In the first post of this series we considered the history and origins of English chalk streams; where they were and where they’d come from. In this post we’ll look at what makes them tick: their characteristics and tell-tale signs.
The technical definition of a chalk stream is any river whose base-flow index (the volume of river flow derived from groundwater aquifers) exceeds 75%, and whose course runs over chalk geology. Beyond this, chalk streams are characterised by stable planforms, low stream densities, and clear, alkaline waters. At the catchment scale, permeable rocks and soils have a high infiltration capacity, leading to dampened flood hydrographs, few tributaries and low connectivity with the landscape. At the reach scale, upstream headwaters (winterbournes) may experience a naturally dry period of low flows at the end of summer and the water table may fall because of insufficient precipitation inputs into chalk aquifers (hence the
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