# Lake retention time

Lake retention time (also called the residence time of lake water, or the water age or flushing time) is a calculated quantity expressing the mean time that water (or some dissolved substance) spends in a particular lake. At its simplest, this figure is the result of dividing the lake volume by the flow in or out of the lake. It roughly expresses the amount of time taken for a substance introduced into a lake to flow out of it again. The retention time is particularly important where downstream flooding or pollutants are concerned.

## Global retention time

The global retention time for a lake (the overall mean time that water spends in the lake) is calculated by dividing the lake volume by either the mean rate of inflow of all tributaries, or by the mean rate of outflow (ideally including evaporation and seepage). This metric assumes that water in the lake is well-mixed (rather than stratified), so that any portion of the lake water is much like any other. In reality, larger and deeper lakes are generally not well-mixed. Many large lakes can be divided into distinct portions with only limited flow between them. Deep lakes are generally stratified, with deeper water mixing infrequently with surface water. These are often better modeled as several distinct sub-volumes of water.

## More specific residence times

It is possible to calculate more specific residence time figures for a particular lake, such as individual residence times for sub-volumes (e.g. particular arms), or a residence time distribution for the various layers of a stratified lake. These figures can often better express the hydrodynamics of the lake. However, any such approach remains a simplification and must be guided by an understanding of the processes operating in the lake.
Two approaches can be used (often in combination) to elucidate how a particular lake works: field measurements and mathematical modeling. One common technique for field measurement is to introduce a tracer into the lake and monitor its movement. This can be a solid tracer, such as a float constructed to be neutrally buoyant within a particular water layer, or sometimes a liquid. This approach is sometimes referred to as using a Lagrangian reference frame. Another field measurement approach, using an Eulerian reference frame, is to capture various properties of the lake water (including mass movement, water temperature, electrical conductivity and levels of dissolved substances, typically oxygen) at various fixed positions in the lake. From these can be constructed an understanding of the dominant processes operating in the various parts of the lake and their range and duration.
Field measurements alone are usually not a reliable basis for generating residence times, mainly because they necessarily represent a small subset of locations and conditions. Therefore, the measurements are generally used as the input for numerical models. In theory it would be possible to integrate a system of hydrodynamic equations with variable boundary conditions over a very long period sufficient for inflowing water particles to exit the lake. One could then calculate the traveling times of the particles using a Lagrangian method. However, this approach exceeds the detail available in current hydrodynamic models and the capacity of current computer resources. Instead, residence time models developed for gas and fluid dynamics, chemical engineering, and bio-hydrodynamics can be adapted to generate residence times for sub-volumes of lakes.

## Renewal time

One useful mathematical model is the measurement of how quickly inflows are able to refill a lake. Renewal time is a specific measure of retention time, where the focus is on 'how long does it take to completely replace all water in a lake.' This is modeling can only be done with an accurate budget of all water gained and lost by the system. Renewal time simply becomes a question how quickly could the inflows of the lake fill the entire volume of the basin (this does still assume the outflows are unchanged). For example if Lake Michigan was emptied, it would take 99 years for its tributaries to completely refill the lake.

