Keeping our gin-clear chalk streams flowing

Riverkeeper Chris de Cani explains what goes into keeping our gin-clear chalk streams productive and the water flowing during the summer months.

Summer work on a chalk stream sees the trout fishing season in full swing, and there is grass and weed to be cut. Chalk streams are unique rivers that receive the bulk of their replenishment from groundwater. Scrape the soil from the hills that form the valley and you would find bright white chalk that absorbs rainwater and releases it slowly into the river via springs. There is much less direct run-off after rain than non-chalk rivers, and this delay in the water entering the river means that they do not flash flood.

Their benign temperament has resulted in them being managed by humans for hundreds of years, to work mills or deliberately allowing them to flood to warm up pasture, thus providing early grazing. They are what they are today because of the way that we have managed them for centuries. Weed cutting has always been an important part of the summer management on chalk streams. A chalk stream is an incredibly productive habitat and weed can grow at a remarkable rate. Weed is a vital part of the habitat, playing host to a rich variety of invertebrates as well as providing cover for fish from predators. It can also be useful in helping control water levels. Cutting back weed in October can drop the water level in a chalk stream by up to 10 inches. During the summer, however, weed can only be cut on designated dates. Large rafts of cut weed heading downstream disrupt sight fishing for trout, but if all keepers cut at the same time the disruption is minimised.

I still cut the weed with a scythe, although some keepers prefer mechanical methods. I face downstream and cut with the water, sharpening the blade every half an hour or so as contact with the gravel and flint riverbed can take the edge off a blade. If there is plenty of water the weed is cut hard. If the there is a dearth of water, weed is cut in a way to hold water up, with bars of weed left to span the river.

The chalk streams are a unique aquatic environment, and there aren't very many of them. Eighty per cent of the chalk streams on earth are in the UK , most of them in the increasingly populated south-east. More hard surfaces are being created in the region, reducing the amount of water soaking into the ground to replenish aquifers. A burgeoning population rely on the groundwater resource for 70 per cent of their supply. The region doesn't receive the rainfall that it once did, and I haven't cut weed in summer with an eye to reducing the water level for some years. The groundwater resource that feeds the chalk streams and the kitchen taps of the region is under immense pressure. The way that we use it is increasingly unsustainable. We therefore need to become more 'water wise' if these chalk rivers are not going to be impacted.