Geograph - photograph every grid square

From the Chess Valley Walk official guide: "The clean warm waters of the Chess create ideal growing conditions for watercress, once a common crop along the river but now grown only here at Sarratt. Water from the Chess is diverted into a series of growing compartments - 'beds'. The cress takes root in the shallow beds, taking minerals from the calcium rich water. The flow is added to by spring water from deep in the aquifer which is at a constant temperature - about 10ºC. In years when the air temperature remains high enough the warm water allows cultivation right through the winter."