So far in this blog series we’ve looked at what English chalk streams are, where they come from, and why they’re important. In this fourth installment (the first of a two-parter), we’ll be discussing three key threats to the chalk stream sustainability: water abstraction; fisheries management; and river channel modification.
In the UK, 55% of all groundwater-abstracted drinking water is sourced from chalk aquifers. The abstraction of water for human consumption increases the likelihood of low flows in winterbournes during summer. Low flows often result in fine sediment deposition and aquatic habitat loss, which is deleterious to submerged macrophytes and spawning fish. It is thought that low-flow alleviation schemes may assist in recovering winterbourne ecology during a time of both increased human demand for water and reduced precipitation inputs due to climate change.
Fisheries management, both for recreational and commercial purposes, has a number of impacts upon chalk rivers. Weed cutting is undertaken to improve unobstructed angling, increase flow velocity (for flood management) and reduce fine sediment trapping. However, indiscriminate weed cutting equates to habitat removal, with the loss of macrophytes, freshwater mussels and common invertebrate species such as the Grannom caddisfly (Brachycentrus subnubilus).
River channel modification
Morphological modification of river form via bed level lowering and dredging for the purposes of flood control, navigation and drainage can have major implications for benthic organisms and their food chain dependents. Although historically such land management practices were common, contemporary changes in river depth are usually only undertaken as part of restoration projects. In some instances manmade chalk river stretches are not restored, either because it is too expensive, their social amenity value is high, or, as is the case with the Brandy Stream in Hampshire, they have acquired ecological value over time. Because of their low-energy nature, many modified chalk rivers do not have the capacity to reassert their natural form.
In the next blog post we’ll be looking at four more threats to chalk stream sustainability: catchment-scale land-use change; allochthonous nutrient inputs; urbanisation; and neglect.