## List of residence times of lake water

The residence time listed is taken from the infobox in the associated article unless otherwise specified.
| Lake | Location | Basin countries | Residence time |
| Stausee Ferden | Ferden, Valais | Switzerland | 36 hours |
| Lake Wohlen / Wohlensee | Canton of Bern | Switzerland | 2.1 days |
| Lake Hévíz | Hévíz | Hungary | 3 days |
| Lake St. Clair | Great Lakes | Canada, United States | 7 days (2-30 days) |
| Lake Arapuni | North Island | New Zealand | 1 week |
| Milton Pond | Strafford County, New Hampshire, York County, Maine | United States | 11.1 days |
| Gibson Lake | Gibson Generating Station, Princeton, Indiana | United States | 2 weeks |
| Råcksta Träsk | Stockholm | Sweden | 2-3 weeks |
| Lake Sagami | Kanagawa | Japan | 18 days |
| Lake Cristallina | Ticino | Switzerland | 2-4 weeks |
| Northeast Pond | Strafford County, New Hampshire; York County, Maine | United States | 23.7 days |
| Brandy Pond | Naples, Maine | United States | 34 days |
| Champagne Pool | Waiotapu, North Island | New Zealand | 34 days |
| Prospect reservoir | Prospect, New South Wales | Australia | 30-40 days |
| Sicklasjön | Stockholm | Sweden | 1.2 months |
| Devils Lake | Lincoln County, Oregon | United States | 1.8 months |
| Lough Derg | Shannon River Basin | Ireland | 1.8 months |
| Derwent Water | Cumbria | United Kingdom | 55 days |
| Lake Biel / Bielersee / Lac de Bienne | Canton of Bern | Switzerland | 58 days |
| Veluwemeer | Flevoland, Gelderland | Netherlands | 2 months |
| Hacksjön | Tullinge | Sweden | 2.4 months |
| Kamloops Lake | British Columbia | Canada | ca. 0.2 years (20-340 days) |
| Malham Tarn | North Yorkshire, England | United Kingdom | 11 weeks |
| Lac d'Anterne | Passy, Haute-Savoie | France | 90 days |
| Graham Lake | Hancock County, Maine | United States | 3 months |
| Grand Lac de Clairvaux | Jura department, Franche-Comté | France | 3 months |
| Laduviken | Djurgården | Sweden | 3 months |
| Esthwaite Water | Lake District, Cumbria | United Kingdom | 95 days |
| Trehörningen | Huddinge, Southern Stockholm County | Sweden | 98 days |
| Lac de Divonne | Divonne-les-Bains, Ain | France | 100 days |
| Lake Lesina / Lago di Lesina | Province of Foggia, Apulia | Italy | 100 days |
| Seeweidsee | Canton of Zurich | Switzerland | 100 days |
| Hüttnersee | Hütten, Canton of Zurich | Switzerland | 120 days |
| Lake Lauerz / Lauerzersee | Canton of Schwyz | Switzerland | 4 months |
| Morris Lake | Halifax Regional Municipality, Nova Scotia | Canada | 4.1 months |
| Sobradinho Reservoir | Sobradinho, Bahia | Brazil | 4.2 months |
| Balch Pond | Carroll County, New Hampshire; York County, Maine | United States | 136 days |
| Magelungen | Huddinge | Sweden | 4-5 months |
| Russell Lake | Halifax Regional Municipality, Nova Scotia | Canada | 4.6 months |
| Loweswater | Lake District, Cumbria | United Kingdom | 150 days |
| Lappkärret | Stockholm | Sweden | 5 months |
| Lough Feeagh | County Mayo | Ireland | 5.6 months |
| Egelsee | Bubikon, Canton of Zurich | Switzerland | 180 days |
| Lac de Vouglans | Jura, Franche-Comté | France | 180 days |
| Mettmenhaslisee | Canton of Zurich | Switzerland | 180 days |
| Albysjön | Stockholm | Sweden | 6 months |
| Chesuncook Lake | Piscataquis County, Maine | United States | 6 months |
