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LABORATORY BLANK—An artificial sample, usually distilled water, introduced to a chemical analyzer to observe the response of the instrument to a sample that does not contain the material being measured. The blank can also detect any contamination occurring during laboratory processing of the sample.

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. [See Appendix W-3 for an explanation of the USFWS Wetland and Deepwater Habitat Classification System and more detailed information on these Systems.]

LADE—To take up or remove water with a ladle or dipper.

LAG (Time)—(Statistics) The difference in time units of a series value and a previous series value. In time series analysis, the lag typically represents the period of time between the change in the independent or predictor (Exogenous) variable and its strongest (most significant) effect on the dependent or predicted (Endogenous) variable. Also see Lead (Time).

LAGGED ENDOGENOUS VARIABLE—(Statistics) Refers to the use of a prior-period Dependent Variable used as an Explanatory Variable in the current period. The model below uses a prior Endogenous value, Yt-1, to explain the behavior of Yt:

Yt = ø1Yt-1 + ð + et

In many instances of social, economic, and natural phenomenon, the behavior of a variable in the current period may be dependent upon or somehow influenced by its prior behavior or level. This constitutes the fundamental underpinning of an Autoregressive Process in the analysis of Time-Series Data.

LAGOON—(1) A shallow lake or pond, especially one connected with a larger body of water. (2) The area of water enclosed by a circular coral reef, or atoll. (3) An area of shallow salt water separated from the sea by sand dunes. (4) (Water Quality) Lagoons are scientifically constructed ponds in which sunlight, algae, and oxygen interact to restore water to a quality equal to effluent from a secondary treatment plant.

LAGOON SYSTEM—(Water Quality) A system of scientifically construction Lagoons or ponds in which sunlight, algae, and oxygen interact to restore water to a quality equal to effluent from a Secondary Treatment Plant.

LAG TIME, also Lagtime—(1) The time from the center of a Unit Storm to the peak discharge or center of volume of the corresponding Unit Hydrograph. (2) (Flood Irrigation) The period between the time that the irrigation stream is turned off at the upper end of an irrigated area and the time that water disappears from the surface at the point or points of application.

LAHAR—A mudflow composed of volcanic debris and water.

LAHONTAN VALLEY WETLAND SYSTEM [Nevada]—An extensive wetland system in northwestern Nevada in Churchill County near the City of Fallon encompassing the Stillwater Wildlife Management Area and the Stillwater National Wildlife Refuge, as well as the Carson Lake and Carson Pasture, which serves as a key migration and wintering area for up to 1 million waterfowl, shorebirds, and raptors. Each spring and fall, it hosts a significant percentage of the Pacific Flyway's migratory birds. The Lahontan Valley Wetland System was named to the Western Hemisphere Shorebird Reserve Network in 1988, and it has been nominated for inclusion under the Convention of Wetlands of International Importance, attesting to the continental significance of this invaluable resource. By one estimate, in the early 1900s the Lahontan Valley Wetland System alone contained about 85,000 acres (34,400 ha) of wetlands visited by millions of waterfowl and shorebirds using the eastern edge of the Pacific Flyway during migration. With the advent of the Newlands Project, fresh water that traditionally charged the wetlands was replaced by a greatly diminished supply of agricultural drain water. Overall, wetland acreage in the Lahontan Valley declined by 85 percent. Because it is one of only three large interior basin wetland systems along the west coast, deterioration of Lahontan Valley wetlands has already markedly reduced the carrying capacity of the Pacific Flyway. In 1990, Congress passed Public Law 101-618 (the Negotiated Settlement) authorizing the purchase and transfer of enough water rights to maintain a total of 25,000 acres of primary wetlands in the Lahontan Valley. The U.S. Department of the Interior, Fish and Wildlife Service (USFWS) estimates this will require up to 125,000 acre-feet of water annually. Also see Newlands Project [Nevada] and Public Law 101-618 [Nevada].

LAKE—A considerable body of inland water or an expanded part of a river.

LAKE BONNEVILLE [Utah and Nevada]—An ancient Ice Age lake which during the last Glacial Epoch covered most of the Utah portion of the Great Basin, encompassing most of northwestern Utah and stretching into eastern Nevada. The present Great Salt Lake is the remaining remnant of this lake.

LAKE EVAPORATION—Normal evaporation such as from a pond or lake.

LAKE LAHONTAN [Nevada and California]—An ancient Ice Age lake which covered an extensive portion of northwestern Nevada during the last Glacial Epoch, a period when the Great Basin was covered with a considerable number of lakes and rivers. Lake Lahontan, along with Lake Bonneville far to the east, represented the major Ice Age lakes which covered vast portions of Nevada and Utah and provided a far more lush and hospitable environment for both flora and fauna. Now, only the Great Salt Lake remains to provide an indication of the prehistoric presence of Lake Bonneville, and only Pyramid Lake and Walker Lake remain to provide testament of the presence of Lake Lahontan. In its day, Lake Lahontan would cover some 8,655 square miles in northwestern Nevada, an area equal to almost 8 percent of Nevada's present surface area. This Ice Age lake was fed by the flows of the Truckee, Carson, Walker, Humboldt, Susan and Quinn rivers, attained a maximum surface elevation of approximately 4,380 feet, reached a maximum depth of at least 886 feet where Pyramid Lake, the lowest point in the system, now remains, covered the Lahontan Valley Wetlands (Stillwater Wildlife Refuge) to a depth of 500-700 feet, stretched from just below Nevada's northern boundary to Walker Lake, and extended well up the lower Truckee Canyon towards the city of Reno to near the present-day location of Lagomarsino Canyon near Lockwood. Lake Lahontan experienced at several peaking enlargements—approximately 65,000, 45,000, 30,000, and as recently as 12,500 years ago—and at times nearly dried up.

LAKE TRUCKEE [California]—In Neocene times, which occurred during the late Tertiary Period approximately 25 million to 13 million years ago and encompassed both the Pliocene and Miocene Epochs, Lake Truckee was formed from a basalt flow that dammed the upper Truckee River canyon just below the present-day site of Hirshdale, California. The lake covered an area of some 73 square miles, its surface level reached an elevation of at least 6,000 feet above mean sea level (MSL), and attained a maximum depth of 465 feet. Lake Truckee remained through part of the glacial (Pleistocene) period until the river eventu