Neap is not (necessarily) the worst river fishing tide…

“Without the moon’s steadying influence,” writes Bill Bryson in his ultimate idiot’s guide to science, A Short History of Nearly Everything, “the earth would wobble like a dying top, with goodness knows what consequences for climate and weather.” It’s hard to believe that something so seemingly small cruising its way across the night sky can have such influence our big old planet. But, as Bill so eloquently explains, the moon is actually the earth’s twin planet. And, at about a quarter the diameter, comparatively large. A kind of a magnetised runt brother that keeps the bigger one in check.

While we should probably be more grateful that twinny keeps us spinning nice and steady, far more visible (and important to us as fly fisherman) is the effect it has on our estuaries. The push and pull of the tides is all the doing of the moon.

River mouth on the drain
African river mouth on the drain

Arab sailors already began relating the timing of the tide to the cycles of the moon way back in the 1100’s, but it was not until 1687, when Sir Isaac Newton first described how every object in the universe exerts a tug on each other, that the how and why became more clear. Well not really, because Philosophiae Naturalis Principia Mathematica – the hefty tome in which he puts forward his universal law of gravitation – has been described as one of the most inaccessible books ever written. Thankfully though, there were those who dumbed it into high school physics for us, so we too could understand that the liquid part of the earth ebbs and flows because of the combined effects of the gravitational forces of the moon, the sun and the rotation of the earth.

But then, that self-same high school physics taught us that there is zero gravity on the moon. How then could the moon push and pull the sea as it does? (Flat earthers can sit down now) According to the clever people at the SA Navy’s Hydrologic Office, the tidal forces of the moon is, at most, one ten-millionth that of the earth’s surface gravity. So, the earth’s gravity keeps the sea on the earth and the moon’s ‘slight’ gravitational strength has a horizontal ‘pulling’ effect.

As the moon orbits us, it creates a slight rise (or bulge) in big bodies of water and ‘pulls’ them along. In the most simplistic terms then: when the moon rises that bulge of water follows it and you get high tide. “An additional high tide is produced at a position on the opposite side of the earth where the centrifugal force (moving away from the centre) of the orbiting system overpowers the gravitational attraction of the Sun and Moon,” so says the IHO Manual of Hydrography. “Low tides are created by a compensating withdrawal of water from regions around the Earth midway between these two tidal bulges.”

Because the moon orbits the earth in the same direction as the earth rotates on its own axis, it takes just more than a day—about 24 hours and 50 minutes—for the moon to return to the same location in the sky. Neatly explaining why the tide moves forward roughly an hour each day.

tidal flow
Sneaky neap tide session on an Southern Cape estuary

Shew…tides hey. A new appreciation for the most consistent variable in the lives of estuary fisherman. More interesting though (and ultimately more applicable to fishing-trip planning), says Dr JD Filmalter, is that the tidal height in an estuary is influenced by other variables too.

“In an estuarine system it (tidal height) also depends on the shape of the mouth, the size of the estuary, the volume of river flow and, the prevailing winds,” he says. According to JD you basically have two major forces colliding: “The one is the flow of the river coming down and the other is the sea pushing up”.

In simple terms then, mouth shape is critical because that determines how much sea water can get in and, more importantly, also get back out.

So, in typical low-flow Western Cape summer conditions, if we start at a neap tide, you have the lowest force from the sea pushing from the mouth.

“As we move away from the neaps, the power of the oceanic push increases daily – the sea water ‘wins’ more and more,” JD says, explaining how every day more sea water comes in than goes out. “This reaches a maximum at Spring tide, when there is a massive push from the sea.”

But, now, and here comes the kicker, the highest tide on the calendar (ie full or new moon) is not the biggest tidal height in an estuary. “I think what is happening with those big pushes, because the mouth is constrained you have leftover water inside the estuary – saltwater and that doesn’t entirely clear on the low tide,” JD dumbs down the hydrology for us. “Because the push is stronger than the gravitational drain, so there is this ‘extra’ water staying in the estuary.”

“Then, on the next tide after spring tide, the sea still dominates in the estuary, so the day after Spring Tide your tidal height is actually higher than it was the previous day,” JD says.

JD is yet to take exact measurements, but he has a hunch that this continues for two or even three days after the Springs, depending on the characteristics of the mouth. This could account for the reason salt water pushes as high as 30km up the Breede in Summer and why you should perhaps plan trips to target certain species in the higher estuarine reaches around those days.

“The highest tide in the estuary after this cycle then rolls down toward Neaps again and you get to an equilibrium – whereby the opposing forces of the fresh river water and sea water are in balance around neaps,” he says.

What is particularly interesting in systems where there is still a decent fresh water flow, is that the day before a neap the water can still be super clean (sea water) right near the mouth on a dead low tide, where the following day it is dirty. “Even though there is very little tidal flow, what happens is that last residual sea water got pushed out on the low tide.”

For fly-fisherman targeting species such as leeries, skipjack and kob this neap situation can be particularly productive. If you time the push of the tide on up-river facing drop-offs and can find the ‘line’ where clean sea water and ‘dirty’ fresh meet and work that, you could be in for some good action. Many boat anglers follow this line for as far up as possible, fishing from clean into dirty.

In addition wind is also going to have an impact on things. Says JD: “If you have a Westerly wind blowing and your river is flowing west to east – like the Breede, and a lot of our East Coast estuaries – that is going to add to the force of the river, so on a neap you might get a lower low because the wind aids the river outflow.”

Conversely then, if the easterly is blowing it will have the opposite effect. “If you have an easterly blowing over a spring, your tide will be even higher on a high tide,” says JD.

Well there you go. Use it, don’t use it.

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