Chalk streams are streams that flow through chalk hills
towards the sea. They are typically wide, 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 geology is porous, and rain falling onto chalk hills percolate directly into the ground, where the chalk acts as an aquifer. The water filters through the chalk, 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, it can look as though steam is rising above the relatively warm river.
Chalk is a soluble geology 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 uniquely mineral-rich. 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 is characteristically free from fine sediment deposits.
The unique characteristics of chalk stream ecology are due to stable
temperature and flow regimes combined with highly transparent water and
lack of sand grade sediment particles.
The chalk streams have been intensively managed for many generations