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. Lake level and lake water composition monthly to annual. Areal extent every 5 years. Limited by availability of gauge data, resolution of photographic and satellite images, and by climatic records for baseline data.