| Clinton Lake | DeWitt County, Illinois | United States | 6 months |
| Flagstaff Lake | Somerset County and Franklin County, Maine | United States | 6 months |
| Lake Coeur d'Alene | Northern Idaho | United States | 6 months |
| Lac de Saint-Point | Doubs department, Franche-Comté | France | 200 days |
| Kyrksjön | Bromma | Sweden | 7 months |
| Newnan's Lake | Gainesville, Florida | United States | 7.2 months |
| Staples Pond | Temple, Maine | United States | 8.2 months |
| Lac Cornu | Chamonix-Mont-Blanc, Haute-Savoie | France | 250 days |
| Lac du Brévent | Chamonix, Haute-Savoie | France | 250 days |
| Lake Silvaplana / Silvaplanersee / Lej da Silvaplauna | Engadin, Grisons | Switzerland | 250 days |
| Loch Awe | Argyll and Bute, Scotland | United Kingdom | 8.4 months |
| Långsjön | Stockholm | Sweden | 9-10 months |
| Lake Sarnen / Sarnersee | Obwalden | Switzerland | 9.6 months |
| Lake Lovozero | Kola Peninsula, Murmansk Oblast | Russia | 10 months |
| Drevviken | Stockholm | Sweden | 10-11 months |
| Mörtsjön | Huddinge | Sweden | < 1 year |
| Judarn | Bromma, Stockholm | Sweden | 11 months |
| Coniston Water | Lake District, Cumbria | United Kingdom | 340 days |
| Ullswater | Lake District, Cumbria | United Kingdom | 350 days |
| Khanka Lake | Heilongjiang Province, China and Primorsky Krai, Russia | China, Russia | 1 year |
| Lake Monona | Dane County, Wisconsin | United States | 1.1 years |
| Long Lake | Maine | United States | 1.1 years |
| Mooselookmeguntic Lake | Franklin County, Maine | United States | 1.1 years |
| Greifensee | Canton of Zurich | Switzerland | 408 days |
| Nainital Lake | Uttarakhand | India | 1.16 years |
| Manasbal Lake | Kashmir Valley | India | 1.2 years |
| Lake Zurich / Zürichsee | Zürich | Switzerland | 440 days |
| Tegernsee | Bavaria | Germany | 1.28 years |
| Deer Creek Reservoir | Utah | United States | 1.3 year |
| Orlången | Huddinge | Sweden | 1.33 years |
| Haweswater Reservoir | Lake District, Cumbria | United Kingdom | 500 days |
| Loch Shiel | Lochaber, Highland, Scotland | United Kingdom | 1.37 year |
| Lake Walen /Walensee / Lake Walenstadt | St. Gallen, Glarus | Switzerland | 1.4258 years |
| Keoka Lake | Maine | United States | 1.5 years |
| Kootenay Lake | British Columbia | Canada | 1.5 years |
| Lake Murten /Lac de Morat /Murtensee | Canton of Fribourg, Vaud | Switzerland | 1.6 years |
| Oeschinen Lake | Bernese Oberland | Switzerland | 1.61 years |
| Lac de Lessy | Haute-Savoie | France | 600 days |
| East Grand Lake | Maine / New Brunswick | Canada, United States | 1.7 years |
| Lake Ikeda | Kyūshū island | Japan | 1.7 years |
| Lake Poyang | Jiujiang | China | 0.173 years |
| Faaker See | Carinthia | Austria | 1.8 years |
| Ossiacher See / Lake Ossiach | Carinthia | Austria | 1.8 years |
| Varese Lake/Lago di Varese | Province of Varese, Lombardy | Italy | 1.8 years |
| Long Lost Lake | Clearwater County, Minnesota | United States | 1.8 years |
| Ältasjön | Älta | Sweden | 1.8 years |
| Lake Thun / Thunersee | Canton of Bern | Switzerland | 684 days |
| Green Lake | Maine | United States | 1.9 years |
| Gömmaren | Huddinge | Sweden | 1.9 yrs |
| Loch Lomond | Dunbartonshire, Central Scotland | Scotland, United Kingdom | 1.9 years |
| Lake Balaton | Hungary | Hungary | 2 years |
| Lake Rotoroa | Hamilton, Waikato, North Island | New Zealand | approx. 2 years |
