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Lake levels and salinity
Revised February 2006 with contributions from Robert Vance and James Teller

BRIEF DESCRIPTION: Lakes are dynamic systems that are sensitive to local climate and to land-use changes in the surrounding landscape. Some lakes receive their water mainly from precipitation, some are dominated by drainage runoff, and others are controlled by groundwater systems. On a time scale ranging from days to millennia, the areal extent and depth of water in lakes are indicators of changes in climatic parameters such as precipitation, radiation, temperature and wind speed. Lake level fluctuations vary with the water balance of the lake and its catchment, and may, in certain cases, reflect changes in shallow groundwater resources.
Especially useful as climatic indicators are lakes without outlets (endorheic), widely distributed in North America, Africa, Central Asia, Middle East, and Australia. In arid and semi- arid areas, the levels and areas of lakes with outflows are also highly sensitive to weather. Where not directly affected by human actions, lake level fluctuations are excellent indicators of drought conditions. For example, lake levels throughout the southern prairie provinces of Canada dropped in response to the warm, dry weather of the 1980s. Ephemerally or seasonally-flooded lake basins (playas) are dynamic landforms, the physical character and chemical properties of which reflect local hydrologic changes, and which react sensitively to short-term climate changes (e.g. rate of evaporation).
Variations in lake water salinity (e.g. CaHCO3, MgHCO3, CO3, MgSO4, NaSO4) and other chemical constituents also provide an indication of changes in conditions at the surface (climate, inflow/outflow relations) and in shallow groundwater; these changes may also be recorded in the sequence of lake bottom sediments [see Sediment sequence and composition]. Water level changes are commonly recorded in shoreline deposits [see Shoreline position] and in the extent and composition of adjacent wetlands [see Wetlands extent, structure, and hydrology].

SIGNIFICANCE: The history of fluctuations in lake levels and water chemistry can provide a detailed record of climate changes on a time scale from 10 to 1,000,000 years. Annual and inter-annual changes can also be measured, and are commonly present in the sedimentary sequence. Lakes can also be valuable indicators of near-surface groundwater conditions.

HUMAN OR NATURAL CAUSE: Both. Can be influenced by human-induced climate change, by engineering works such as dams, channels, near shore construction, irrigation systems, and even local farm activities. Urban activity, including industrial effluent and aerosols, and application of salt to streets in the winter, can alter the natural chemical composition of lakes and even groundwaters feeding the lake [see Surface water quality]. Reduction of infiltration by paving and roofing-over natural landscapes, as well as forest clearance and disturbance to native vegetation, increases runoff (and sediment yield) into the lake and may alter lake chemistry. The record of natural changes in water chemistry, sediment geochemistry, and lake level through short to long periods of time can provide a natural .baseline. of past conditions that allow comparisons with potential human-induced changes. For example, as a result of diversion into irrigation projects of rivers that flowed into the Aral Sea between Kazahkstan and Uzbekistan, the volume and extent of this huge inland lake has been dramatically reduced: between 1960 and 1989 its level dropped by 14 m, its volume decreased by 68%, and its salinity tripled. It is now less than half its size in the 1960s.

ENVIRONMENT WHERE APPLICABLE: Mainly arid and semi- arid regions, continental mid- latitudes and tropical and subtropical latitudes. Changes may be recorded more subtly in wetter areas.

TYPES OF MONITORING SITES: Shallow and, in particular, saline and hyposaline lakes, and lakes where groundwater responds rapidly to climate.

SPATIAL SCALE: Patch to mesoscale / regional

METHOD OF MEASUREMENT: Lake levels are generally measured with shoreline gauges. Areal extent is assessed primarily using successive air photos, supplemented with ground- level surveys, radar altimetry, and satellite images. High-precision GPS instruments allow centimeter-accuracy in measuring elevation. Salinity is measured by standard analytical means. Past variations in levels and salinity can be recognized by studying old shorelines, lakeside archaeological sites, and the geochemistry, mineralogy, isotopic composition and fossil content of sediment cores. Remains of diatoms, chrysophytes, chironomids, ostracods and other bio-indicators in lake sediments are widely used to infer past lake water salinity.

FREQUENCY OF MEASUREMENT: Lake level and lake water composition monthly to annual. Areal extent every 3-5 years.

LIMITATIONS OF DATA AND MONITORING: Limited by availability of gauge data, short-term .event. changes (e.g. thunderstorms), the availability, timing, and resolution of photographic and satellite images, and by availability of climatic or past sedimentary records for baseline data.

APPLICATIONS TO PAST AND FUTURE: Good index of water balance and changes in precipitation and evaporation. Can be good monitor of short-term changes and a predictor of trends. Records of lake dynamics in historic and pre- historic periods provide baseline data on past responses to climate change. With the establishment of threshold values, lakes may provide an early warning of shallow groundwater depletion.

POSSIBLE THRESHOLDS: When evaporation exceeds precipitation and runoff into lake (e.g. in semi-arid environments), lake area and salinity can change markedly. The utility of lakes as sources of water for human use depends on water availability and quality: thresholds for human health can be rapidly crossed as chemical concentrations (salinity) increase with evaporation.

KEY REFERENCES: Cohen, A. 2003. Paleolimnology. The history and evolution of lake systems. New York: Oxford University Press
Last, W.M., J.P. Smol, and H.J. Birks (eds) 2002. Tracking environmental change using lake sediments. Springer. A four volume set.
Mason, I.M., M.A.J.Guzkowska, C.G.Rapley & F.A.Street-Perrott 1994. The response of lake levels and areas to climate change. Climatic Change 27: 161-197.
Rosen, M.R. (ed) 1994. Paleoclimate and basin evolution of playa systems. Geological Society of America Special Paper 289.
Vance, R.E. & S.A.Wolfe 1996. Geological indicators of water resources in semi-arid environments: Southwestern interior of Canada. In Berger, A.R. & W.J.Iams (eds). Geoindicators: Assessing rapid environmental changes in earth systems:237-250. Rotterdam: A.A. Balkema.

OTHER SOURCES OF INFORMATION: Environment, water/hydrological agencies, geological surveys, INQUA, IGBP, World Data Center-A for Paleoclimatology (Lake level databases).

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Lake levels are important for regional hydrological investigations, and for a wide range of issues concerning lakeshore land use. Increases in lake salinity by evaporative concentration (or human-introduced components) may be transmitted into groundwater system by lake bed infiltration [see Groundwater quality].

OVERALL ASSESSMENT: Monitoring lake extent, depth and salinity provides a convenient and simple guide to changes in climate and hydrological conditions.

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