Chalk stream

Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk they are alkaline and clear. There are 210 chalk streams in the world, and 160 of those are in England. Chalk streams are popular with fly fishermen who fish for trout on these rivers.

Chalk is a highly porous and permeable rock and rain falling onto chalk topography percolates directly into the ground, where the chalk acts as an aquifer. The groundwater flows through the chalk bedrock, re-emerging lower down the slope in springs. The chalk acts as a temporary reservoir by regulating the amount of water supplied to the springs. This is why many chalk streams in the UK have stable flow regimes that vary only slightly over time. The temperature of the emerging surface water is fairly stable and rarely deviates from 10°C (50°F). On cold winter mornings, water vapour from the relatively warm stream condenses in the cold air above to form fog.

Chalk is a soluble in rainwater because rain is naturally slightly acidic. The products of chalk weathering are dissolved in rainwater and are transported in stream flow. Chalk streams transport little suspended material (unlike most rivers), but are considered mineral-rich due to the dissolved calcium and carbonate ions. The surface water of chalk streams is commonly described as 'gin clear'. The channel bed consists of angular flint gravel derived from the natural flint deposits found embedded within the chalk geology that contains relatively low amounts of clay and silt deposits.

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