View of the Letcombe Brook chalk stream that rises from a spring at the foot of the Berkshire Downs in southern England, eventually to become a tributary of the River Thames. This wide and shallow stream flows over a gravel bed. Chalk streams are fed from the groundwater held in the underlying chalk bedrock aquifers of the downland geology. Due to the filtering and purifying effect of the chalk, their waters are alkaline and very clear. With the stable temperature of the water, and when the flow is constant, the conditions are ideal to support a rich diversity of wild life such as fish and invertebrates. Of the 210 chalk streams in the world, 160 of them are in England, Chalk streams are considered highly important ecosystems and a fundamental feature of the English countryside. In the UK, they have been designated as a ???abitat of Principle Importance??under the NERC Act of 2006.