Dorset's Chalk Rivers

Many of central Dorset’s rivers run over chalk, this can be seen on a map by the sparse valley bottom watercourses that run from central West Dorset, north-east up through the county. Watercourses that lie on other substrates look different. This is illustrated clearly by the clay beds of the north of the county where the water cannot seep under ground and so forms a more visible close knit network of streams.

The content of the streams is also affected; chalk streams are fed by springs and have clear water, which is alkaline with a high calcium content. This affects the flora and fauna, for example Dorset has some white-clawed crayfish populations that require calcium for their exoskeletons, as do many invertebrates.

Many chalk streams have low riverbanks with wide wetland margins, which are beneficial to many species and help to support good vegetation growth.

Some examples of Dorset’s chalk rivers include the river Avon with it diverse fish populations. These include salmon, brook lamprey, sea lamprey and bullhead, as well as Desmoulin’s whorl snail, snipe, redshank and lapwing.

The Moors River has around thirty species of dragonfly including white-legged damselflies, hairy dragonflies and chaser dragonflies.

White-clawed crayfish are found on the river Piddle and Allen, which have sufficient calcium ions for their exoskeletons.

Water voles like the vegetation found on the Frome and some Stour tributaries.

Rivers and streams on other substrates can and do support many of the species mentioned above, but this does not detract from the rich ecology of rivers on chalk. These special habitats are susceptible to damage from pollution and inappropriate management, but the work of the Dorset Otter Group assists in monitoring and raising their profile.