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. 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. The history of fluctuations in lake levels provides a detailed record of climate changes on a scale of ten to a million years.
Lakes can also be valuable indicators of near- surface groundwater conditions.
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. Salinity is measured by standard analytical means. Past variations in levels and salinity can be recognized by studying old shorelines, lake-side 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 lakewater salinity.