Links:

• Slow-no-wake
• Yahara Lakes Water Level Advisory Group (YLAG2)
• Dane County Lake Level Management Guide for the Yahara Chain of Lakes (1.86 MB PDF)
• Lake Level Management on the Yahara Chain of Lakes (7.3 MB PDF)

Lake Locations and Current and Target Lake Levels*

Mendota     [Lake Info] 6/19/2013 Summer Minimum Target max 100-Year** 850.25 849.60 850.10 852.8 Monona     [Lake Info] 6/19/2013 Summer Minimum Target max 100-Year** 845.81 844.70 845.20 847.7 Waubesa     [Lake Info] 6/19/2013 Summer Minimum Target max 100-Year** 845.46 844.50 845.00 847.0 Kegonsa     [Lake Info] 6/19/2013 Summer Minimum Target max 100-Year** 843.56 843.00 843.50 845.2

Readings are updated daily from the United States Geological Survey (USGS) and are expressed in feet above sea level. For additional information, see the USGS web site Real-Time Data for Wisconsin: Yahara River and Lakes or the University of Wisconsin Extension booklet Understanding Lake Data.

To view more lake level readings, check out our Lake Level Search Page.

* Elevations are reported in feet above the National Geodetic Vertical Datum of 1929 (NGVD29). Prior to 1973, this datum was the Sea Level Datum of 1929, commonly referred to as mean sea level (MSL).

More on Vertical Datums

All elevation data is reported in relation to a vertical datum and it is critical to know the vertical datum when evaluating elevation information, such as lake levels and topography. Failure to take vertical datum into account can result in erroneous conclusions about water level impacts on shorelines and shoreland structures.

The two most commonly referenced vertical datums are the National Geodetic Vertical Datum of 1929 (NGVD 1929) and the North American Vertical Datum of 1988 (NAVD88). In addition, in 2007 the National Geodetic Survey (NGS) readjusted NAVD88 in Wisconsin but did not officially rename it. Effectively they are NAVD88(pre-2007) and NAVD88(2007).

These datums were developed using different methodologies and consequently they vary spatially. There is not a single exact elevation shift to convert from one to another. In addition, any point's reported elevation in a certain datum may be incorrect, for example if the point has moved since the date of survey, or the survey or starting bench mark was erroneous. The National Geodetic Survey's VERTCON tool estimates that NGVD29 elevations are 0.2 ft larger than NAVD88(pre-2007) elevations in the Madison area. Comparing NGS bench marks in the Madison area shows NAVD88(2007) elevations are on average 0.1 ft larger than NAVD88(pre-2007) elevations. Therefore, an approximation sufficient for many purposes, using sample elevations, is: 850.0 ft NGVD29 = 849.8 ft NAVD88(pre-2007) = 849.9 ft NAVD88(2007).

The Federal Emergency Management Agency's Flood Insurance Rate Maps use the NAVD88(pre-2007) datum, based on the 0.2 ft shift between NGVD29 and "NAVD88" assumed for Dane County in Flood Insurance Study Number 55025CV001B revised January 2, 2009. The lake level orders for the Yahara Lakes and the USGS gauging station water elevations are reported in NGVD29. Using the conversions above, subtract 0.2 ft from the county/USGS NGVD29 lake levels to compare them to the NAVD88(pre-2007) elevations in the 2009 FEMA flood maps.

For more information on Madison-area vertical datums please see here: http://danrodman.tripod.com/

1% Probability of Flooding Every Year (also known as the "100-year floodplain")

**100-Year Flood Elevation: the 100-year flood elevation establishes the extent of the 100-year floodplain. The 100-year floodplain is the area that has a 1% probability of flooding every year, and where predicted flood water elevations above mean sea level have been established. Properties in the floodplain are considered to be at high risk of flooding under the National Flood Insurance Program (NFIP). Construction in these areas must meet local floodplain zoning ordinance requirements, including evidence that principle structures are above the 100-year flood elevation as shown on the adopted floodplain maps. For more information on floodplains, please visit Dane County Planning & Development's website.