|Scroll Bars are the curving lines visible along this stretch of river bank, stacked from oldest on the right to youngest on the left.|
Wednesday, June 22, 2016
Adventures With Professor Tooth
Last week I was once again lucky enough to spend some time in the field with renowned hydrogeomorphologist, Professor Stephen Tooth of Aberystwyth University. Among other things (he is a very very busy man), he is currently working with Dr Kathleen Alexander and her Alexander Research Group on a study of the extremely complex and relatively under-examined Chobe-Zambezi River System in North-Eastern Botswana, the Zambezi Region (formerly Caprivi Strip) of Namibia, and South-Western Zambia.
The Alexander Research Group are attempting to gain a better understanding of the manner in which this system functions, including its annual flood cycles, sedimentology, vegetation patterns, ecosystem services and the dynamics (including agriculture and other human and non-human inputs) which affect the quality of the water circulating within it, and therefore, by extension, affect the health of the human and wildlife populations which depend upon it. This dryland wetland system is not only poorly understood, it is also incredibly unique and equally important from the perspective of conservation and biodiversity – some of the richest and most diverse ecosystems on earth depend upon it.
Professor Tooth is bringing his expertise as a hydrogeomorphologist to bear on these issues by helping to understand – perhaps counterintuitively – the manner in which this system changes and has changed over time. To him a landscape like the area around the Chobe and Zambezi Rivers is not only a picture of the present, but is positively riddled with clues to its past. To be out in the field with him is a bit like entering a time-machine of sorts, as he combs through it all exposing little bits of evidence here and there which together paint a picture of a system continuously changing, water-courses altering, sediments being deposited and washed away, lakes forming and disappearing thousands of years later. There is nothing static about this view, nothing permanent, and it is absolutely fascinating in its extraordinary scope and complexity.
Last week I went out with him to look for two things in particular. The first of these was the bank of a now extinct lake, which lay along the present-day course of the Chobe River some 40,000 years ago. We were looking for a certain kind of rocky outcrop along the riverbank, itself composed of the sediment of from this ancient body of water. The rock is called calcrete, and it can be found along a stretch of the Chobe River on the Botswana side, close to a rest-point in the Chobe National Park called Serondela (which also has an interesting history – it was the hub of the old Chobe timber industry). This rock is not only interesting for the fact that it is composed of the sediment from this ancient lake; it also actually contains the shells of two snail species which lived in it.
I went looking for 40 000 year-old snail shells with Professor Tooth, in other words; and after combing hundreds of meters of calcrete banks around Serondela, we finally came across a spot containing the fabled shells – or at least, a single shell to begin with. Once we'd found this first shell, we began to find others, more and more of them until we realized we were standing on a real trove of them. We even found one or two specimens that had broken free of there calcrete casings and were able to hold them, beautifully intact, in our hands. What's more, the area around these shells was littered all about with ancient stone tools, cutting blades of quartz and other colourful stones with clear indications of knapping. We'd found the Southern Bank of the ancient lake, and traces of ancient human habitation to boot!
The following day we went out again, this time on the Chobe River itself. This time we were looking for something a little more recent in geological terms, but something no less fascinating. We were looking for a hydrogeological feature known as a Scroll Bar – a sedimentary deposit which accumulates as a river gradually alters its course, wearing away the bank on the outer edge of a curve, and depositing sediment on the inner edge. This process is quite well known – it is the same process which can cause the formation of ox-bow lakes, and is the greatest driver behind the subtle but continuous changes in the way rivers undulate.
We had our eye on a certain section of the river already; the real challenge lay in working out how to go about sampling the scroll bars Professor Tooth had identified using satellite imagery (for reasons I'll describe below). We had to contend with several challenges, the first being the manner in which we might access these sites, and the second being the abundant large animals like elephant, hippo, and buffalo, which frequent the area and might make any attempt to sample the formations an exceedingly dangerous undertaking.
The reason we had to work out a sampling strategy is that, while these hydrogeological features are relatively easy to identify once you know what you're looking for [see embedded map, depicting the area we examined], they tell a far more interesting and complex story if you're able to date them. It is one thing to be able to tell that a river has altered its course in such and such a way; it is quite another to be able to draw an accurate time-scale of these events, to be able to say that the river looked like this five thousand years ago, and this is how it's changed since and when. We could then tell how fast this process is happening, for example, or, by correlating this data with other data sets, we might be able to discern other factors at play. We might, for example, begin to look at the manner in which floods of varying intensities either favour or inhibit the formation of these interesting features of the hydrogeological landscape.
So we had to figure out a way of getting to them in the first place, and then of surviving while we did so in the second. We found them easily enough – they are, as I say, quite easy to spot when you know what to look for – and, surrounded on all sides by the spoor and droppings of quite big and quite dangerous animals, actually clambered atop several of them. Menaced by crocodiles and taunted by troops of baboons, careful all the while of potential lurking hippo and buffalo, we surveyed the area. We found several places we might later use to gain access to the layered Scroll Bars, and we made a detailed assessment of the outer bank too, which might, in much the same way, allow us to determine the age of the flood plain through which the river meanders and which it slowly but surely eats away.
All told these excursions were a great success. Professor Tooth managed to find and survey the two hydrogeological features he was most interested in, and I had the excellent fortune to accompany him while he did it. All of this helps the Alexander Research Group to develop a more nuanced understanding of the processes and dynamics at play in the Chobe-Zambezi River System too, and will allow us in the near future to begin collecting the samples we'll need to begin the process of dating the changes in the Chobe River's course and assessing the ages of the surrounding floodplains.
at 3:34 AM