can be contrasted with Effluent Limitations, which are set for individual pollutants by Standard Industrial Classification (SIC) Code.

Elutriate — To purify, separate, or remove by washing, decanting, and settling.

Elutriation — Separation of solid waste into heavy and light fractions by washing.

Eluviation —
(1) The removal of soil material in suspension (or in solution) from a layer or layers of a soil.
(2) The transportation of dissolved or suspended material within the soil by the movement of water when rainfall exceeds evaporation.

Embankment — An artificial deposit of material that is raised above the natural surface

Emergent Hydrophytes — Erect, rooted, herbaceous Angiosperms that may be temporarily to permanently flooded at the base but do not tolerate prolonged inundation of the entire stem or plant. Familiar examples are cattails, bulrushes, and saltmarsh cordgrass.

Empirical — (Statistics) Based on experience or observations, as opposed to theory or conjecture.

Encroachment —
(1) Any physical object placed in the floodplain that hinders the passage of water or otherwise affects flood flows, such as fill, excavation, storage of equipment and materials, or buildings.
(2) The advance or infringement of uses, plant growth, fill, excavation, buildings, permanent structures or development into a floodplain which may impede or alter the flow capacity of a floodplain.

Endamoeba Histolytica — A waterborne disease organism causing amoebic dysentery.

Endemic — (Ecology) Confined to, or Indigenous in, a certain area or region, as an endemic plant or animal.

Endogenous — (Geology) Derived from within; geologic processes originating from internal causes within the earth or magma.

Endoreic — A term used to describe areas with terminal lakes and an interior drainage basin. Approximately 27 percent of the earth’s total land surface is endoreic; only about 5 percent of the North American continent is endoreic.

Endosmosis, also Endosmotic — The inward flow of a fluid through a permeable membrane toward a fluid of greater concentration.

Endrin — A pesticide toxic to freshwater and marine aquatic life that produces adverse health effects in domestic water supplies.

Enforceable Requirements — Conditions or limitations in permits issued under the Clean Water Act (CWA), Section 202 or 404, that, if violated, could result in the issuance of a compliance order or initiation of a civil or criminal action under federal or applicable state laws.

Enterococcus Bacteria — Bacteria commonly found in the feces of humans and other warm-blooded animals. Their presence in water is considered to verify fecal pollution. They are characterized as gram-positive, cocci bacteria that are capable of growth in brain-heart infusion broth. In the laboratory, they are defined as all the organisms that produce red or pink colonies within 48 hours at 35 degrees centigrade plus or minus 1 degree centigrade on KF-streptococcus medium (nutrient medium for bacterial growth). Their concentrations are expressed as number
of colonies per 100mL of sample.

Entrainment — (Streams)
(1) To be moved by water motion involuntarily.
(2) The incidental trapping of fish and other aquatic organisms in the water, for example, used for cooling electrical power plants or in waters being diverted for irrigation or similar purposes.

Entrapment Zone — An area of an estuary or other watercourse where seaward-flowing fresh water overlays more dense, saline ocean water resulting in a two-layer mixing zone characterized by Flocculation, aggregation, and accumulation of suspended materials from upstream.

Environmental Defense Fund (EDF) — A national, non-profit environmental and conservation organization active in legal, economic, and scientific aspects of environmental issues. The EDF employs scientists, attorneys, economists, computer modelers, and other environmental professionals whose purpose is to propose practical and economically feasible solutions to major environmental problems. The EDF has been responsible for a number of important environmental law cases coming to the attention of the courts in the United States. The EDF is headquartered in New York City and has six other offices across the United States. The EDF was founded in the
early 1970s when scientists documented the effects of the pesticide DDT on humans, wildlife, and the environment. The EDF subsequently joined with scientists and attorneys and successfully campaigned to have DDT banned nationwide in 1972. Currently, major EDF projects include: (1) limiting the greenhouse effect and climate change; (2) improving air quality; (3) tracing and blocking the sources of ocean pollution; (4) enforcing and extending the Endangered Species Act (ESA); (5) limiting chemical pollution and its effects on human health and the environment; (6) promoting water and energy conservation; (7) encouraging recycling and the reduction of solid
waste; and (8) protecting endangered land areas such as Antarctica and the rain forests in Brazil, West Africa, and Indonesia.

Environmental Impact Statement (EIS) — A report required by Section 102(2)(c) of Public Law 91–190, National Environmental Policy Act (NEPA), for all major projects which significantly impact on the quality of the human environment or are environmentally controversial. The EIS is a detailed and formal evaluation of the favorable and adverse environmental and social impacts of a proposed project and its alternatives. A tool for decision making, the EIS describes the positive and negative effects of an undertaking and cites possible, less environmentally disruptive alternative actions. Also see Environmental Assessment (EA).

