Enjoy the Sound: Tides and Currents

Blogged by Dick Wood in 2009:

One of the really exciting things about Puget Sound is how and why the water moves. Water movement comes about through tides, currents, and wind.

TIDES

Tidal action causes the salt waters of the Pacific Northwest to move along two axes. They move up and down and they flow back and forth. This tidal action is the result of a complex but predictable relationship between the earth’s oceans and the moon and sun.

Our tides are in constant motion. The up and down movements of our salt waters reflect differences in water depth. When the depth of the water reaches zero at a particular location, then beach is exposed. High tide is when the water is the deepest at any particular location with the least amount of beach exposed. Low tide is when the water is the shallowest at that location with the most beach exposed.

We measure the level of our tides in tenths of a foot, with an average of low tides as our zero point. A typical range of tide levels in Puget Sound varies between a high of 12 feet above this zero point and a low of 3 feet below this zero point, called minus tides.

Our tides move in daily, monthly, and yearly cycles. There are two high tides and two low tides every day caused by gravitational attraction and centrifugal force of the earth-moon system.

The levels of the first high tide and the second high, however, are rarely the same; nor are the levels of the first low and the second low the same. The time interval for the first daily tidal cycle and the second daily tidal cycle also can vary.

A lowering, outgoing, or ebb tide is that water action occurring between a high tide and a low tide. The rate at which the tide moves out increases from slack water, usually just after the high tide, to its maximum rate (maximum ebb), and then decreases to slack water again, around the time of the low tide.

From day to day the height of the tides and the time of the tides vary reflecting monthly lunar cycles. When the moon is new and when the moon is full, the moon, sun, and earth are aligned and the tides (spring tides) are higher and lower. When the moon is in its quarter phase, our tides (neap tides) are less extreme.Tides changes occur about 50 minutes later from one day to the next because the moon is rising some 50 minutes later each day. The difference in tide level can be over a foot on successive days.

Our lowest and highest tides of the year occur near the summer and winter solstices. Fortunately, for clam diggers, the low tides of late May to early July happen around noon. The low tides of late fall and early winter occur near midnight.

High tides and low tides do not occur at the same time for all locations in a region. The height of the tide also varies by location. For example on June 25, 2002 the time and height of the tide for five locations was as follows:

See here for tides in your part of Puget Sound

Actual water depth can vary from predicted tide level because of weather conditions.  Storm surges, heavy rains and river runoff, and low-pressure weather cells produce higher than predicted tides.

A cautionary note to tuck in the back of your head—a tsunami is a very high tidal wave caused by an undersea earthquake. Tsunamis warning systems and evacuation routes have been established for the Washington Pacific coast beaches. A tsunami approach is signaled by a rapid and extreme very low tide. If suddenly the bay you are digging clams in suddenly empties, exposing never-before-seen beach, instead of being drawn out to investigate, head for high ground or towards open water in your boat.

WHY ARE TIDES IMPORTANT OR OF INTEREST?

• The tide level will determine how much of a particular beach is exposed which in turn will determine what shellfish we can gather.  Very low tides happen only a few times a year.
• Tide action affects fish feeding behavior.
• Whether the tide is going out or coming in will determine if the boat you just beached will float or become grounded.

CURRENTS

The back and forth movement of tidal bodies of water produce currents. Currents move in different directions and at different speeds for different times and locations.

An outgoing or ebb time occurs between high and low tide as water empties out of Willapa Bay, Grays Harbor, and Puget Sound, heading towards the Pacific. An incoming or flood tide occurs between low and high tide as water returns.

Currents are not to be confused with wave action.  Wind waves may move with or against the current.  Water tends to be flatter when the two forces are aligned and choppier when they are opposed.  This action can impact launching or landing a boat. Afternoon conditions at the Mukilteo boat launch can get very exciting in the late afternoon during the summer with a strong north wind and the max flood tide interacting to produce large waves.

The direction a current flows and the speed that it moves is determined by the physical structure of the tidal region. Typically, currents move faster where structure is constricted by narrows or passes.

Puget Sound is a very complex system. Fortunately there are tide tables and current charts available which provide detailed information for seafood gathers.

WHY ARE CURRENTS IMPORTANT

• Fish feed around periods and places of slack water. They will be found in different places depending on whether the tide is ebbing or flooding.
• Currents will affect your boat.  If you are rowing with the current, it could be a long struggle to return to your starting point.
• Rip tides occur where currents meet, concentrating baitfish and attracting larger fish.