Have you ever wondered how estuary tides work? I certainly have! That regular movement of saltwater into the estuary and then back out to the ocean, creating a dynamic estuary environment. It’s a wonderful thing to have been learning about over the last year since I started working with MGET, but it does often have me wondering how and why does it happen?
I have been teaching primary school children the basics about Mersey estuary tides in assemblies and workshops. They learn that the estuary can look very different at different times of day. When the tide is out, there is lots of the sandy mud that lines the estuary to be seen, with some freshwater coming from the river. When the tide comes in, the estuary quickly fills with saltwater and all of the sandy mud is covered, with lots of birds flying away to the banks of the estuary, waiting for the next low tide.
Then of course there is the influence the Mersey tides have on the colour of the Mersey. People (myself included) have been surprised to learn that when the tide comes rushing in, it churns up the sandy mud that lines the estuary causing it to float in the water, giving the lower Mersey estuary its infamous brown colour. It is completely normal for the estuary to be brown, it does not mean the water is dirty. The Mersey estuary has the second highest tidal range in the UK, which is why the tide is so powerful and very good at disturbing the sandy mud. This means that there is a huge difference between the height of the low and full tides. If we think of low tide as being 0 metres high, the highest Mersey tides can be over 10 metres high (National Oceanography Centre, 2026), meaning there is more than 10 metres difference between low and high tide!
Tides are incredibly important in coastal environments, with benefits from maintaining the health of coastal environments through distributing nutrients, to playing an important role in the life cycle of many organisms. For us as humans, the predictability of the tides mean we can plan around them when navigating estuary waterways on ships, boats, and even kayaks! Tide times can be predicted sometimes years in advance, and only really change with landscape changes. We are always looking at tide times here at MGET as high tide influences when we are able to conduct work on the saltmarsh.
So why does this tidal movement happen? Come with me on a journey as I teach myself why we have 2 high tides every day. I should note I have had this explained to me several times by various people, but something in my independent brain likes to not understand things until I have worked it out myself!
We look to the moon for our answer. Simply put, the moon has its own gravity which pulls ocean water toward it, creating a ‘tidal bulge’ on the side of the earth closest to the moon – this causes a high tide. A second high tide occurs on the opposite side of the earth due to a centrifugal force in the earth-moon system.
If you are satisfied with that answer, please feel free to skip the next few highlighted paragraphs as I explore the above in more detail and understand how each part works…
The moon has a gravitational pull, so wherever the moon is closest to on earth, the tide will be pulled toward the moon. This causes a ‘tidal bulge’ where lots of ocean water gathers – hence the high tide. This high tide on the side of the earth closest to the moon makes sense to me, but where I have gotten lost in the past is understanding why we have 2 high tides a day. Surely if the moon moves around the earth once per day, we would have just 1 high tide per day?
To answer this question, we must consider something called the centrifugal force. My research has found that the physics involved is way beyond my limited GCSE knowledge, but I’ll give it a shot!
The moon and earth are rotating around each other, rather than just the moon rotating around the earth as we would expect. Or I should say, both are rotating around a central point. When objects rotate a ‘centrifugal force’ occurs which throws things outwards. Professor Brian Cox explains this as imagining you’re on a roundabout at a park; as you spin around, you hold on tight as it feels you are going to be flung off the roundabout! On earth, the centrifugal force is pushing ocean water away from the earth. On the side of the earth furthest from the moon, the centrifugal force gets a bit more power to push ocean water away from the earth (due to less influence from the moon’s gravity) – this causes another tidal bulge on that side of earth, the second high tide.
I hope you’ve stayed with me on this one, what makes sense in my head may well only make sense to me! If you’d like to understand things a bit more, or see things in a different format explained by people with brains much bigger than mine, I have provided some links to YouTube videos below:
Neil deGrasse Tyson explains the Earth’s ocean tides 🌎 🌊 #science #space
Why do we have tides? – Forces of Nature with Brian Cox: Episode 2 – BBC Here, Brian Cox explains the centrifugal force
The last thing I wanted to understand (on this topic, there are always about a hundred tabs open in my mind of things I’d like to understand!), is why the height of the tide changes throughout the month. Gravity is strongest when the moon, earth, and sun are in a straight line (as shown in the photo to the right). This happens at a full moon, as shown in the photo, and a new moon, when the moon would be on the right side of the earth, in-between the earth and sun. Because the gravity is stronger, more water is pulled into the ‘tidal bulges’ causing a higher high tide.
Tidal movement is a fascinating topic, to which I hope I have done justice. So many things come into play to give us the tidal movements we experience in the Mersey estuary, which is central to this dynamic environment. If you’d like to know more on this topic, you’re in luck! Next month, our Biodiversity Manager, Andy, will explain the difference between spring and neap tides, what influence weather can have on tide height, and why the shape of the Mersey creates its unique tidal patterns.
Until next time, Hannah.
References:
National Oceanography Centre. (2026, January 21). National Tidal and Sea Level Facility. Retrieved from https://ntslf.org/tides/about-tides/river-mersey