thumb|upright=1.3|Priorslee Lake in Shropshire, England. Originally known locally as the Balancing Lake, it was created to manage water run-off from the roofs and streets of [[Priorslee.]]
A balancing lake (also flood basin) is a term used in the UK to describe a retention basin used to control flooding by temporarily storing flood waters. The term balancing pond is also used, though typically for smaller storage facilities for streams and brooks.
In open countryside, heavy rainfall soaks into the ground and is released relatively slowly into watercourses (ditches, streams, rivers). In an urban area, the extent of hard surfaces (roofs, roads) means that the rainfall is dumped immediately into the drainage system. If left unchecked, this has the potential to cause flooding downstream. The function of a balancing lake as part of a sustainable urban drainage scheme is to contain this surge and release it slowly. Water sports and wildlife habitats do not mix well, though a scheme can have both in linked basins where the recreational basin fills first and the wildlife basin is only used in exceptional conditions.
- A recreational use facility can have relatively steep banks (perhaps with a footpath inside the bank next to the permanent lake for aesthetics and safety). The water level can rise substantially without a significant increase in overall area.
- A basin that is intended for use by wildlife and for visual amenity needs to be relatively shallow for maximum plant life. It must be designed with the assumption that it will be invoked very rarely, especially during the nesting season.
Case study: Willen Lake, Milton Keynes
thumb|300px|right|Willen Lake (north basin), with spill-way in the background
Willen Lake is one of the largest (400,000 m<sup>2</sup>) <!-- http://willen-lake.co.uk/ --> purpose-built urban stormwater balancing lakes in the UK. The lake is designed to take surface run-off from Milton Keynes, the largest of a number designed to do so. The lake has capacity for an additional level increase of 1.3 metres, equivalent to a once in 200 years event. Unlike most of the rest of the UK, the city has separate storm and foul sewers, so sewage pollution is not a significant problem. Additionally, there are facilities to prevent accidental oil spills and the like from reaching the lake.
As well as local storm drains, the lake's primary purpose is to intercept the river Ouzel, a tributary of the river Great Ouse. The catchment area is Oxford Clay that tends to get saturated easily, so field run-off has always been a problem.
The South Basin is designed for recreational use, mainly dinghy sailing and wind surfing, with a circumference path and banks as described above. It is linked to the North (Wildlife) Basin and can be drawn on to manage the level of the latter more finely. The North Basin has a large, undisturbed, central island. Canada geese have become naturalised and they are permanent residents. Both basins have deep ponds to maintain the fish population during droughts. The lake is and its surrounds is owned by the Milton Keynes Parks Trust and is a public open space.