The Ouse and the nile

The Ouse and The Nile

Simon Rogerson

March 2, 2025

As I write this we are approaching the end of another flood season in York and (touch wood) as of yet it has been a relatively quiet affair. The winter of 2024/25 was the first winter since 2018/19 not to see any 4m floods in the city centre (as measured by the infamous Viking Recorder) and everyone is very thankful that there has been less of the disruption, damage and dirt associated with these floods that has been so relentless in recent years.

     King's Staith Under Flood Conditions in February 2022 and from the same spot 3 months later

The River Ouse in York is far from the only river in the world to regularly flood, nor is it the first river in the world to leave people nervously watching on every year, wondering how high it will reach. For evidence we can go all the way back to ancient Egypt where the annual calender was based all around the regular cycle of flooding from the River Nile. 

The flood season of The Nile differed from The Ouse in a couple of major ways. One being that The Nile flooded in summer and The Ouse in winter and the other being instead of being greeted with worry and annoyance, flooding in Egypt was greeted with celebration and joyfulness. As anyone who has ever tried to build a successful civilisation in a computer game can tell you, it's not an easy thing to do. Leaders constantly have to deal with the competing demands for time and money, external threats and natural disasters. Building one of the most advanced and successful civilisations the world had yet seen in the middle of a desert would be like playing the game on the hardest setting imaginable. If it wasn't for the fact that the world's longest river snaked its way through the land providing an otherwise unavailable source of water. Not only that but The Nile could be relied upon each summer to rise up, burst it's banks and spill out onto the surrounding land. In places the flood water could cover up to several miles away from the river banks. Gradually, the waters would fall again and the river would return to its channel. However, crucially, the flood plains would be left with a nice layer of fertile silt proving perfect land for the growing of all of the crops needed to feed a flourishing nation. Without this cycle of flood and silt, there could not have been an Egyptian civilisation in quite the way that we know it and we may never have heard of names such as Rameses, Thutnose and Nefertiti. These names are not relevant to the following discussion, they are just fun names to say. 

The Nile fueled Egyptians were so ahead of the game that by the 5th Century BCE the likes of Rameses and Nefertiti were already being taught as ancient history. In around the 440s BCE the Greek writer Herodotus wrote his ambitious work describing all that had gone before him and simply entitled Histories. Herodotus' work is a healthy mix of hard facts, speculation and colourful tales and he was clearly of the belief that the truth should not always get in the way of a good story. Personal favourites include the claim that Persians would never make a decision before discussing it twice, once whilst sober and once whilst drunk and the claim that Babylonian women could not get married until they had visited the temple of Aphrodite and found a man willing to pay to spend the night with them. Some women find someone before they'd even had time to sit down, whereas others are still waiting years after first arriving. However, in amongst this, I found one particular discussion absolutely fascinating because when Herodotus got on to the subject of Egypt he began to ask an important question. It's one thing to say that the civilisation could only flourish if The Nile flooded every summer, it is another thing to understand the hows and the whys this happens. As a quick spoiler, Herodotus' conclusions are completely wrong, but in his journey to get to the wrong destination he does go via some fascinating ideas that we can still learn from today. 

There were clearly existing theories floating around before he began his writing, some of which he immediately dismisses. One theory that gets short thrift from Herodotus is that the winds near the mouth of The Nile tend to blow from the north in the summer and this might cause the water to back up and flood further upstream. This is quickly dismissed for two reasons, the first being being that the winds don't always blow from the north but even when they don't The Nile floods anyway and secondly that there are other rivers in the area that flow against the wind direction but don't flood. So far so good from our writer; he has considered an argument, observed the data and come to a logical and convincing conclusion.

Other discussions covered by Herodotus are harder to fully understand due to the fundamental difference in the understanding of what constituted the Earth at that time compared to now. One theory he briefly considers is the claim that The Nile flows from the river called Ocean and that causes the unusual flow patterns. This model of the Earth that some people at the time believed in contains a much smaller amount of land than we're used to which was centred on the aptly name Mediterranean Sea and a body of water named Ocean that went around the outside of the land. This might be difficult for us to fully understand but fortunately Herodotus quickly dismisses this theory anyway so we don't have to worry too much about it.

