If you’ve ever spent any amount of time in the English countryside, you’ve probably come across a chalk stream or two. Spanning from the River Hull in Humberside to the River Frome in Dorset, English chalk streams are an integral part of our landscape history. Their distribution and unique characteristics are principally a function of their chalk geology; the result of chalk sedimentation during the Cretaceous period approximately 100-65 million years ago. Today, English chalk streams have great cultural, ecological and economic importance, which is tempered by a legacy of human intervention, natural environmental change and 21st century ecosystem demands.
This introduction to English chalk streams will be the first in a series of blog posts discussing these groundwater-dominated rivers. In this post we will consider where chalk streams have come from and why it is essential to understand their history.
Although England’s chalk geology has been around for tens of millions of years, contemporary chalk rivers are a relic of more recent glacial outflow events that occurred at the end of the last Ice Age. At the beginning of the Holocene (approximately 10,000 years ago) our chalk streams were laden with glacial debris and sediment, causing the channels of these rivers to braid and anastomose. Water from melting glaciers in the retreating British and Irish Ice Sheet cascaded across lowland England, transporting with it the gravels that now characterise many English chalk streams. Once the energy and material from the deglaciation had been exhausted, England’s chalk rivers began to develop their now familiar single-channel planforms.
And that was, more or less, where it ended in terms of the natural processes that created English chalk streams. Never since have these rivers had sufficient energy to significantly alter their location, shape or size.
So why do some modern chalk streams not have simple channels? Why do we have water meadows? Why aren’t there trees on many chalk river banks? And the answer is, of course, humans.
Neolithic peoples began the process of river alteration indirectly when they initiated large-scale deforestation around 5,000 years before present. Thereafter, records show that the Romans were building bridges and changing the shape of southern England’s rivers (such as the Kennet) in the 1st century AD. In Medieval times Anglo-Saxons constructed water mills (of which there were over 6,000 in England in 1086 according to the Domesday Book) to harness the power of English chalk streams. The water meadows of the 16th-19th century, which extended the grazing season and reduced the likelihood of crop failure, added numerous channels and ditches to chalk river floodplains that can still be seen today. Contemporary issues in chalk river systems, including fisheries management, water abstraction and flood defence, add further to the complex picture.
With such a long history of human-induced changes, it’s clear that English chalk streams are a fundamental part of our landscape history. In the next blog post we’ll look at what it is that characterises today’s chalk streams and why this makes them important.