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4 | How levels and flows are measuredWater levels are measured and recorded at many locations around the Great Lakes and on their connecting channels.
The National Ocean Service of the National Oceanic and Atmospheric Administration (NOAA) presently operates 31 water level gages on the Great Lakes and 18 gages on the connecting channels. In Canada, the Canadian Hydrographic Service maintains 29 water level gages on the Great Lakes and 27 on the St. Lawrence River. Other agencies, including the U.S. Army Corps of Engineers, New York Power Authority and Ontario Power Generation, also operate recording gages at various locations on the lakes.
Great Lakes water levels are officially measured from the International Great Lakes Datum 1985 (IGLD 1985). This datum is referenced to sea level, as measured at Rimouski, Quebec, near the mouth of the St. Lawrence River. Because the crust of the earth in the Great Lakes region is continuously rising with respect to sea level, and the rate of movement is not uniform throughout the region, the IGLD must be updated every 25 to 30 years.
The rate of flow, or discharge, in a river or Great Lakes connecting channel is determined by measuring the channel depth and width, and the velocity of the flow. Measurements can be made by boat, from a bridge, or from a cableway strung across the river. With sufficient measurements of flow over a range of water levels, mathematical relationships can be developed between levels and discharges for various points along the connecting channels and the St. Lawrence River. These equations are essential to the coordination of outflow data, particularly related to hydroelectric power usage of Great Lakes waters.
Graphics: Water level gaging station; measuring outflows at the Niagara Cableway.