Why Is That River So Clear? It Came Out of the Rock, Not Off the Land

Chalk is porous and holds an enormous volume of water in the spaces within it, functioning as a vast reservoir rather than as a surface through which rain runs off. Rain falling on chalk downland soaks in almost entirely and travels slowly through the rock for months or years before emerging at springs where the chalk meets less permeable ground. The water arriving in the river has therefore been filtered by its passage, has had time to reach a stable temperature, and has dissolved calcium carbonate along the way. Almost none of it has run across a field surface, which is the fact that determines everything else.

The resulting river differs from a surface-fed one in every respect:

• •Water that is clear, since it carries almost no suspended sediment
• •A stable flow, with no sudden rise after rain
• •A narrow temperature range, cool in summer and mild in winter
• •Alkaline mineral-rich water that supports abundant plant growth
• •A gravel bed kept clean rather than silted
• •A gentle gradient with weed beds rather than rapids

Stable temperature, clean gravel and abundant plants together produce a system that is unusually productive. Water crowfoot grows in trailing beds that shelter invertebrates and concentrate the flow into channels that keep the gravel clean, which is precisely what trout and salmon need to spawn in. Mayfly and other insects emerge in enormous numbers. Water voles burrow in the soft banks, and they support the largest remaining populations of several declining species. The fishing tradition built on this is the origin of dry fly angling as a discipline, which developed on these rivers in the nineteenth century and was exported worldwide.

Repair work on these rivers is well developed and the measures follow directly from what was damaged. Removing or lowering the weirs and mills that ponded the water restores the flow needed to keep gravel clean, and hundreds have been taken out. Narrowing an over-widened channel with gravel and brushwood speeds the water and scours the bed. Adding gravel where it has been lost restores spawning ground directly. Fencing livestock away from banks stops trampling and the fine sediment it releases. Reducing abstraction, by moving supply boreholes away from headwaters, addresses the flow itself, which is the hardest measure because it means finding water elsewhere.

These rivers are globally rare, with roughly two hundred and ten identified worldwide and the great majority of those in southern and eastern England, which places an unusual concentration of responsibility there. The pressures are severe. Water is abstracted from the chalk for public supply in quantities that reduce the flow and in dry years leave headwaters dry for months. Nutrients from farmland promote algae that smother the plants and the gravel. Fine sediment from cultivated land clogs the bed. Sewage discharges during heavy rain release untreated waste. And the combination is worse than any single pressure, since a river with reduced flow dilutes pollution less effectively.

Water that has soaked into porous chalk and travelled through it for months emerges filtered, at a stable temperature and rich in dissolved minerals, which produces a clear river with steady flow and clean gravel. Trailing weed beds concentrate the flow, keep the gravel clean and shelter the insects that trout need. Around two hundred and ten exist worldwide and most are in southern England.