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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

lag (basin)

The measure of the time between the center of mass of precipitation to the center of mass of runoff (on the hydrograph). Basin lag is a function of not only basin characteristics, but also of storm intensity and movement. Some hydrologic texts define lag from the center of mass of rainfall to the hydrograph peak.

lag (time)

The time it takes a flood wave to move downstream.

laminar flow

Streamline flow in which successive flow particles follow similar path lines and head loss varies with velocity to the first power.

Limited Automatic Report Collector (LARC)

An electronic device which interfaces a river or precipitation gage with a telephone line making it possible for remote computers to call a gaging site and retrieve data.

length

The distance in the direction of flow between two specific points along a river, stream, or channel.

lentic system

A nonflowing or standing body of fresh water, such as a lake or pond. Compare lotic system.

levee (dike)

A long, narrow embankment usually built to protect land from flooding. If built of concrete or masonary the structure is usually referred to as a flood wall. Levees and floodwalls confine streamflow within a specified area to prevent flooding. The term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.

lining

A coating of concrete, rubber, or plastic to a canal, tunnel, shaft or reservoir to provide water-tightness, prevent erosion, reduce friction, or support the periphery of the structure.

lithosphere

That part of the earth which is composed predominantly of rocks (either coherent or incoherent, and including the disintegrated rock materials known as soils and subsoils), together with everything in this rocky crust.

littoral zone

The area on, or near the shore of a body water.

live capacity

The total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation storage and exclusive flood control storage.

local flooding

Flooding conditions over a relatively limited (localized) area.

log and safety boom

A net-like device installed in a reservoir, upstream of the principal spillway, to prevent logs, debris, and boaters from entering a water discharge facility or spillway.

long-term storage dams

Reservoirs used for recreational use or storage of irrigation, municipal or industrial water. Because water is impounded on a "permanent" basis, the design of these dams is more complex than for tailings or flood control detention dams. A long-term storage dam may include an impermeable core surrounded by shell materia, have many types of drains and filters, outlet works, with gates and valves, seepage collection boxes, and possibly several spillways. The capacity of the spillway is dependant upon the downstream hazard potential.

lotic system

A flowing body of fresh water, such as a river or stream. Compare lentic system.

lowland flooding

Inundation of low areas near a river, often rural but may also occur in urban areas.

lysimeter

A device to measure the quantity or rate of downward water movement through a block of soil usually undisturbed, or to collect such percolated water for quality analysis.