Marching to the Tide's Beat: Life in Coastal Alaska
One of the things you come to learn about Alaska is that it’s bigger than you. No, I’m not making a size reference (stand down, Texas). I mean that Alaska is intrinsically made up of forces—constant, fluid, rushing change from one extreme to the other—and it is all so much bigger than you. It’s a learned reality, because while you may know it, it takes experience to really drive that lesson home. This is one of the things Alaskans fall in love with; it makes us feel small and a part of something big all at the same time. And to live in Alaska or just to enjoy it during your summer vacation, you need to plan according to Alaska’s rules. You need to march to the beat of Alaska’s drum because it’s bigger than you and doesn’t care about what you planned. For life on Alaska’s coast, one of the greatest forces that has final say over how each and every one of your days is going to look is the tides.
We wrote this article (1) to help our guests plan in accordance with Alaska’s tides and (2) for us all, resident and guest alike, to know the scientific underpinnings of the ocean that feeds us, is our happiest playground, and is home to marine life of every size and shape.
Kachemak Bay sees some of the largest tidal fluctuations in the world, with an average difference of 15 feet in water levels between low tide and high tide. That means on an average day, in our 40 mile long bay, the surface of the ocean rises and falls 15 feet. Twice. Millions of cubic feet of ocean gushes in and out of the bay every 12 or so hours. Why?
Tides are formed most significantly, though not exclusively, by the gravitational pull exerted on the earth by the sun and the moon. The tides that result from this are predictable and happen like clockwork. However, the effects of gravitational tides can be dramatically enhanced by factors like local topography and the weather.
The “Scientific Underpinnings”
Everything that has mass exerts, to some degree, a gravitational pull on the objects around it. How much of a gravitational pull will be exerted on nearby bodies is directly proportional to the mass of the object. The larger the body, the greater the gravitational effect. For example, the earth’s gravitational force exerted on us (and everything else on the surface) keeps us grounded. And, as we all learned in science class, the Sun is the gravitational center of our solar system because of its mass, holding all of the planets in orbit around itself.
The sun, the earth, and the moon all exert a gravitational pull on the earth’s oceans, which explains all of the variation we get: two high tides a day (the sides of the earth closest to and furthest from the moon) and two low tides a day, not to mention bi-monthly spring and neap tides. When the earth is parallel to the sun, during a new and full moon, we get what is called a spring tide. When solar and lunar forces are aligned, the tidal variation is the most extreme (lower low tides and higher high tides). Seven days after a spring tide, the sun and moon are perpendicular to one another, producing mediocre tidal variation (higher low tides and lower high tides). This is called a neap tide.
Tidal Resonance and Oscillation
Water can store energy in the form of momentum. When water is displaced, like by the hull of a ship, you can see the energy travel in the form of waves, and they continue for long periods of time until the energy disperses in one way or another (this would be the wake that the boat leaves behind). We can think of the lunar tidal force, the Moon’s gravitational effect on the Earth’s waters, as something like a form of displacement as it draws the waters to itself. In the case of Kachemak Bay, it takes roughly 12–13 hours for waters entering its mouth to reach the head of the bay and begin oscillating back in the other direction. This natural movement of energy traveling back and forth within the bay’s waters, in combination with current tidal forces, creates even greater tidal effects. It is the same thing as water sloshing back and forth in a basin. And the shape of the basin matters: enter topography.
The Topographical Factor
Topography, the arrangement of an area’s physical features (i.e., mountains and valleys), causes tides to vary. Kachemak Bay's tidal differences is in part due to the geological nature of the bay itself. The bay is shaped like a natural funnel (gradually growing shallow at the head), causing more dramatic tide levels on the shores. Topography’s effects are exotically illustrated in Turnagain Arm’s shallow, narrow inlet during a new or full moon. After a minus low tide, a significant high tide comes from the Cook Inlet in the form of a single 6 to 10 foot wave. There are few places in the world where this phenomena, called a bore tide, happens and Alaska is home to one of them.
A rip current’s existence is entirely owing to topography. They typically occur when water from a withdrawing wave is bottle-necked by a gap in a sandbar, creating a strong underwater current. Rip tides can produce the same dangerous currents, but are formed when topography and gravitational tides collaborate with each other. On a falling tide (gravitational), a strong current is formed by the constraints of a beach’s contours (topographical). It takes an experienced and watchful eye to read the lay of the land and spot rip currents and tides.
Weather Gets the Last Say
You may have accounted for the gravitational tides when making your plans, but what will make or break them will likely be the local weather. The weather can have an affect on the rise and fall of the ocean, most notably in the case of a storm-surge induced by a low pressure weather system. A storm surge can affect water levels over and above the predicted astronomical tides. It is thought that the 2020 whale in Hallo Bay (we haven't been talking about much else on our social media lately) was picked up and pushed into the middle of the coastal sedge meadow during a winter storm-surge. Remember, a whale is a small thing for an angry ocean to throw around when you have weather, topography, and gravitational tides working in concert.
The Importance of Planning Ahead
Keeping track of the tide is the difference between wet and dry, between stranded (with your fingers crossed for the next high tide to be as big) and home for dinner. Given a bad set of circumstances, it can be the difference between life and death. Because the risk is real and nature’s rules win every time, you march to the beat of the tide's drum, and eventually it becomes your own. It may take some getting used to, but check the tides when planning an excursion. Every time I take my dog for a walk on the beach (or am thinking about it), I check the tides and, because I know the topography of the local beaches, make my choice of where and when to go.
Gravitational tides are very predictable, and you can look up any given day in a tide table. I especially like the app “Tides” (the icon is a wave) because it maps out the arch of rising and falling tides so you know water height at any given time. You may be going where there is no cell service, in which case, the old-fashioned pocket tide book is (and always will be) your friend.
How Tides Affect Emerald Air Service
Everything we do in coastal Alaska is affected by—sometimes very large—but always significant tide swings. Your bear viewing trip is being done on a float plane. From June 1st until some time in late July, we will be operating on and off of the ocean, and thus affected by the tides. Tide height can make a difference with respect to where we can land and how far you may have to walk or wade once we do land. In some places, a tidal difference of 5 feet (vertically) can expose as much as a half mile of beach(horizontally). In a shallow bay on some stages of the tide, you may have to wade knee deep in your hip boots for a hundred yards or more. On some stages of the tide, we can lose or gain 5 inches of water per minute (in a flat bay, this means you will really have to hustle getting on and off the plane if your pilot or guide asks you to—otherwise the plane goes dry and we have to wait 12 hours for it to float again!). Tides can even affect when our trip departs, but this is not common. It can also, on extreme tides, make a difference what coastal locations we can actually work—again, this is uncommon.
Tides are yet another one of the many wonderful, exciting extremes we live with here in Alaska. They bring variety to our lives and remind us how small we really are when the activity of our day depends on these cosmic gravitational forces.