The oceanic climate of Wales has ensured that reservoirs are well distributed throughout Wales. Some of the larger reservoirs in Wales are in the mountain areas and are oligotrophic, i.e., low in nutrients. While these tend to be of limited interest for biodiversity purposes, the scenery can be spectacular, e.g., at the Claerwen Dam in the Elan Valley, and there can be a range of recreational activities available, typically fishing and boating, with walks and cycle trails around the edges, e.g. at Llyn Brenig, Llyn Brianne and Lake Vyrnwy.

From a biodiversity perspective, the better reservoirs tend to be at a lower altitude and in more nutrient-rich environments. For example, some of the grassland adjoining Llandegfedd Reservoir has been designated as a Site of Special Scientific Interest (SSSI) for its flora and the embankments of Llanishen Reservoir in Cardiff have been designated as SSSI for the diverse assemblage of grassland fungi, with at least 25 species of waxcap recorded. Eglwys Nunydd Reservoir near Port Talbot and Lisvane Reservoir in Cardiff are examples of reservoirs that have been designated as SSSI for breeding and wintering wildfowl and Talybont Reservoir in Breconshire is another reservoir popular with birdwatchers, with a bird hide overlooking one of the more natural and interested sections of bank.