Retention time - Understanding Lake Data

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The average length of time water remains in a lake is called the retention time or flushing rate. The lake's size, water source, and watershed size primarily determine the retention time.

Rapid water exchange rates allow nutrients to be flushed out of the lake quickly. Such lakes respond best to management practices that decrease nutrient input. Impoundments, small drainage lakes, and lakes with large volumes of groundwater inflow and stream outlets (groundwater drainage lakes) fit this category.

Longer retention times occur in seepage lakes with no surface outlets. Average retention times range from several days for some small impoundments to many years for large seepage lakes. Lake Superior has the longest retention time of Wisconsin lakes-500 years!

Nutrients that accumulate over a number of years in lakes with long retention times can be recycled annually with spring and fall mixing. Reserve nutrients in lake sediments can continue to recirculate, even after the source of nutrients in the watershed has been controlled. Thus, the effects of watershed protection may not be apparent for a number of years. Nevertheless, lakes with long retention times tend to have the best water quality as shown by the lower levels of the plant nutrient phosphorus in Table 1. Better water quality results from both their greater depth and relatively smaller watersheds.

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TABLE 1. Several characteristics of lakes with different retention times. (Adopted from Lillie and Mason, 1983.)

Retention time in days	    0-14  15-60	61-180 181-365 366-730	>730
Mean depth (ft.)	       6     8    11	  11	  13	  23
Max. depth (ft.)	      16    21    25	  27	  35	  57
Mean total phosphorus (ug/l)* 94    85    56	  48	  33	  25
Mean DB:LA ratio**	    1166   142    42	  15	   8	   6

*Summer values; ug/l = micrograms per liter or parts per billion
**DB:LA = Drainage basin/lake area

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212


Lakes Partnership | Watershed Management | Fish Wisconsin | Fisheries Management

Last Revised: Thursday January 25 2007