Tides and Currents

Depending on the time of year and position of celestial bodies, we have a very wide tidal variance here on Hilton Head, ranging as low as 6 feet and as high as 10. This is evidenced along the beaches and marshes around the island. In the morning you can find yourself as a beach goer sprawled out along the packed wet sand, plenty of space to distance yourself from other sunbathers. Ghost crabs popping out of their burrows, foraging for a bite. Little plovers zigzag back and forth in the surf, pulling worms out as they move.  Hours later, most of the beach has disappeared, and space along the dry powdered sand is at a premium as the water level creeps up with every wave.

The tide changes every 6 hours, 12 minutes and 30 seconds, meaning HHI will have 3-4 tides a day, depending on the timing. A ten foot tidal variance between high and low means that the water level rises or lowers about an inch every five minutes. As the waters move into or recede from the narrow rivers and creek, the water speeds up creating currents exceeding 4 miles per hour! This is considered a very fast moving current and is the main reason our water isn’t clear: sediment and pluff mud can’t settle when the water moves that quickly.

Spring Tides occur when the sun and moon are in alignment, as is the case with a full or new moon.  Their combined gravitational pull results in higher high tides and lower lows.  Neap tides occur when the sun and moon are at right angles to each other, creating a weaker gravitational pull, resulting in less dramatic tidal range.  King tides are a popular non-scientific term used to describe exceptionally high tides that occur when the full or new moon corresponds with either the moon or the sun being in closer proximity to the earth.

So why are our tide levels so extreme? In the Virginian Chesapeake, the tide ranges between 1 and 3 feet. In the Florida Keys, the same. What’s the big deal about here?

Take a look at a map of the United States. One hundred miles south of Hilton Head lies the westernmost coastline of the eastern seaboard.  This area near Brunswick, Georgia is the absolute center of a giant coastal arc between Cape Hatteras, NC and Cape Canaveral, FL, known as the South Atlantic Bight. The entire South Atlantic Bight is considered to be one of the most diverse ecosystems in the world, from salt marshes to coral reefs, and extends out hundreds of miles out to the continental shelf.

Old House Creek at low tide

At high tide the inward-curving coastline forces water along the entire Southeastern coast to pile up, creating that ten foot tidal exchange near the arc’s center, and we reap the benefits of being so close.  The salt marshes here attain their greatest width, nearly 8 miles inland, between Brunswick and Beaufort, SC. This area harbors in excess of eight hundred thousand acres of salt marsh, more than half of all the marsh land along the entire US east coast.  Nearly every acre of the wetlands is drained and refilled during each tidal cycle, with over 1.3 trillion gallons each cycle!

Each cycle exchange brings in sediment, moves detritus, aids shellfish and mollusk larvae to move about, and provides a continuous food source for bait and pelagic fish to feed off, as well as a host of marine mammals, including our Atlantic bottle nose dolphins.

So the next time you’re out on a tour with us watching these amazing creatures, you can be thankful of the tides that help bring them here.

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