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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. 

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 Table for 7/22/09

This chart 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 7/22/2009 occurred at 1:52 PM with low tides occurring at 6:57 AM and 6:50 PM.  On this date, the second high tide occurred at 12:41 AM on July 23rd, so on certain days there may not be two high and two low tides.  The lowest tide of the day occurred at 0657 on 7/22/09.  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 feet, while some tide tables are given in meters.  Thus, in this example, the tide at 0657 would be a low tide, and listed as -1.5 feet above MLLW.  The high tide at 1352 would be listed as 6.0 feet above MLLW and the next low tide at 1850 would be listed as 2.3 feet above MLLW.

Tidal Data for Alameda California

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 by going to NOAA's tidal data section.  This link is for the California coastal region. This may help in your next planned trip by allowing you to calculate bridge clearances, and time of maximal tidal currents.

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