Frequently Asked Questions

Who provides water level data in the Great Lakes?

U.S. Great Lakes water level data at 51 locations are provided by the National Oceanic and Atmospheric Administration’s (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS). CO-OPS collects, analyzes and distributes historical and real-time observations and predictions of water levels, coastal currents and other meteorological and oceanographic data. This is part of an integrated National Ocean Service program supporting safe maritime navigation, more productive water-borne commerce, and the needs of the National Weather Service, coastal zone management, engineering and surveying communities. The Center manages the National Water Level Observation Program and the national network of Physical Oceanographic Real-Time Systems in major U.S. harbors. CO-OPs conducts its programs through university, industry, Federal and State partnerships as appropriate. CO-OPS resides within NOAA's National Ocean Service.

Are there tides in the Great Lakes?

The water levels of the Great Lakes have long term, annual, and short term variations. Long term variations depend on climatology and water storage over many years. Annual variations occur with the changing seasons. There is an annual high in the late spring and low in the winter. These rate changes are measured in feet per month.

True tides, changes in water level caused by the gravitational forces of the sun and moon, do occur in a semi-diurnal pattern on the Great Lakes. The spring tide, the largest monthly tides caused by the combined forces of the Sun and Moon, is less than 2 inches (5cm) in height. These minor water level variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes. Consequently, the Great Lakes are considered to be essentially non-tidal.

Other factors may create quite large fluctuations in lake levels, but for only short periods of time lasting from minutes to several days. The two most dramatic are storm surge and seiche.

Storm surge is a sudden, and often unexpected, rise in lake level associated with a rapid decrease in atmospheric pressure, or a strong storm. If high winds are steady for a long period, several hours, a change in water level may occur known as wind set-up. Once the winds cease or shift direction, a standing wave called a seiche may occur. This can result in significant increases in water level. Example, a strong northeast wind can increase the water level on the east side of Lake Erie while decreasing on the west end. This is very similar to the wave created by sloshing water in a bathtub. The lake surface will oscillate from higher to lower with decreasing amplitude over a period of up to several days. It is this oscillating water level that is most often confused with a tide as that the period of the oscillation in some lakes is very similar to the period of the tides.

In summary:
Great Lakes water levels are primarily influenced by long term changes in wolume of water and meteorological events.

How can water level data be used to determine the depth of the water?

A “datum” is a base elevation used as a reference.  Chart Datum, the datum to which soundings are referenced,  for NOAA charts in the Great Lakes is Low Water Datum (LWD).  LWD  is a fixed datum for each lake relative to International Great Lakes Datum 1985 (IGLD 1985). CO-OPS real-time water level data are presented in inches above or below LWD.  Historical data are presented in feet or meters above IGLD85.

Is water level data available in other formats?

Real-time water level and meteorological data are disseminated from NOAA’s Great Lakes Online Interactive Voice Response System (301) 713-9596. A brochure on the voice response system is available HERE.

How can historical water level data be retrieved?

For current and historic levels in the Great Lakes, see the Great Lake Online or Water Level Observationssections of our web site. The information available includes six minute data, hourly heights, daily means, monthly means, max/min reports, and 7-day reports.

How are water levels measured?

A typical water level station consists of a gauge house mounted on top of a six foot diameter sump. The house contians the water level sensors and data collection platforms (DCP).  Data are transmitted hourly via Geostationary Orbiting Environmental Satellite (GOES).  Real-time data are acquired every six minutes by connecting to the gauge via a telephone modem.    In addition to a primary and secondary water level sensor (see Figure 2), some stations also acquire meteorological data including wind speed and direction, air temperature, water temperature, and relative humidity.

The primary water level sensor consists of float and a shaft angle encoder system.  The floats rest in wells below the gauge house.  The wells are connected to an intake in the lake or river.  As the water level of the lake or river rises or falls, so does the water level in the sump.  The shaft angle encoder measures how much the floats move up or down which is used to determine the water level relative to the datum.  (Click HERE to see an animation of how water levels are measured.) The data are stored in the DCP.  The backup water level sensor uses pressure to determine the water level.  The backup water level data are stored in back up DCP.  The Electric Tape Gauge (ETG) is used as a reference gauge and for a precise check of the primary and backup gauges.  The ETG is attached to a battery and voltage meter.  When the bottom of the tape touches the surface of the water, current is produced.  The tape is read and compared with the primary gauge reading at the same time.

Water level gauges are tied into a network of benchmarks. Annual leveling between benchmarks is conducted to maintain geodetic control of the gauge.

Figure 1:  Rock Cut, MI water level station

Figure 2: Inside Rock Cut, MI water level station

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