| Lake Tūtira | Hawke's Bay, North Island | New Zealand | 2 years |
| Türlersee | Canton of Zurich | Switzerland | 730 days |
| Pfäffikersee | Canton of Zurich | Switzerland | 2.085 years |
| Caniapiscau Reservoir | Quebec | Canada | 2.2 years |
| Lake Sils / Silsersee /Lej da Segl | Engadin, Grisons | Switzerland | 2.2 years |
| Wilersee | Canton of Zug | Switzerland | 2.2 years |
| Great Pond | Maine | United States | 2.3 years |
| Moberly Lake | British Columbia | Canada | 2.4 years |
| Päijänne | Jyväskylä | Finland | 2.5 years |
| Lake Erie | North America | Canada, United States | 2.6 years |
| Rangeley Lake | Franklin County, Maine | United States | 2.6 years |
| Lake Brienz / Brienzersee | Canton of Bern | Switzerland | 2.69 years |
| Ammersee / Lake Ammer | Upper Bavaria | Germany | 2.7 years |
| Großer Plöner See | Schleswig-Holstein | Germany | 3 years |
| Paudash Lake | Haliburton County, Ontario | Canada | 3 years |
| Moosehead Lake | Maine | United States | 3.1 years |
| Trekanten | Stockholm | Sweden | 3.1 years |
| Great East Lake | Carroll County, New Hampshire; York County, Maine | United States | 3.3 years |
| Lake Champlain | North America | Canada, United States | 3.3 years |
| Flathead Lake | Montana | United States | 3.4 years |
| Lake Lucerne /Vierwaldstättersee | Central Switzerland | Switzerland | 3.4 years |
| Lake Winnipeg | Manitoba, Canada | Canada | 3.5 years |
| Lac de Paladru | Isère | France | 3.6 years |
| Lake Hallwil / Hallwilersee | Aargau | Switzerland | 3.9 years |
| Flaten | Southern Stockholm | Sweden | 4 years |
| Lake Annecy / Lac d'Annecy | Haute-Savoie | France | 4 years |
| Lake Maggiore / Verbano / Lago Maggiore | Lombardy and Piedmont, Italy; Canton Ticino, Switzerland | Italy, Switzerland | 4 years |
| Todos los Santos Lake | Los Lagos | Chile | 4 years |
| Lake Baldegg / Baldeggersee | Canton of Lucerne | Switzerland | 4.2 years |
| Lake Iseo / Lago d'Iseo | Lombardy | Italy | 4.2 years |
| Lake Constance / Bodensee | Germany, Switzerland, Austria | Germany, Switzerland, Austria | 4.3 years |
| Corey Lake | Michigan | United States | 4.4 years |
| Elk and Beaver Lakes | Saanich, British Columbia | Canada | 4.4 years (Elk Lake), |
| Lake Como | Lombardy | Italy | 4.5 years |
| Lake Mendota | Wisconsin | United States | 4.5 years |
| Sea of Galilee | Galilee | Israel | 5 years |
| Walden Pond | Massachusetts | United States | 5 years |
| Bannwaldsee | Allgäu, Bavaria | Germany | 5.2 years |
| Sebago Lake | Cumberland County, Maine | United States | 5.1 to 5.4 yrs |
| Kennisis Lake | Haliburton Highlands, Ontario | Canada | 5.26 years |
| Bay Lake | Crow Wing County, Minnesota | United States | 4-7 years |
| Lake Biwa / 琵琶湖 | Shiga Prefecture | Japan | 5.5 years |
| Jordan Pond | Hancock County, Maine | United States | 6 years |
| Lago di Mergozzo | Piedmont | Italy | 6 years |
| Lake Ontario | North America | Canada, United States | 6 years |
| Lac la Nonne | Alberta | Canada | 6.5 years |
| Lago di Candia | Province of Turin | Italy | 6 or 7 years |
| Ägerisee | Canton of Zug | Switzerland | 6.8 years |
| Loch Morar | Morar, Lochaber, Highland, Scotland | United Kingdom | 6.87 years |
| Lake Powell | Utah/Arizona | United States | 7.2 years |
| Millstätter See / Lake Millstatt | Carinthia | Austria | 7.5 years |
