Tides

Daily Oscillations

Because the moon is so close, its gravitational effect is most noticeable. The moon takes about 24 hours and 50 minutes to return to the same position in the sky from one day to the next. Thus the tides of one day repeat themselves (with minor variations) about 50 minutes later on the following day.

Monthly Tidal Effects

When there is a new moon or a full moon the gravitational effect of the sun augments that of the moon. This produces the spring tides that are the greater of the two monthly tidal ranges. The opposite occurs when the moon is in the first or third quarter. In this configuration, the two gravitational forces oppose one another and neap tides occur. The word "spring" in this context has nothing to do with the season: both spring (an outflow of water) and neap (Anglo-Saxon for "scanty") come to us from Old English.

Tide Measurements

Water levels are measured constantly at numerous tide stations along Canada's mainland coast and on the major sea islands. A tide gauge must be in place for at least 19 years to record all the possible ranges of tides, because that is the cycle of the relative motions of the earth, moon and sun.

Tidal Definitions

Since the dawn of history, the rise and fall of tides has been studied by people near the oceans. Not surprisingly, therefore, the terminology of tides has become a part of our everyday language. Spring and neap tides have already been defined. Flood tide is the incoming or rising water, and ebb tide is the reverse. The tide turns when the water has reached its maximum and starts to fall or has reached its minimum and starts to rise. On old nautical charts the terms spring and neap were commonly used to describe the tidal range of a port or anchorage. Although these terms are firmly embedded in nautical lore they are not used in modern tide tables or on modern charts. Modern terms are more precise than the older terms in describing tidal surfaces which, in the real world, are affected by both astronomic attraction and local effects of water flow and weather.

Chart Datum

The Canadian Hydrographic Chart Datum (CHCD) is the lowest tide that can be expected in a given locality considering gravitational effects alone. Even lower tides may occur rarely if strong or persistent winds and storm surges work to further lower the water.

MWL

Mean water level. Average of all hourly water levels over the available period of record.

HHWLT

Higher high water, large tide. Average of the highest high waters, one from each of the 19 years of prediction.

HHWMT

Higher high water, mean tide. Average of all the higher high waters from 19 years of prediction.

LLWMT

Lower low water, mean tide. Average of all the lowest low water, one from each of the 19 years of prediction.

LLWLT

Lower low water, large tide. Average of the lowest low water, one from each of the 19 years of prediction.

LNT

Lowest normal tide. In present usage, it is synonymous with LLWLT, but on older charts it may refer to a variety of low-water chart datums.

Recorded Extremes

Highest and lowest tides recorded at a tidal station.

Geodetic Datum

Mean sea level as determined from readings taken at tide stations around the coast of Canada and the United States.

The shape of the bays and estuaries has a decided effect on the size of tides. The long narrow funnel of the Bay of Fundy produces the greatest tidal range in the world. Burntcoat Head, in the Bay of Fundy, has the largest tidal range in Canada (16.1 metres). Eureka, on Ellesmere Island, probably has the smallest tidal range (0.1 metres).