Why did the wave cross the ocean? To get to the other tide…sorry…Now that we are familiar with waves, let’s turn our attention to tides and currents. Tides are basically waves controlled by gravitational pull of the sun and moon. High tides are when the pull is strongest; for many dives this is when we tend to arrange our splash times, when we enter the water. As high tide approaches, we suit up and get ready, then dive the rest of the incoming tide as well as the immediate slack tide that follows high tide.

Rachel Carson Salt Pond at extreme low tide, entry channel to deeper water is to the right, all other surrounding rocks become slick with marine growth and waves washing over them (Photo by Charles H. Lagerbom)

At a site like Rachel Carson Salt Pond near New Harbor, we try to enter at high tide since anything lower means crossing the actual salt pond and then slippery, wrack-covered boulders to the waters beyond. Difficult, if not impossible to traverse, this stretch is more underwater during the higher tides and therefore easier to negotiate.

One time we worked towards the exit at Rachel Carson Salt Pond, only to have one member of our group get tired out. They decided to swim towards shore rather than back through the gap by the salt pond. Of course, we stayed with him and so powered our way into thick seaweed and slippery rocks which were just awash with the falling tide. These are pretty hardy living things. As we have seen, tides most dramatically affect the biota living in this intertidal zone.

That time out, we all took quite a few diggers trying to get to shore, walking was nearly impossible in thigh deep water washing over barnacle covered rocks slick with seaweed. I fell a couple of times in an ungraceful heap, one time banging my shin on a rock. Our tired dive buddy even lost his fin in the effort, although someone recovered it later and left it hanging by the Salt Pond Preserve sign where we park. I picked it up on a later dive there and got it back to him.

Entry rocks at Beauchamp Point in Rockport at high tide and at less than high tide (Photos by Charles H. Lagerbom)

That is what makes Beauchamp Point in Rockport such a great dive, since the two sets of rocks you can enter from provide a nice, easy entry pretty much at any tide level other than absolute low. The series of ledges make it easy to sit and don your fins and mask, then roll into the water. We did try going in one time at absolute low, just to see how different the site is. The slippery rocks made it no fun and we have not done that again. Same thing at Duck Trap Harbor, where low tide means a long slog out to even knee-deep water.

At lower tides, Beauchamp Point’s entry rocks can be slick with seaweed and marine life, exercise caution when entering or exiting (From collection of Charles H. Lagerbom)

The highest high tides occur when earth and sun align and thus exert their pull together. These are known as Spring tides. They tend to happen on full and new moons. For years, shipbuilders would use these seasonal tides as well as the spring freshets of meltwater to launch their ships from what otherwise would look like dinky streams that could no way launch a vessel.

When sun and moon are at right angles, water is pulled in both directions. This results in smaller differences of high and low tides. These are known as neap tides, and they occur between the Spring tides. Semidiurnal tides happen twice a day, while a Diurnal tide happens only once a day.

Then there are Bore tides, these actually produce a wave as it travels upriver. A Brown Tide is a type of algal bloom that occurs in the water. While it is not harmful, the discoloration is very noticeable. Then there is the infamous red tide, which is a harmful algal bloom.

On the surface, winds push the water around in giant circular patterns called gyres. These currents form by steady winds such as the famous oceanic trade winds. In the northern hemisphere, water is moved towards the right, relative to the wind direction. In the southern hemisphere, this is to the left and is known as the Coriolis effect.

I made a note to check this out my first time in New Zealand, before I deployed to the Antarctic Continent with a glacial geology team from UMaine. In the hotel, I watched how the water circled the drain. Opposite of what it did in the northern hemisphere! How cool is that?

As for currents, however, this moving water bumps into the continents and gets deflected. As a result, they form different gyres in the Atlantic, Pacific, and Indian Oceans. And remembering Coriolis and its effects, in the northern hemisphere, these flow clockwise, while below the equator the flow is in a counterclockwise motion.

For scuba divers, one of the most infamous examples of ocean waves impacting the coastline is that of the longshore or littoral current. These are ocean currents created by waves refracted, or bent backward angularly, as they reach the shoreline.

Longshore current off Rachel Carson Salt Pond, it moves you parallel to shore and can be dangerous (Photo by Charles H. Lagerbom)

They are generated in the surf zone when the front end of a wave is pushed onto shore and slows down. The back of the wave is still in deeper water so is moving faster than the front of the wave. The water then moves faster and flows parallel to the coast. With the arrival of more water, a newer part of longshore current is pushed onshore. The result is a zigzag pattern created in the direction from which the waves are coming in.

The energy released when waves break on the beach creates these longshore currents. When waves approach the beach at an angle rather than straight on, two things happen. One of them is that part of the wave’s energy goes perpendicular to the shore. The rest of it is directed parallel to the shore. It is this parallel energy, which powers the longshore current. So, it runs parallel along the shoreline. If you’ve ever been swimming in the ocean and felt the ocean tugging you farther down the shore, then you’ve felt the impact of a longshore current.

We felt one at Rachel Carson Salt Pond. Going out to the area where at 70 to 80 feet you come upon some pretty incredible Sea Anemones, we surfaced to find ourselves in a longshore and moving parallel to the preserve. We went back down and worked our way in angles, trying to cut across the current. Luckily, we made good progress and were able to return to the gap at the Salt Pond entrance.

Rip currents are yet another type of coastal current that can be a problem for divers. They form when underwater land, forming rock piles or ledges, prevent the waves from flowing straight back out to sea. You’ve probably seen signs posted at a beach warning of rip currents. They can be pretty dangerous. Getting caught in one, it is difficult to fight the current and swimmers are sometimes swept pretty far out before the current dissipates. Rip currents result from waves that have already crashed, called spent waves, being channeled out through a narrow opening between reefs, or ledges or a sandbar. This funneling effect makes a rip current race along with great force.

One more type of coastal current a diver might encounter is an upwelling. These occur when winds literally blow away the surface water which allows deeper water to rise and take its place. The opposite is called downwelling, an even more serious condition for divers. These occur when winds blow the surface water towards the coastline or some such barrier. The accumulating water then forces the water on top to sink, pulling all down with it. Divers encountering downwellings have been pushed pretty deep in a very short time. There is considerable danger with this from pressure and equalization of the air cavities in you.

The important consideration for scuba divers is to pay particularly close attention to water and weather conditions. These can rapidly change; we have driven long distances to a dive site only to arrive and agree that the conditions are not good for diving there that day. But that is how it goes. We console ourselves knowing we can always come back by saying “the ocean will always be there.”

Charles Lagerbom teaches AP US History at Belfast Area High School and lives in Northport.  He is author of “Whaling in Maine” and “Maine to Cape Horn,” available through Historypress.com.

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