All river systems are complex and unique. Understanding how and when they flow, or overflow, and what can be done to control or mitigate the results is becoming ever more important. Not only are more homes being built along rivers or in floodplains but climate change is making weather events more erratic and extreme. Much work has been done on this: more is needed.

This area is noted for its chalk streams, a specific type of river that have particular features, support particular eco-systems and – in the case of the seasonal or bourn part of these – have particular life cycles. Two of these are the Letcombe Brook and the Lambourn. Although superficially similar, research by Dr Colin Lloyd, one of the founders of Future Flood Resilience, has shown that they have an important differences which requires distinct types of mitigation.

As this report (“Report 6 Letcombe Brook Streamflow Groundwater Anomaly 13 Mar 2025”, which can be found here) explains, the differences in the topsoil above the chalk aquifer leads to a markedly different reaction to surface water. As anyone living on the Lambourn (as we do) will know, even very heavy rainfall generally has little effect on the river levels. One case the report highlights, from 2010, shows the river-flow falling even as rainfall was increasing.

The reason for this would seem to be that the topsoil is very porous. Even decades of monoculture farming and the use of heavy equipment which tends to compact the soil have had little effect on the ability of the land to absorb rainfall. The Lambourn’s flow is thus almost entirely a result of groundwater from rain several months before, rather than that which fell twenty minutes ago. (If the river continues to run high after a storm, this is often because of blockages by debris such as branches or river plants at bridges or other choke points.)

The Letcombe, however, is different. This has, the report concludes, “a heavy clay superficial soil layer over the chalk aquifer” which acts almost as a partially permeable dampcourse. Penetration into the aquifer happens, but far more slowly. After heavy rainfall, the Letcombe is thus more likely to be full and to remain so until the water has run off from the nearby land.

The topography is also a factor here: the gradients are steeper and the collecting area smaller than in the Lambourn valley further south which given even less time for any infiltration to occur on its journey down towards the Letcombe.

Further rises and falls will follow depending on how much water has penetrated the clay but this is less influential. In fact, more of the replenishment comes from horizontal water transport from other contiguous aquifer regions where the rainwater has infiltrated more easily.

One question is how much this is due to human intervention, and thus human remedy. A recent trend is that of minimum tilling (min-till) which reduces the depth of annual ploughing. This has many benefits for the soil and for carbon emissions. It could be, however, that disturbing less of this top 50cm or so has reduced the soil’s ability to absorb water compared to previous periods before about 1980 when as much soil as possible was turned each year. Is this the problem?

The report indicates that, although this is a minor factor, the main issue is the largely impervious clay layer. It goes on to suggest some ways by which farmers might help mitigate this, such as by excavating ”V” shaped trenches at the downslope edge of fields. Whether this advice will be accepted and acted upon is another matter. However, the evidence is here for those who choose to study it: and who are, unlike me, capable of understanding all its implications.

What does all this tell us?

• Firstly that I find this report to be important, relevant and (by its very nature) hard to understand: hence this summary, which is provided with the approval of its author.
• Secondly, that local groups involving experts are essential in our understanding of how river flows, and thus flooding, happen – only by doing this can we hope to mitigate the results in a sustainable way.
• Thirdly, all these problems are becoming more extreme and thus more immediate. It’s never been more important to understand how the natural landscape works and therefore to learn how we can live in harmony with it, rather than in conflict or ignorance.

That is surely something on which, regardless of our political persuasion or our views of climate change, we can all agree about. After all, even in the twenty-first century, those of us who live near the Letcombe or the Lambourn are constantly aware of their behaviour. Our medieval predecessors were even more so but probably instinctively understood their threats and opportunities far better.

We now need to re-acquaint ourselves with these and, in our much more fast moving world, understand the essential forces which we cannot fully tame but can only find a way of mitigating, hopefully benevolently. This report helps provide one clue as to how this might happen.

Photos

• The left one (taken by Colin Lloyd in September 2024) is at the more northern end of the “urbanised” stretch of the Letcombe Brook: beyond this point the Brook crosses agricultural land before joining the Childrey Brook and then going under the A338 south of the South Oxfordshire Crematorium. Notice the line of sandbags on the far bank. During rainstorms, especially during high groundwater supply, this stretch regularly overtops its banks and floods areas of East Hanney.
• The right one (taken by Penny Locke in March 2020) is of the River Lambourn in full flow in the middle of the village of East Garston, in the seasonal or bourn seasonal section of the river. On occasions – particularly in 2007 and 2014 – the land on the far bank is required to act as a small but very effective natural flood plain. A couple of miles upstream, the Eastbury flood defence scheme (completed in 2015) has provided this down-stream section with some protection, as was particularly evident in early 2024.