Lakes and Reservoirs

Lakes and reservoirs are large expanses of water that accumulate in a low-lying area. Lakes are naturally occurring water bodies that form as a result of more water flowing into the area than what flows out. Reservoirs, on the other hand, form behind man-made structures that are designed to restrict the flow of a river – dam walls are constructed across a river to hold back the water in a storage facility, or reservoir, for human use – usually as a source of drinking water, and/or hydroelectric power. 

Large lakes and reservoirs very often provide a source of drinking water for local communities, and for towns and cities further afield. With a surface area of 31,000 square miles, Lake Superior, is the world's largest natural freshwater lake - it is purported to hold 10% of the earths' surface fresh water - supplying water to sixteen different communities in four different states. Obviously, when water in lakes and reservoirs is used as a source of drinking water, careful attention needs to be paid to water quality. However, water quality issues in lakes and reservoirs also affects the ecology of these aquatic ecosystems, and can even cause aesthetic problems, and health issues, which can affect tourism, and recreational activities on these water bodies.

Common Water Quality Issues

Because lakes and reservoirs have restricted water flow, they typically consist of large bodies of standing water. A wide variety of pollutants from many different sources may flow into the lake or reservoir, where they may accumulate. The type of pollutants found in lakes and reservoirs are determined to a large degree by land practices in the watershed from which the water originates.


Bacteria, protozoa, and viruses can enter the water body from runoff as a result of animal feces or sewage being washed into the system. Cryptosporidium and Giardia are two disease-causing microorganisms that may enter lakes and reservoirs from runoff. As these pathogens can survive water treatment processing, they are commonly found in drinking water, where they can cause severe gastrointestinal upsets.

Silting – Turbidity

Lakes and reservoirs may also be prone to silting, especially after periods of heavy rainfall, and when surrounding areas, and/or riparian zones are degraded by soil erosion. While sediment does not pose a direct health risk, it can increase turbidity, reducing water clarity. Clear water is always more appealing than murky water, both for recreational activities, and for drinking. Clear water not only looks more appealing, it generally tastes better too.

Nutrient Loading

Lakes and reservoirs may also be susceptible to nutrient loading. Nitrates and phosphates from animal manure and fertilizers used on crops can be washed into rivers and streams, or leach through the soil to the groundwater. Lakes and rivers may accumulate nutrients from various rivers, streams and tributaries within the watershed, and as a result, these nutrients may become quite concentrated in a water body whose flow is restricted by a natural or man-made barrier. Excess nutrients in lakes and reservoirs can cause algal blooms to develop – some of which can be toxic to fish, wildlife, pets, and humans too. 

 Addition Resources

Dams and Water Quality


Water Purification Systems and Filters

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