What happened to negative tides?


The All Point Bulletin has received inquiries asking how come the tide tables don’t show negative tides anymore. The sad truth is, due to global warming, the glaciers have melted to the extent that the seas have risen to the point there are no negative tides anymore.

You don’t believe in global warming? Actually, we have changed our tidal station from the Tsawwassen ferry terminal to Point Roberts. NOAA and the Canadian Hydrographic Service use different reference planes for measurements and calculations.

Canadian charts and tidal data use Lower Low Water Large Tide (LLWLT) chart datum (see charts.gc.ca) while NOAA uses Mean Lower Low Water (MLLW) for chart datum (NOAA Tides & Currents).

As a result, Canadian tide charts may show negative tides but typically, U.S. charts won’t.


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