A marble bust of our man Herodotus (picture from Wikicommons)

He then goes on to discuss the theory that seasonal cycles of flooding may be causes by seasonal cycles of precipitation further upstream. Once again our intrepid writer goes off to investigate and once again comes to a logical conclusion that he can dismiss the theory. You can travel The Nile a long way upstream using local pilots as your guide, he explains, out of Egypt, into Libya and out towards Ethiopia. The trouble is that the further you go upstream, the hotter and drier the world becomes until it becomes so hot and dry that there aren't even locals willing to act as your guide. How, he asks no doubt with a confident flourish, could you go through the hot and dry zones and suddenly, magically even, find yourself in a wet zone with enough rainfall to flood a huge river miles and miles downstream. Nowadays, with a better understanding on the layout of the Earth, global circulation patterns, Hadley Cells, convergent and divergent zones etc. we can see that this is exactly what does happens. The highlands of Ethiopia where the river begins, and tropical regions throughout the world, not only have lots of precipitation but also very distinct and dramatic wet and dry seasons. In this case it is exactly the seasonal cycle of wet seasons in the upstream areas that causes summer flooding in Egypt with rains in the Southern Ethiopian Highlands peaking in summer and creating those life giving floods all the way in Egypt. As an aside, at this point Herodotus scoffs at the suggestion that snow melt may add to summer floods. Once again, a fuller understanding of the globe shows us that even close to the equator, if you go up high enough you will find snow and even ice forming that will subsequently melt and add to river flows in the low lands. The most famous snow and ice in tropical Africa is found a long way from The Nile on the slopes of Kilimanjaro and Mount Kenya but there are a small number of glaciers in Uganda and The Democratic Republic of Congo that do melt into The Nile, albeit only adding a tiny fraction of the total flow. At least that was the case around 15 years ago when I was constructing global maps of a glacier meltwater contribution to river flows as part of a previous life in the fabulous Department of Meteorology in the University of Reading. Since I did this work they may have all melted away but that is a story for another time.

So what did Herodotus conclude caused The Nile to flood every summer? Well, rather than the rain he considers The Sun to be the most important meteorological factor. The Sun, according to our writer's view on the world, loves water and draws it up wherever He goes. Nowadays we'd describe this as evaporation but the result is the same. In winter, The Sun sits above Africa and draws up all the water he requires from there. However, in summertime he moves a bit north so that He is closer to Europe. In these months He draws an awful lot of water up from countries such as Greece and Italy and if He requires a fixed amount of water, Herodotus argues, surely that means He must be drawing up less water from Africa than He does in the winter. If that is the case then that must mean that even if there is no more water falling in summer than winter, there is less water being evaporated and therefore more water finding its way into the river and hence flooding. His final conclusion is that it is not excess water due to precipitation that causes flooding at some times of the year, it is a deficit water caused by evaporation that prevents flooding at other times of the year. Once again, a better understanding of the world casts doubt on his idea that there is a deficit of evaporation in Egypt in summer months but it is still a fascinating discussion and a theory that could be applied elsewhere.

Before we move away from Africa it should be pointed out that the high mountains in Ethiopia still see a clear annual precipitation cycle of wet and dry seasons and The Nile would continue to flood every summer if it were not for the construction of several dams along its length in the 20th Century that have had the impact of stopping the annual floods.

Herodotus would have loved The Ouse Catchment. It's a shame he never got to visit. There is definitely no tropical style seasonal cycle of precipitation in Yorkshire. Or anywhere in Britain for that matter. The nature of the rainfall does change, with slow and steady frontal rainfall peaking in the winter and short and sharp convective showers peaking in the summer but those peaks mostly cancel each other out so that the mean monthly precipitation totals in the country remain pretty much consistent throughout the year. Despite this, there is a very clear seasonal cycle of river flows and flooding.

Monthly precipitation averages in North East England and number of flood days of the River Ouse in the centre of York. This graph has been constructed by the author of the blog in a very unscientific manner and should not be considered authoritative.

No matter how you measure it (mean river height, number of flood days or any other metric) the rivers of Yorkshire and the rest of Britain routinely rise in the winter and fall in the summer. So if this isn't caused by precipitation cycles then can we learn from the work of Herodotus? 

Well, evaporation dramatically increases in a British summer compared with winter. As summer progresses the days get longer, the sun gets stronger and the water starts to disappear into the atmosphere. First the puddles dry up, then the soil gets drier, then the ground deeper and deeper below the topsoil gets drier and drier. This all has the effect of reducing the amount of water entering into the river systems. There are also other factors preventing water reaching the rivers in summer such as the increased amount of vegetation as summer advances in the form of tree canopies and crops intercepting the falling rain and drawing up the water from the soil in order to grow big and strong. Whereas, as winter progresses the lack of vegetation and decreased evaporation means that first the ground and then the topsoil get gradually saturated and finally those pesky puddles come back year after year as all that water has no where else to go.  Furthermore, when heavy rain does fall in summer it is like falling onto a dry sponge that soaks up all of the excess moisture without any great drama. However, if the same amount (or even less) rain falls in winter it is like falling onto a wet sponge that can't soak up any more water and before you know it the river has flooded once again.

So all in all we can conclude with The Ouse, just like Herodotus concluded about The Nile more than 2,500 years ago, that we shouldn't think about conditions in some parts of the year causing flooding such much as conditions in other parts of the year preventing flooding. Of course, in another 2,500 years this theory could also be proved entirely wrong but hopefully somebody can use this blog to illustrate their points. If that is the case, you have my blessing to use my words with due reverence and whilst we're at at I would like to apologise for all the things we did to the Earth in our day. Sorry.