Tidal Waters

If you are mostly an inland water boater, you may not be familiar with ocean boating and tidal waters. Reno is so close to the Bay area that sooner or later, most of us are tempted to explore those waters. The reach of ocean tides is long, even affecting the waters of the Sacramento River Delta region. This Boating Tips of the Month will go into some common aspects of tides and tidal tables.

Tides are produced by the gravitational effects of the sun and the moon. However, because the moon is much closer to the earth than the sun, the moon's effect on the earth is much more dramatic. In the United States, there is usually a semi-diurnal tide, i.e. two high and two low tides each day. Because the lunar day ( the time required for the moon to orbit one time around the earth) is 24 hours and 50 minutes, the high and low tides are 50 minutes later each day.

Tide Chart

This chart, from the NOAA website/a>, demonstrates the tidal change in Alameda, California near Coast Guard Island. Tides are generally referenced to MLLW or mean lower low water. This phrase may sound redundant, but really gives useful information. Mean low water (MLW) is the average height of water at a specific location over the previous 19 years at low tide. In the table above you can see of the two low tides, one of them is lower than the other. Thus MLLW is the average of the lower tide at this location over the last 19 years. If you are crossing under a bridge, for example, the bridge clearance height is listed based on MLLW.

In this table, note that the high tide on 2/3/2002 occurred at noon with low tides occurring at 6 AM and 7 PM. On this date, the second high tide occurred shortly after midnight, so on certain days there may not be two high and two low tides. The lowest tide of the day occurred at 0600 on 2/3/02. This value will be utilized and averaged in with data from other days to give a historical mean lower low water. Note also in this table that the values are given in meters, while most tide tables are given in feet. Thus, in this example, the tide at 0600 would be a low tide, and listed as 0.66 feet above MLLW. The high tide at noon would be listed as 6.55 feet above MLLW and the next low tide at 7 PM would be listed as 1.31 feet above MLLW.

Ib>n this sample from a tide table, on January 1st, the lowest low tide occurred at 1 minute after midnight. This tide height was -.8 feet. The high tide of the day occurred at 6:42 AM with a height of 4.8 feet above MLLW. The next low tide occurred at 12:55 PM and was a - .7 feet tide height. Negative tide heights mean that that tide was slightly lower than the average lower low tide over the last 19 years. The final high tide for the day was at 7:06 PM with a height of 3.9 feet above MLLW. Note that as previously mentioned, some days like January 16th in this example, do not have two high and two low tides.

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In this table, from the NOAA website, actual observations are compared to predicted tide heights. The results are impressive in their accuracy! This kind of data truly is a great use of taxpayers' money. This data can be accessed for many coastal locations on the NOAA web site. Use this information in your next planned trip to calculate bridge clearances, and time of maximal tidal currents.

Thanks to Gregory Hernandez, Webmaster for NOAA for permission to post these Tide Tables.p>