| Moose Lake | Alberta | Canada | 7.5 years |
| Schaalsee | Schleswig-Holstein | Germany | 5 to 10 years |
| Iliamna Lake | Lake and Peninsula Borough, Alaska | United States | 7.8 years |
| Lac du Bourget | Savoie, France | France | 8 years |
| Odell Lake | Klamath County, Oregon | United States | 8 years |
| Lake Lugano /Lago di Lugano | Ticino, Lombardy | Switzerland, Italy | 8.2 years |
| Lake Neuchâtel / Lac de Neuchâtel / Neuenburgersee | Romandy | Switzerland | 8.2 years |
| Lake Orta /Lago d’Orta /Cusio | Piedmont | Italy | 8.9 years |
| Weissensee | Carinthia | Austria | 9.2 years |
| Quesnel Lake | British Columbia | Canada | 10.1 years |
| Schweriner See | Mecklenburg-Vorpommern | Germany | 10.1 years |
| Lake Taupō | Taupō District, Waikato region, North Island | New Zealand | 10.5 years |
| Wörthersee / Lake Wörth | West of Klagenfurt | Austria | 10.5 years |
| Lake Chelan | Washington | United States | 10.6 years |
| Lake Geneva | Switzerland, France | Switzerland, France | 11.4 years |
| Lake Wakatipu | Queenstown-Lakes District, Otago Region, South Island | New Zealand | c. 12 years |
| Higgins Lake | Michigan | United States | 12.5 years |
| Crescent Lake | Klamath County, Oregon | United States | 13 years |
| Yellowstone Lake | Yellowstone National Park, Wyoming | United States | 14 years |
| Lake Zug / Zugersee | Central Switzerland | Switzerland | 14.7 years |
| Müritz | Mecklenburg-Vorpommern | Germany | 15 years (?) |
| Western Brook Pond | Gros Morne National Park, Newfoundland, Newfoundland and Labrador | Canada | 15.4 years |
| Lake Sempach / Sempachersee | Canton of Lucerne | Switzerland | 16.9 years |
| Skaneateles Lake | New York | United States | 18 years |
| Cayuga Lake | New York | United States | 18.2 years |
| Bear Lake | Idaho, Utah | United States | 19.6 years |
| Green Lake | Green Lake County, Wisconsin | United States | 21 years |
| Lake Starnberg /Starnberger See | Bavaria | Germany | 21 years |
| Lake Huron | North America | Canada, United States | 22 years |
| Lake Garda / Lago di Garda | Northern Italy | Italy | 26.8 years |
| Waldo Lake | Cascade Mountains | United States | 32 years |
| Crystal Lake | Benzie County, Michigan | United States | 30 to 60 years |
| Lake Columbia | Jackson County, Michigan | United States | 47 years |
| Okanagan Lake | British Columbia | Canada | 52.8 years |
| Lake Ohrid | | Albania, North Macedonia | 70 years |
| Lake Michigan | Michigan | United States | 99 years |
| Arendsee | Saxony-Anhalt | Germany | 100 years |
| Gull Lake | Alberta | Canada | >100 years |
| Lake Yosemite | California | United States | 121 years |
| Great Bear Lake | Northwest Territories | Canada | 124 years |
| Crater Lake | Oregon | United States | 157 years |
| Lake Superior | North America | Canada, United States | 191 years |
| Caspian Sea | | Azerbaijan, Iran, Kazakhstan, Russia, Turkmenistan | 250 years |
| Lake Baikal | Southern Siberia | Russia | 330 years |
| Lake Tahoe | California, Nevada | United States | 650 years |
| Lake Titicaca | | Peru, Bolivia | 1343 years |
| Lake Eucumbene | Snowy Mountains, New South Wales | Australia | 1953 years |
| Lakes Qaban | Kazan | Russia | 2000 years |
| Lake Tanganyika | | Burundi, DR Congo, Tanzania, Zambia | 5500 years |
| Lake Vostok | | Antarctica | 13300 years |