Reservoirs are man-made lakes used for storing water, usually for drinking. They fall into two categories; impounded lakes and dammed rivers. Impounded lakes are commonly constructed in flat areas by removing soil and sub soil and piling the excavated earth around the margins of the site to create an artificial lake (usually river fed) the surface of which is usually several meters above the height of the surrounding land. One of the main problems with impounded reservoirs is that, because they are typically filled with enriched river water they grow large amounts of phytoplankton which makes the water difficult and expensive to filter before putting into supply.
Damming as the name suggests uses existing rivers or lakes and raises the height of them by restricting their outflow. This technique is employed in upland areas throughout the world where reservoirs are often used from water storage and power generation. In some parts of the world most of the suitable valleys have already been turned into reservoirs often to the detriment of migratory fish.