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O

OASIS—A fertile or green spot in a desert or wasteland, made so by the presence of water.

OBLIGATE HYDROPHYTES—Species that are found only in Wetlands, e.g., cattail (Typha latifolia) as opposed to Ubiquitous Hydrophytes, species that grow either in wetland or on upland areas—e.g., red maple (Acer rubrum).

OBSERVATION WELL—A well used to monitor changes in water levels of an aquifer and to obtain samples for water quality analyses. Also see Wellhead Protection Program.

OBSIDIAN HYDRATION STUDIES—A method of determining the approximate age of an obsidian artifact by the measurement of the thickness of a microscopically visible "rind" on a flaked edge, resulting from the absorption of water.

OBSTRUCTION—Includes, but is not limited to, any dam, wall, wharf, embankment, levee, dike, pile, abutment, protection, excavation, channelization, bridge, conduit, culvert, building, wire, fence, rock, gravel refuse, fill, structure, vegetation or other material in, along, across or projecting into any watercourse which may alter, impede, retard or change the direction and/or velocity of the flow of water, or due to its location, its propensity to snare or collect debris carried by the flow of water, or its likelihood of being carried downstream.

OCAP (OPERATING CRITERIA AND PROCEDURES) [Nevada]—Operating criteria originally instituted in 1967 for water diversions and irrigation of the Newlands (Irrigation) Project [Nevada] in the Carson River Basin and designed to maximize use of Carson River flows to satisfy project requirements and minimize diversions from the Truckee River. Current OCAP requirements for this project were set in 1988 and according to Public Law 101-618 (the Negotiated Settlement) are to remain in effect at least through December 31, 1997 at which time a new Truckee River Operating Agreement (TROA) [Nevada and California] will be implemented.

OCEAN—Generally, the whole body of salt water which covers nearly three fourths of the surface of the globe. The average depth of the ocean is estimated to be about 13,000 feet (3,960 meters); the greatest reported depth is 34,218 feet (10,430 meters), north of Mindanao in the Western Pacific Ocean. The ocean bottom is a generally level or gently undulating plain, covered with a fine red or gray clay, or, in certain regions, with ooze of organic origin. The water, whose composition is fairly constant, contains on the average 3 percent of dissolved salts; of this solid portion, common salt forms about 78 percent, magnesium salts 15-16 percent, calcium salts 4 percent, with smaller amounts of various other substances. The density of ocean water is about 1.026 (relative to distilled water, or pure H2O). The oceans are divided into the Atlantic, Pacific, Indian, Arctic, and Antarctic Oceans.

OCEANAUT—A person trained to live in underwater installations and conduct, assist in, or be a subject of scientific research. Also called Aquanaut.

OCEAN DISCHARGE WAIVER—A variance from Clean Water Act (CWA) requirements for discharges into marine waters.

OCEAN DUMPING PERMIT—Under the Ocean Dumping Ban Act of 1988, which amended the 1972 Marine Protection, Research, and Sanctuaries Act (MPRSA), no new permits for the dumping of sewage sludge or industrial waste are allowed, and all holders of existing permits were forced to cease ocean dumping of these materials on December 31, 1991.

OCEAN FLOOR SEDIMENT—Unconsolidated materials that settle and accumulate on the floor of the deep ocean. These materials can be fine muds and clays, quartz grains, dust, glacial debris comprised of microscopic shells of plants or animals, and substances precipitated directly from seawater.

OCEANS (World)—The oceans of the world include the North and South Atlantic Oceans, the North and South Pacific Oceans, the Arctic Ocean, the Antarctic Ocean, and the Indian Ocean. Figuratively, these are referred to as the Seven Seas.

OCEAN THERMAL ENERGY CONVERSION (OTEC)—Electricity generation by making use of the temperature difference (as much as 20°C, or 68°F, in the tropics) between the top and bottom layers of the ocean to convert a fluid to vapor, which in turn powers a turbine generator. Low efficiency and saltwater corrosion are two current technical problems with the implementation of OTEC.

OCEAN THERMAL GRADIENTS—The temperature difference between deep and surface water in the oceans.

OCEANOGRAPHY—The science relating to the study of the ocean.

ODOR THRESHOLD—The minimum odor of a water sample that can just be detected after successive dilutions with odorless water. Also referred to as Threshold Odor.

OFF-CHANNEL USE—See Offstream Use.

OFF-LINE RESERVOIR—A reservoir constructed to the side of the main canal, usually in a natural drainage channel used to store surplus water runoff during the winter season for use during the irrigation season.

OFF-PISTE—(Sports) Existing or taking place on snow that has not been compacted into tracks.

OFFSET—(Irrigation) The difference between the controlled variable and the referenced input, for example, in a canal system, the difference between the actual water level in the canal and the water level at design flow.

OFFSHORE—Situated off the shore but within waters under a country's control, as offshore fisheries.

OFFSTREAM USE—Water withdrawn from a surface water source for uses such as irrigation, municipal and industrial (M&I) water supply, steam electric power generation, etc.

OFFSTREAM USE—Water withdrawn or diverted from a ground or surface-water source for use at another place. Examples of offstream use include public-water supply, industry, irrigation, livestock, thermoelectric power generation, and other uses. Also referred to as Off-Channel Use.

OGEE—A reverse curve shaped like an elongated letter S. The downstream faces of overflow dams are often made in this shape.

OIL SKIMMER—A device that collects and removes oil from a water surface. Ropes, belts, rotating drums, and similar devices are used as adhering surfaces for the oil, and the oil is pressed out or scraped off into a holding tank.

OIL SLICK—A layer of oil floating on the surface of water.

OIL SPILL—An accidental or intentional discharge of oil which reaches bodies of water. Can be controlled by chemical dispersion, combustion, mechanical containment, and/or adsorption. Spills from tanks and pipelines can also occur away from water bodies, contaminating the soil, getting into sewer systems and threatening underground water sources.

OLD—(1) (Geology) Having become slower in flow and less vigorous in action. Used of a river. (2) (Geography/Topography) Having become simpler in form and of lower relief; well advanced toward reduction by running water to the lowest level possible. Used of topographic features or a landform.

OLD FIELD—Cropland that is no longer used to produce an agricultural crop and that has been allowed to revert to natural plant cover.

OLD GROWTH—Forests that either have never been cut or have not been cut for many decades. Forests characterized by a large percentage of mature trees.

OLIGOHALINE—Term to characterize water with salinity of 0.5 to 5.0 0/00 (parts per thousand), due to ocean-derived salts.

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