Low Water Datum

The Vertical Control - Water Level Subcommittee (SC) is considering an update to the existing Low Water Datum (LWD) as part of the IGLD (2020) update. Known as chart datum, LWD was established in 1933. Over time Glacial Isostatic Adjustment, the addition of controlling structures and deepening of channels have caused a shift in water level relationships across the Great Lakes. The IGLD update now presents an opportunity to evaluate the existing LWD value.

What is the LWD? The LWD identifies a surface so low that the water level will seldom fall below it. Different lakes and rivers use different LWD surfaces.

What will change if the LWD is revised? Depths on navigation charts and navigation channels refer to LWD. On the upper Great Lakes, calculations indicate the LWD may be too high. On the lower Great Lakes, calculations indicate that the LWD may be too low. Preliminary results indicate that LWD could be adjusted anywhere from -6.3 inches (-16 cm) to 18.9 inches (48 cm), depending on the lake.

What is the estimated timeline for the LWD recalculation?

  • 2020: propose range of new LWD values
  • 2021: engage stakeholders to communicate changes
  • 2022: finalize proposed LWD values
  • 2025: implement changes in conjunction with release of IGLD update

To obtain LWD for a Great Lake or connecting channel, please visit the Great Lakes Low Water Datums web pages.

Water levels across the Great Lakes - St. Lawrence River System