Chalk Rivers of the Thame Catchment

November 17, 2021

Brown trout 2015, Chalgrove Brook. c. Roselle Chapman

I wonder how many people are aware that the Thame catchment is home to approximately 1.5% of the global stock of chalk rivers, which arise from the chalk aquifer under the Chiltern hills?

Chalk rivers were originally formed by the racing flows of melting ice sheets, which brought with them distinctive gravels that still occupy the beds of these rivers and influence their ecology. Their main characteristics comprise being predominately fed by springs arising from a chalk aquifer and that they flow over a chalk surface geology, but the category ‘chalk river’ encompasses a wide range of rivers with different physical and chemical conditions, from ephemeral winterbournes to large chalk rivers. Frequently they have a range of geological influences, and it can be difficult to define exactly where and when a chalk river stops being a chalk river (in that the chalk influence has become so diluted by flows from other sources and a changed base geology that significant changes in the chemistry and ecology of the channel occur).

So, in the light of this do we really know how many chalk rivers are in the Thame catchment? Currently there is no single definitive list, but it is clear that at least in their upper reaches, the Chalgrove Brook, Lewknor Brook and Horsenden Stream are all chalk rivers. This uncertainly over classified may seem a minor one, but it is important when it comes to understanding their ecology and protecting them. The springs, headwaters and upper reaches of our chalk rivers are particularly poorly mapped across the country, and work is being done to rectify this at local and national levels (See Iain Naismith’s article above).

Chalk-filtered water should be very pure as it emerges, rich in minerals, and a fairly constant temperature year-round. Unfortunately pollution (from a variety of sources) means the emerging water is not as pure as it should be. In the Chalgrove Brook these unique chalk river characteristics do still support a rich variety of specialist plants and wildlife, including populations of brown trout, brook lamprey and bullhead, and the diminutive fine-lined pea mussel. All these species are threatened by pollution, loss of habitat, and over-abstraction of water.

Pollution comes in many guises: in 2015 deaths of brown trout were recorded in the Chalgrove Brook when saponins entered the watercourse. These compounds, poisonous to fish, can be found in a range of plants; in this case it was traced to a heavy fall of horse chestnuts on the road alongside the brook, which released the chemical when crushed by cars and was subesquently washed into the brook by heavy rainfall. More trout were killed in 2019 when a channel of the brook dried up suddenly, leaving fish stranded. Members of the local community mounted a rescue and saved up to 40 trout, ranging in size from 2-14 inches long.

These losses remind us of the fragility of our rivers and the oranisms they support, and how much more needs to be done to halt their decline and help develop more resilient systems able to cope with a future of increasing impacts from development and climate change.

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