What Are Lakes?

Lakes are inland bodies of standing water occupying topographic depressions and sustained by a balance of inflows from rivers, streams, groundwater, and precipitation against outflows through surface drainage and evaporation. They represent a small fraction of the Earth's surface freshwater by area but account for a disproportionately large share of accessible liquid freshwater resources and host highly productive aquatic ecosystems. Lakes differ from reservoirs, which are artificial impoundments created by dams for water supply or hydroelectric generation, and from wetlands, which are shallower transitional environments with emergent vegetation. The scientific study of lakes, encompassing their physical, chemical, and biological properties, constitutes the discipline of limnology, a subdiscipline of hydrology that has grown significantly in practical importance as freshwater availability has become a global concern.

Hydrology and Water Resources

The hydrological behavior of a lake is governed by its water budget: inputs of precipitation and inflow from tributary rivers and groundwater seepage must balance outputs of evaporation, surface outflow, and groundwater recharge. Water residence time, the average period a water molecule spends in the lake before exiting, is a fundamental parameter that ranges from days in small, flow-through systems to thousands of years in ancient tectonic lakes such as Lake Baikal. Residence time controls the concentration of dissolved substances, the temperature response to climate variability, and the recovery rate from pollution events. Lakes also function as natural storage buffers in regional water supply systems, regulating downstream river flows during drought periods and reducing flood peaks after intense precipitation. Engineered reservoirs extend this function artificially, but natural lakes often provide comparable buffering services without infrastructure cost. The US EPA's Water Quality Portal, operated in cooperation with USGS and the National Water Quality Monitoring Council, provides publicly accessible databases of in situ chemical, physical, and biological measurements from lakes and rivers across the United States.

Water Quality and Pollution

Lake water quality is determined by the physical, chemical, and biological processes operating within the lake and its contributing watershed. Key chemical parameters include concentrations of dissolved oxygen, nitrogen and phosphorus nutrients, pH, conductivity, and toxic contaminants from agricultural runoff or industrial discharge. A major contemporary concern is eutrophication: the enrichment of lakes with phosphorus and nitrogen from fertilizer runoff and wastewater, which stimulates excessive algal growth. Dense algal blooms deplete dissolved oxygen as they decompose, creating hypoxic zones that eliminate fish and invertebrate populations. Harmful cyanobacterial blooms produce toxins hazardous to drinking water systems and recreational users. The EPA Understanding Lake Ecology module explains the relationships among nutrient loading, primary productivity, stratification, and oxygen depletion that govern lake condition across trophic states from oligotrophic to hypereutrophic.

Remote Sensing and Environmental Monitoring

Satellites and airborne sensors have transformed the ability to monitor lake conditions at spatial and temporal scales that traditional field sampling cannot achieve. Multispectral imagery from platforms including Landsat 8 and 9, Sentinel-2, and Sentinel-3 enables estimation of surface chlorophyll-a concentrations, turbidity, total suspended solids, and surface temperature across lakes too large or remote for dense in situ networks. NASA's Applied Remote Sensing Training program offers resources on monitoring water quality of inland lakes using remote sensing, including satellite-derived cyanobacterial bloom detection through the CyAN early warning system. Sediment transport from tributary rivers and shoreline erosion contributes to lake turbidity and records watershed land use history in accumulated bottom sediments, which paleolimnologists analyze to reconstruct past climate and pollution events.

Applications

Lakes have applications in a range of fields, including:

• Municipal and agricultural water supply and drinking water treatment
• Hydroelectric power generation through connected reservoirs
• Environmental monitoring for pollution assessment and ecosystem health
• Climate research using sediment core paleoclimate records
• Aquaculture and commercial fisheries
• Flood regulation and regional hydrological buffering