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La Niña — (Oceanography and Meteorology) [From the “little girl”] The name given to the weather phenomenon characterized by abnormally cold ocean surface water temperatures in the eastern Pacific Ocean near the equator. According to the National Oceanic and Atmospheric Administration (NOAA), during a La Niña event, temperatures are typically warmer than normal in the southeast United States and cooler than normal in the northwest, bringing drier than normal conditions to southern California and the Southwest U.S. With the cold water in the Pacific tropics characterizing a La Niña event, the chill, west-to-east high-altitude winds known as the jet stream no longer move southward attracted by the temperature differential which exists during the El Niño warm-water event. Therefore, instead of being “pulled” downward as the jet stream hurls across the United States, it tends to shift northward, producing unusually wet springs in the Northwestern U.S. and summer droughts in the mid-Atlantic region. It also means that there are no strong upper elevation winds in the middle Atlantic to blow the tops off of any big tropical storms forming, consequently allowing for the formation of more hurricanes.

Lacustrine — Pertaining to, produced by, or inhabiting a lake.

Lacustrine Deposits — Stratified materials deposited in lake waters and later become exposed either by the lowering of the water level or by the elevation of the land.

Lacustrine Wetlands — According to criteria of the U.S. Fish and Wildlife Service (USFWS), Lacustrine Wetlands are greater than 20 acres and have less than 30 percent cover of persistent vegetation. Also see Wetlands.

Lahontan Basin — A major basin within the Great Basin that is fed by the Truckee, Carson, Walker, Susan, Quinn and Humboldt Rivers. It has a drainage or hydrologic area of about 45,000 square miles and during the Pleistocene Epoch (11,000 years B.P. to approximately 1.8 million years B.P.) Contained the 8,600 square mile Lake Lahontan. The Lahontan basin encompasses much of northern Nevada and parts of eastern California and southern Oregon.

Lake Whitening — A phenomenon which occurs in moderately productive lakes when photosynthetic uptake of carbon dioxide (CO2) causes the precipitation of small particles of calcite (mostly calcium carbonate, CaCO3). Since small particles have a greater effect on water transparency and typical calcite particles are only 1–2 micrometers (µm) in diameter, lake water takes on a milky appearance, hence lending to its name.

Land Subsidence — The sinking or settling of land to a lower level in response to various natural and man-caused factors, for example:
[1] earth movements;
[2] lowering of fluid pressure (or lowering of ground water level);
[3] removal of underlying supporting materials by mining or solution of solids, either artificially or from natural causes;
[4] compaction caused by wetting (Hydrocompaction);
[5] oxidation of organic matter in soils; or
[6] added load on the land surface. With respect to ground water, subsidence most frequently results from overdrafts of the underlying water table or aquifer and its inability to fully recharge, a process termed Aquifer Compaction.

Large organic/ woody debris — Relatively large piece of stable, woody material that intrudes into a water body; complex or irregular woody debris is better for fish.

Leachate — Liquid which has percolated through the ground, such as water seeping through a sanitary landfill, wastes, pesticides, or fertilizers. Leaching may occur in farming areas, feedlots, and landfills, and may result in hazardous substances entering surface water, ground water, or soil.

Lemna Gibba (Duckweed) — The genus and species name of a small, stemless, free-floating plant used in experiments to determine the toxicity of pollutants to aquatic plant life. Commonly called duckweed.

Lentic — Characterizing aquatic communities found in standing water. Compare to Lotic.

Lentic System — A non-flowing or standing body of fresh water, such as a lake or pond. Compare to a Lotic System.

Lentic Waters — Ponds or lakes (standing water).

Lenticles — Tiny pores, into which oxygen passes, on the roots or branches of trees. For example, both red and black mangrove trees have lenticles on some of their roots.

Levee —
(1) A natural or man-made earthen obstruction along the edge of a stream, lake, or river. Also, a long, low embankment usually built to restrain the flow of water out of a river bank and protect land from flooding. If built of concrete or masonry, the structure is usually referred to as a flood wall. The term Dike is commonly used to describe embankments that block an area on a reservoir rim that are lower than the top of the main dam.
(2)(FEMA) A man-made structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control or divert the flow of water so as to provide protection from temporary flooding.

Levee (Natural) — Bank of sand and silt built by a river during floods, where the Suspended Load is deposited in greatest quantity close to the river. The process of developing natural levees tends to raise river banks above the level of the surrounding flood plains. A break in a natural levee is sometimes called a Crevasse.

Levee (Manmade) — An embankment, generally constructed on or parallel to the banks of a stream, lake or other body of water, for the purpose of protecting the land side from inundation by flood water or to confine the stream flow to its regular channel.

Light-and-Dark Bottle Technique — A method used to determine the extent of Photosynthesis in an aquatic Ecosystem. Duplicate portions of a water sample are collected. One portion is Incubated in a clear bottle, and the other is incubated in a dark, light-impermeable bottle. Following incubation for a prescribed time period, the net uptake of carbon dioxide in each is measured and compared.