Environmental Indicator — A measurement, statistic or value that provides a proximate gauge or evidence of the effects of environmental management programs or of the state or condition of the environment.

Ephemeral (Stream) — A stream that flows only in direct response to precipitation, and thus discontinues its flow during dry seasons. Such flow is usually of short duration. Most of the dry washes of more arid regions may be classified as ephemeral streams.

Epilimnion — The warm upper layer of a body of water with thermal stratification, which extends down from the surface to the Thermocline, which forms the boundary between the warmer upper layers of the emilimnion and the colder waters of the lower depths, or Hypolimnion. The epilimnion is less dense than the lower waters and is windcirculated and essentially homothermous. Also see Thermal Stratification, Fall Overturn, and Spring Overturn.

Equal Discharge Increment (EDI) — A method used in measuring suspended sediment in a stream wherein samples are obtained at the centroids of equal discharge increments. This method requires knowledge of the flow distribution in the stream cross section, but can save time over the Equal Transit Rate (ETR) method because fewer verticals are required.

Equal Transit Rate (ETR) — A method used in measuring Suspended Sediment in a stream wherein the sample volume taken is proportional to the streamflow at each of several equally spaced verticals. This technique results in a gross sample proportional to the total streamflow.

Equinoctial — A violent storm of wind and rain occurring at or near the time of the equinox.

Equipotential Surface — A surface (or line) in a three-dimensional ground-water flow field such that the total hydraulic head is the same everywhere on the surface.

Erosion Ballon — A metaphorical term for commonly obovately shaped, eroded sideslope areas that normally empty into an incised drainageway and are surrounded by non-eroded sideslopes.

ESA (Endangered Species Act) — An act passed by Congress in 1973 intended to protect species and subspecies of plants and animals that are of “aesthetic, ecological, educational, historical, recreational and scientific value.” It may also protect the listed species’ “critical habitat”, the geographic area occupied by or essential to the species. The U.S. Fish and Wildlife Service (USFWS) and the National Marine Fisheries Service (NMFS) share authority to list endangered species, determine critical habitat and develop recovery plans for listed species. As of July, 1993,
nationwide, some 728 plants and animals were on the federal threatened or endangered list. Further, under a settlement with environmental groups, USFWS has agreed to propose listing another 400 species over the next few years.

Euphotic — Of, relating to, or being the uppermost layer of a body of water that receives sufficient light for Photosynthesis and the growth of green plants. Also see Euphotic Zone.

Euphotic Zone — An area, particularly in regard to lakes, where there is sufficient light for Photosynthesis to take place. Contrast with Bathyal Zone and Abyssal Zone.

Eurybathic — Capable of living in a wide range of water depths. Use of an aquatic organism.

Euryhaline — Capable of tolerating a wide range of salt water concentrations. Use of an aquatic organism.

Eutrophic (Water) — Pertaining to a lake or other body of water characterized by large nutrient concentrations such as nitrogen and phosphorous and resulting high productivity. Such waters are often shallow, with algal blooms and periods of oxygen deficiency. Slightly or moderately eutrophic water can be healthful and support a complex web of plant and animal life. However, such waters are generally undesirable for drinking water and other needs. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum
chlorophyll–a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll–a, maximum phosphorus). Also see Carlson’s Trophic State Index (TSI) and (Mean) Trophic State Index (TSI).

Eutrophication —

(1) The degradation of water quality due to enrichment by nutrients, primarily nitrogen (N) and phosphorus (P), which results in excessive plant (principally algae) growth and decay. When levels of N:P are about 7:1, algae will thrive. Low dissoved oxygen (DO) in the water is a common consequence.
(2) The process of enrichment of water bodies by nutrients. (3) Over-enrichment of a lake or other water body with nutrients, resulting in excessive growth of organisms and the depletion of oxygen. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll–a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll–a, maximum phosphorus). Eutrophication of a lake normally contributes to its slow evolution into a Bog or Marsh and ultimately to dry land. Eutrophication may be accelerated by human activities and thereby speed up the aging process. Also see Carlson’s Trophic State Index (TSI) and (Mean) Trophic State Index (TSI).

Exposure Indicator — A characteristic of the environment measured to provide evidence of the occurrence or magnitude of a response indicator’s exposure to a chemical or biological stress.

Extinction Depth — The minimum depth from the surface to the groundwater table at which plant species that rely on groundwater can no longer survive.

Extirpated Species — A species rendered extinct in a given area.

Extirpation — (Biology) To destroy or remove completely, as a species from an particular area, region, or habitat.