Chalk stream

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Chalk streams are rivers that rise from springs in landscapes with chalk bedrock. Since chalk is permeable, water easily percolates through the ground to the water table and chalk streams therefore receive little surface runoff. As a result, the water in the streams contains little organic matter and sediment and is generally very clear. The beds of the rivers are generally composed of clean, compacted gravel and flints, which provide good spawning grounds for Salmonidae fish species. Since they are primarily fed by aquifers, the flow rate, mineral content and temperature range of chalk streams shows less seasonal variation than other rivers. They are mildly alkaline and contain high levels of nitrate, phosphate, potassium and silicate. In addition to algae and diatoms, the streams provide a suitable habitat for macrophytes (including water crowfoot) and oxygen levels are generally supportive of coarse fish populations. Of the 210 rivers classified as chalk streams globally, 160 are in England. A list of chalk streams in England gives a total of 224.