Historical water level observations are shown for the Great Lakes, and, for comparison, for The
Battery, New York City, NY. Annual average water levels are shown as black dots and monthly
average water levels are light blue dots. Water levels for both the Great Lakes and the ocean
coasts are monitored by NOAA’s National Ocean Service. The average level for the period of
record for each is indicated by a horizontal red line. Water levels are surface water elevations
above either the 1985 International Great Lakes Datum (for the Great Lakes), or mean sea
water level (for the gauge at The Battery) in both meters (left axis) and feet (right axis). To
facilitate direct comparisons, elevations are presented on the same scale. Although much
attention is called to global sea level rise, as this figure reveals, the Great Lakes are a unique
example of a large coastal system that continuously experiences interannual water level
fluctuations of a greater magnitude than those experienced by similar coastal communities over
the past century.
Historical gauge-based basin-wide precipitation estimates in millimeters for the Great Lakes.
Annual precipitation estimates are shown as quantities above (green) or below (orange) the
long-term average for each lake for the period of record. The precipitation data is from landbased gauges in the U.S. and Canada.
Simulated annual over-lake evaporation in millimeters based on GLERL’s Large Lake
Thermodynamics Model. Orange vertical bars represent annual evaporation greater than the
long term average while green vertical bars represent annual evaporation rates below the
average.
Total annual over-land runoff in millimeters over the lake surface. The runoff is computed from
streamflow site data (U.S. and Canadian) using a simple interpolation method developed at
GLERL.