What Causes the Daily Rise and Fall of Water Levels Known as Tides?
Tides result from the gravitational pull of the moon and sun on the earth's ocean, resulting in daily fluctuations in the water levels of bays, inlets, gulfs, and oceans. This causes the formation of large periodic waves that move through the ocean. According to the National Oceanic and Atmospheric Administration (NOAA), the height of these tides can vary depending on the day and location, with most coastal areas experiencing two high tides and two low tides each day. However, in some areas, such as the Gulf Coast, only one high and one low tide may occur daily. The height difference between high and low tide is referred to as the tidal range and can vary greatly, ranging from barely noticeable to quite dramatic.
What Does a Tide Table Represent?
A tide table provides a daily forecast of the expected times and heights of low and high tides for a specific coastal region. Typically displayed in a tabular format, the tide table is a valuable tool for those who need to plan their activities around tidal conditions. The US's leading source of tide predictions is the National Oceanic and Atmospheric Administration's (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS), which provides predictions for over 3,000 locations. The Tides and Currents website, operated by NOAA, allows users to access and download tide information for up to two years, both past and future. It's important to note that tide tables should not be confused with tide charts, which provide hourly water level information for a bay or estuary but are only available for a limited number of locations in the US.
How to Read a Tide Table?
Step 1: Retrieve Daily Tide Information for Your Location
There are various methods to access information about the daily tides for your particular location.
Utilise NOAA's free online tool, Tide Predictions, which offers past and future tide information for 3,000 locations. If utilising this web-based tool, choose the closest tide station from the list provided in your state or use the map-based search option. Once you have located the nearest station, click on it to view the relevant information and print it.
Purchase a tide table booklet specific to your area. These can often be found at fishing shops, hardware stores, marinas, or sporting goods stores.
Step 2: Check the Dates of Your Trip
Once you have obtained the tide information for your desired location, you'll need to check the tides for the specific dates of your trip, whether for hiking, fishing, or beachcombing.
For instance, let's say you plan to go on a backpacking trip along the coast of Washington state in mid-October and need to know the high and low tides. You may want to know if it's possible to hike, camp along the beach, or cross a rocky point at specific times of the day.
The tide information can be presented in various forms, such as graphs or tables. Below is an example of tide information for two days: Oct. 12-13, 2019 (taken from NOAA, but other sources may provide similar information in a different format or layout).
Step 3: Record the Timing and Heights of High and Low Tides
Make a record of the times and heights of high tides and low tides and the height difference between them.
For example, at Cape Alava in Washington, there are two high tides on October 12: the first high tide at 12:19 a.m. and the second, even higher, at 12:42 p.m. The first high tide of 7.96 feet means that the water level is 7.96 feet above the average of the lowest low tides (NOAA uses MLLW, or Mean Lower Low Water, which is the average height of the daily lowest tide observed over a 19-year period. It's important to note which level you're using, as sometimes tide predictions are referenced to different levels). There are two low tides, one at 0.92 feet at 6:30 a.m. and another at 1.02 feet at 6:53 p.m.
Looking at the information in graph form gives you a visual representation of the rise and fall of water levels.
Watch a video on how to read Tide Tables
Using the Tide Table Information
The tide table provides you with the height and timing of high and low tides and the direction of the tide's movement. In our example from Cape Alava, Washington, you can see that from 6:53 p.m. on Oct. 12 to 12:57 a.m. on Oct. 13, the tide will rise by approximately 7 feet. If you plan to camp on the beach in the area, it's essential to ensure that you set up camp well above the high tide line, so you don't wake up to waves crashing near your tent. Keep in mind that a high tide of 7 feet only indicates that the tide will be 7 feet above the MLLW reference level, and it doesn't specify how far inland the water will reach horizontally. If unsure, you can look for signs such as debris or a "wet beach" to determine the high tide water line, or you can consult with a park ranger or coastal official.
Similarly, if you need a low tide to navigate around a headland on the beach, you'll need to consider the two low tide options available: either early morning (6:30 a.m.) or evening (6:53 p.m.). Plan your trip accordingly to take advantage of the low tide conditions.
Additional Considerations and Suggestions for Using Tide Tables
A detailed topographic map is advisable as it may indicate areas along the coastline that can only be passed during low tides or specify the necessary tidal heights for navigating headlands.
Olympic National Park recommends rounding headlands within an hour or two before low tide as the tide is receding rather than during an incoming tide.
The impact of a 7-foot tide rise can vary depending on the slope of your beach or coastline. A noticeable rise in water level may occur on a shallow slope but not necessarily on a high cliff.
Tide predictions are based on astronomical calculations and can be affected by other factors such as weather, wind patterns, topography, and river runoff. The actual tides in a location may differ from the predictions, so it's essential to remember that tide predictions may not reflect the actual water levels in some areas.
However, the timing of the tide's rise and fall can still provide valuable information even if the water levels are different from what is predicted. The cyclical nature of the tides (up and down) remains consistent.
If you plan to stay in an area for a while, you can observe the changes in the water level during high and low tides. For example, the roots of trees or rocks may be visible at low tide but covered with water during high tide.
It is also advisable to check water levels in a specific area, which can be obtained from sources such as NOAA, the U.S. Geological Survey, or state agencies.
Collectors of beachcombing treasures such as shells, sea glass, agate, etc., may want to visit the beach between high and low tides when the water is lower and more items are exposed.
On the other hand, clammers may have the most success during low tides when water levels recede to reveal clam beds. In Oregon, for example, the best clamming opportunities occur during "negative tides" or "minus tides," which are tides below the MLLW.