Outdoor Glossary: I

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igneous rock
Rocks formed when high temperature, molten mineral matter cooled and solidified.

impermeable groin
A groins through which sand can not pass.

incident wave
Wave moving landward.

incremental shaft encoder
A component of a tide gauge for converting length to a shaft angle on a rotating disk. The position of the rotating disk is determined by single or dual optical or magnetic sensors to provide an electrical output. No electro-mechanical components or gears are used, so extremely low torque is required to move the float wheel, wire, and float mechanism.

Indian spring low water
A datum originated by Professor G. H. Darwin when investigating the tides of India. It is an elevation depressed below mean sea level by an amount equal to the sum of the amplitudes of he harmonic constituents M2, S2, K1, and O1. Indian tide plane: Same as Indian spring low water.

Indian tide plane
Same as Indian spring low water.

indicator species
A plant or animal species related to a particular kind of environment. Its presence indicates that specific habitat conditions are also present.

indigenous (species)
Any species of wildlife native to a given land or water area by natural occurrence.

individual tree selection
The removal of individual trees from certain size and age classes over an entire stand area. Regeneration is mainly natural, and an uneven aged stand is maintained.

inequality
A systematic departure from the mean value of a tidal quantity. See diurnal inequality, parallax inequality, and phase inequality.

inertial flow
A solution of the relative hydrodynamic equations of motion in which only the horizontal component of the Coriolis and centrifugal forces are balanced. This anticyclonic flow results from a sudden application and release of a driving force which then allows the system to continue on under its own momentum without further interference.

infragravity waves
Waves with periods above about 30 seconds generated by wave groups breaking in the surf zone. See LONG WAVES.

inlet
(1) A narrow strip of water running into the land or between islands. (2) An arm of the sea (or other body of water) that is long compared to its width, and that may extend a considerable distance inland.

inlet gorge
Generally, the deepest region of an inlet.

inner primaries
Group of primaries closest to the body.

inner secondaries
Group of secondaries closest to the body.

inner wing
Shoulder, secondaries and secondary coverts.

inshore
(1) The region where waves are transformed by interaction with the sea bed. (2) In beach terminology, the zone of variable width extending from the LOW WATER LINE through the BREAKER ZONE.

inshore current
Any current inside the surf zone.

instream flow
The quantity of water necessary to meet seasonal stream flow requirements to accomplish the purposes of the National Forests, including, but not limited to fisheries, visual quality, and recreational opportunities.

integrated pest management
an ecologically based pest-control strategy that relies on natural mortality factors such as natural enemies, weather, and crop management, and seeks to disturb these factors as little as possible. IPM evaluates alternatives for managing forest pest populations, based on consideration of pest-host relationships.

interdisciplinary team
A team of individuals with skills from different disciplines that focuses on the same task or project.

intermediate cut
The removal of trees from a stand sometime between the beginning or formation of the stand and the regeneration cut. Types of intermediate cuts include thinning, release, and improvement cuttings.

intermittent stream
A stream that flows only at certain times of the year when it receives water from streams or from some surface source, such as melting snow.

intermountain region
The portion of the USDA Forest Service, also referred to as Region Four, that includes National Forests in Utah, Nevada, southern Idaho, and southwestern Wyoming.

internal tide
A tidal wave propagating along a sharp density discontinuity, such as a thermocline, or in an area of gradually changing (vertically) density.

International Great Lakes Datum (1955) [IGLD (1955)]
Mean water level at Pointe-au-Pere, Quebec, on the Gulf of St. Lawrence over the period 1941 through 1956, from which geopotential elevations (geopotential differences) throughout the Great Lakes region are measured. The term is often used to mean the entire system of geopotential elevations rather than just the referenced water level. See low water datum (1).

International Hydrographic Organization (formerly Bureau)
An institution consisting of representatives of a number of nations organized for the purpose of coordinating the hydrographic work of the participating governments. It had its origin in the International Hydrographic Conference in London in 1919. It has permanent headquarters in the Principality of Monaco and is supported by funds provided by the member nations. Its principal publications include the Hydrographic Review and special publications on technical subjects.

international low water
A hydrographic datum originally suggested for international use at the International Hydrographic Conference in London in 1919, and later discussed at the Monaco Conference in 1926. The proposed datum, which has not yet been generally adopted, was to be "a plane so low that the tide will but seldom fall below it." This datum was the subject of the International Hydrographic Bureau's Special Publication No. 5 (March 1925) and No. 10 (January 1926), reproduced in the Hydrographic Review for May 1925 and July 1926.

intertidal
The zone between the high and low water marks.

intertidal zone (technical definition)
The zone between the mean higher high water and mean lower low water lines

interval
See lunitidal interval and lunicurrent interval.

intrinsic rate of increase (rm)
the maximum rate at which a population will grow if resources are unlimited.

inverse barometer effect
The inverse response of sea level to changes in atmospheric pressure. A static reduction of 1.005 mb in atmospheric pressure will cause a stationary rise of 1 cm in sea level.

iris
Colored part of eye.

iris
Thin, contractile disc with a central opening (the pupil) in front of the lens in vertebrate eyes

irminger Current
A North Atlantic Ocean current setting westward off the south-west coast of Iceland.

irregular waves
Waves with random wave periods (and in practice, also heights), which are typical for natural wind-induced waves.

irretrievable
One of the categories of impacts mentioned in the National Environmental Policy Act to be included in statements of environmental impacts. An irretrievable effect applies to losses of production or commitment of renewable natural resources. For example, while an area is used as a ski area, some or all of the timber production there is irretrievably lost. If the ski area closes, timber production could resume; the loss of timber production during the time that the area was devoted to winter sports is irretrievable. However, the loss of timber production during that time is not irreversible, because it is possible for timber production to resume if the area is no longer used as a ski area.

irreversible
A category of impacts mentioned in statements of environmental impacts that applies to non-renewable resources, such as minerals and archaeological sites. Irreversible effects can also refer to effects of actions that can be renewed only after a very long period of time, such as the loss of soil productivity.

isanostere
An isopleth of either specific volume anomaly or thermosteric anomaly.

isobar
An isopleth of pressure.

isobaric surface
A surface of constant or uniform pressure.

isobath
Line connecting points of equal water debth on a chart; a seabed contour.

isohaline
An isopleth of salinity. Constant or uniform in salinity.

isopleth
A line of constant or uniform value of a given quantity. See isanostere, isobar, isohaline, isopycnic, and isotherm.

isopycnic
An isopleth of density. Constant or uniform in density.

isostasy
The tendency of the Earths crust to maintain a state of near equilibrium, i.e., if anything occurs to modify the existing state, a compensation change will occur to maintain a balance. Isotope: An atom with a specified number of protons and a specified number of neutrons.

isotherm
An isopleth of temperature.