Where does river water go?

It might seem like a simple question, but just think about it for a second… Water falls from the sky as rain, it flows over and under the ground and enters into a river. The river flows downstream, maybe passing through a few lakes along the way, until it reaches the ocean. Now what happens? The water has to enter the sea and will eventually be evaporated by heating from the sun and end up back in the atmosphere to form rain again. A lovely full-circle route called the water cycle – you probably learnt about it in Geography class. But, what if we could track a raindrop from the sky, into a river and then into the ocean. What would happen? Does it just flow into the ocean and then get mixed up and thrown around in the wind and waves? Or do tides drag it out further to sea? And what about the fact that the Earth is rotating? Perhaps not so simple after all…


Image credit: https://pmm.nasa.gov/education/water-cycle

This is essentially what my thesis is about. When water from a river enters the ocean, where does it go? It took me almost 4 years to figure it out – or more accurately to understand more about what’s actually happening… I certainly do not have all of the answers (as you’ll see). My plan is this series of articles is to try and explain 4 years’ worth of laboratory experiments, fieldwork, computer simulations and of course maths, so that anyone can understand what I’ve done and why it is important.

That’s probably as good a place to start as any: why is it important to understand more about where river water goes? Also known as the classic question faced by all researchers: why should we care? The grand big-picture answer is of course that by understanding more about the world around us, then and only then, can we begin to answer the fundamental questions about the meaning of life, the universe and our very being. That all sounds a little too Brain Cox to me so let’s try a simpler reason… pollution.

From waste outflows leaving factories to fertilisers used by farmers, it all flows into our rivers. And we want to know where this pollution will end up so that we can try to stop it causing too much damage. If we know where river water goes, then we know where pollution goes – easy (or so we hope). Pollution from fertilisers is a particular problem because it’s difficult to stop. If a factory is pumping out pollution into a river we can tell them to stop and to dispose of the waste properly by some other means. If we tell farmers to stop using fertilisers then they produce less crops, which means less food for us – hopefully you can see the issue.

The fertilisers used on crops seep into the soil and enter the underground water supply. This then flows into rivers, which flow into the sea. Fertilisers contain lots of nitrogen and this is great for growing plants – they love the stuff. The problem with having lots of nitrogen in the ocean is that it causes huge plankton blooms. Plankton are little plant-like things floating everywhere in the ocean, basically tiny sea plants. If you get lots of plankton blooming at the surface of the ocean, it blocks the sunlight from reaching the plankton beneath the surface and so they can’t photosynthesise to produce food, which means that they die. Lots of dead material means lots of bacteria. These little critters break down the dead plant material and when doing so use up oxygen in the water until eventually there’s none left. This is very bad news for fish – they need oxygen to breathe – and so they end up dying too. It’s basically a big circle of death which we scientists call eutrophication.

The good news is that if we know where the river water ends up and therefore where the fertiliser ends up, we can put measures in place to stop eutrophication from happening. So no more dead fish – hooray! At least not until they are caught in giant nets, but that’s a whole other kettle of fish… (pun very much intended).

So there you have it: knowing where river water goes means we can control pollution and stop fish from suffocating to death. And of course by understanding more about the world around us we can begin to answer the fundamental questions… nope I can’t do it. I’ll leave the star-gazing to Brain Cox.



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