Light Water Reactor (LWR) — A nuclear power plant which uses ordinary Water (H2O) as distinguished from one that uses Heavy Water (D2O) or Deuterium Oxide. Fission energy is released in the form of heat and is transferred to a conventional steam cycle which generates electric energy. Heat generated by the fission of the uranium fuel raises the temperature of the water, which is then pumped to heat exchange units for the production of steam and subsequent generation of electricity. The process results in a continuous transfer of heat from the reactor to the outside. The water also functions as a moderator to reduce the energy level of neutrons released by the fission process in order to allow the neutrons to promote additional fission events. The light-water reactor is the most
common type of nuclear reactor operated in the United States.

Limicolous — Living in mud.

Limnetic — Referring to a standing water Ecosystem (ponds or lakes); of, relating to, or inhabiting the open water of a body of fresh water, as a limnetic environment or Limnetic Zone.

Limnetic Zone — The open water of a pond or lake supporting Plankton growth. Contrast with Profundal Zone.

Limnology — The branch of Hydrology pertaining to the study of freshwater, the aquatic environment and its life; the study of the physical, chemical, hydrological, and biological aspects of fresh water bodies. Related terms include Limnological, Limnologic, and Limnologist.

Limnology Hydrobiologist — A person who undertakes the biological study of bodies of water.

Lindane — A pesticide that causes adverse health effects in domestic water supplies and is toxic to freshwater fish and aquatic life.

Lineament — (Geology) An essentially rectilinear topographic feature resulting from a fault or zone of faulting. Frequently such areas provide indications of available groundwater sources.

Liquefaction —
(1) (General) The act or process of making or becoming liquid; especially the conversion of a solid into a liquid by heat, or of a gas into a liquid by cold or pressure.
(2) (Soils) The sudden and spontaneous large decrease of the shearing resistance of a cohesionless soil, caused by a collapse of the structure from shock or other types of strain and associated with a sudden but temporary increase in the pore-fluid pressure resulting in the temporary transformation of the material into a fluid mass.

Littoral — The region along the shore of a non-flowing body of water; corresponds to Riparian for a flowing body of water. More specifically, the zone of the sea flood lying between the tide levels.

Littoral Transport — The movement of material along the shore by waves and currents.

Littoral Water Rights — The equivalent of Riparian Water Rights for a lake, reservoir, or other non-flowing body of water. As with riparian water rights, littoral water rights allow persons who own land adjacent to a body of water to make reasonable use of those waters on lands within the watershed. Littoral users share the waters among themselves and the concept of priority use (Prior Appropriation Doctrine) is not applicable. Under drought conditions, the lake or waterfront users also share shortages. Littoral rights cannot be sold or transferred to use on other (nonriparian) lands. Also see Riparian Doctrine, Riparian Water Rights, Appropriative Water Rights, Prescribed Water Rights, and Reserved Water Rights.

Littoral Zone —
(1) The shallow area near the shore of a non-flowing body of water; that portion of a body of fresh water extending from the shoreline lakeward to the limit of occupancy of rooted plants.
(2) A strip of land along the shoreline between the high and low water levels.

Load — The amount of material that a transporting agency, such as a stream, a glacier, or the wind, is actually carrying at a given time. Also, the amount of power delivered to a given point. In this respect:
[1] Base Load = The minimum load in a stated period of time.
[2] Firm Load = That part of the system load which must be met on demand.
[3] Peak Load = Literally, the maximum load in a stated period of time. Sometimes the term peak load is used in a general sense to describe that portion of the load above the base load.

Load Allocation (LA) — (Water Quality) The portion of the pollution Load of a stream attributable to human Nonpoint Sources (NPS) of pollution. The amount of pollution from each point source is the Wasteload Allocation.

Loading — The quantity of a substance (a contaminant) entering the receiving waters. Synonym for the pollution Load of a stream.

Loading Capacity — The greatest amount of chemical materials or thermal energy that can be added to a stream without exceeding water quality standards established for that stream.

Long-Term Acceptance Rate (of Soils) — A term used to describe the permeability or porosity of various soils and their ability to drain water; usually expressed in gallons per square foot per day.

Longitudinal Profile — A graphic presentation of elevation versus distance; in channel hydraulics it is a plot of water surface elevation against upstream to downstream distance.

Losing Stream — A stream or reach of a stream that is losing water by seepage into the ground. Also referred to as an Influent Stream. Also see Stream.

Losses Incidental to Irrigation — The quantity of water depleted by irrigation in excess of the beneficial irrigation consumptive use.

Lotic —
(1) Of, relating to, or living in moving water.
(2) Referring to a running water Ecosystem (streams and rivers). Compare to Lentic.

Lotic Environment — Characterizing aquatic communities found in running water. Also referred to as a Lotic Habitat.

Lotic System — A flowing body of fresh water, such as a river or stream. Compare to Lentic System.

Lotic Waters — Describing the waters of rivers and streams (flowing waters) as compared to Lentic Waters of ponds or marshes (standing waters).

Low-Level Drawdown — A discharge feature of a dam allowing water to be removed from the bottom of a reservoir.

Low-Level Outlet — An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases. Also referred to as Bottom Outlet or Sluiceway.

Lowest Annual Mean — A value used for river flow readings representing the lowest total annual volume (in acre-feet per year) and the corresponding lowest annual average rate of flow (in cubic feet per second) recorded at a specific gauging station location over a specific period of record. Also referred to as the Low Water Year.