Tides and Currents 101
At 2 AM one summer morning, I crawled out of my tent and heard a gentle bumping sound. I went to investigate. Our four kayaks were about to float away: the water had risen while we slept—but we were on and island in the Columbia River 80 miles from the sea. I hastily dragged the kayaks further up the beach and tied them to a tree before we became stranded like Gilligan and the Skipper. Welcome to the world of tides and ocean currents. The ocean was making itself known, even far inland.
We all know the ocean has tides, and that the moon is involved somehow. But beyond that, tides and currents are a mystery to most people. But they’re what make the ocean a living, breathing thing. Here’s how they work.
There’s a Bad (or Good) Moon on the Rise
Most people know that the moon causes tides. Its gravity pulls the ocean toward it, creating a oceanic bulge that sloshes across ocean basins like a disturbance through a bathtub. Since the moon is the tidal driver, tides and currents are more extreme when the moon is new or full and weaker in the middle of the lunar cycle.
Tides vs Currents
The first distinction most people miss is that tides and tidal currents are different things. Tides are the change in the height of the water: high or low. This controls whether your boat runs aground, whether your beached kayak floats away, and when to take the kids to poke around in tidepools. Currents are the sideways movement of the ocean as a result of the tide coming in or going out. These cause the ocean to flow like a river, and if you’re sailing or paddling, they’ll control where you can go. Let’s talk about tides first.
Tides Aren’t The Same Everywhere
In a continent-less world like Kevin Costner’s famously awful movie Waterworld, the tidal bulge would move across the globe uninterrupted, and tides would be the same everywhere wherever. But continents block the bulge, which creates different tidal patterns in different parts of the world. For example, on the West Coast of North America, we have “mixed semi-diurnal tides”. This is a fancy way of saying that there are two low and two high tides every day, and one is higher and lower than the other.
Changes in Latitude, Big Changes in Attitude
On the day I’m writing this, the height difference between high and low on Kodiak Island in Alaska is 12 feet. In Micronesia, a mere 5 degrees north of the equator, it’s only two feet. Tidal difference tends to be more extreme as we get closer to the poles.
Tides are also local. The particular shape of landforms and the bottom of the ocean affect tides. The Bay of Fundy in Canada’s Maritimes is famous for the highest tides in the world, where water has to squeeze between New Brunswick and Nova Scotia. On the Columbia River, it takes a while for the high tide in the ocean to make it’s way upriver.
How To Read a Tide Chart
Tide charts are different depending on who makes them, but they’re all predicting the vertical height of the water. There will be more subtle differences. In the US you’ll see some negative numbers for really low tides. This is because the US uses “average lower low water.” NOAA takes all the lower of the two daily low tides and averages them. So a -1 tide is one foot lower than the average lower low tide. In Canada, you’ll never see a negative number because instead of averaging the low tides, they pick the lowest one and set that at 0. So a “1” low tide is one meter (remember, this is Canada) above the lowest low tide recorded for that spot.
Going Sideways: Currents
Currents are the sideways movement of the ocean as the bulge of tidal water either enters with a rising tide (the flood) or leaves with a falling tide (the ebb). Currents are more complex than simple vertical tides, because water does strange things when it meets inlets, headlands, shallow bays, and constrictions. Currents are also more important, because they can make moving across the ocean either very easy or very difficult. And if you’re in the wrong spot at the wrong time, currents are dangerous.
Currents change as the tide changes and you’ll quickly find that current tables are more difficult to interpret than tide tables which just have time and height. Current tables have direction, called set (the compass bearing the current is flowing towards at that time) and time and the maximum speed of ebb, flood and slack water when the current stops before it starts flowing the other way. They’ll have a precise spot where the current is being measured (latitude and longitude). Currents can change direction when they encounter landforms, and will form eddies behind headlands, just like a river current does. So you’ll want to know exactly where that current station is. And current tables only exist for some places in the world: those used by commercial shipping. Many places sailors and kayakers go have no tidal current information whatsoever—so you have to chat with the locals or figure it or learn to predict currents from simple tide tables a nautical chart, and experience on the sea.
Some places in the world—Great Britain, for example—have rotary currents: where instead of flowing one direction on the flood and another on the ebb, currents go in all different directions at different points in the tidal cycle. These can be mind-boggling to figure out, and leave a narrower window for getting from A to B.
Overfalls, Rapids, and Races
In the Odyssey, Odysseus had to choose between taking his boat close to a sea monster or risking his ship in a ship-eating whirlpool named Charybdis, believed to be the whirlpool that lurks in the Strait of Messina. He chose the sea monster. Where tidal currents squeeze between islands, over shallow shelves, or around headlands, the ocean often behaves like a whitewater maelstrom, often with giant whirlpools. Even if you’re not escaping from the Siege of Troy, these spots deserve caution. Places like Scotland’s Gulf of Corryvrecken and Falls of Lora, British Columbia’s Skookumchuck Rapids, Washington’s Deception Pass , and Wales’ Penrhyn Mahr have earned their reputations for fearsomeness and thrills, depending on what you’re looking for.
And remember that both tide and current tables are predictions. They may well not occur when and where the internet says. I’ve been surprised many times.