The underlying rocks of Dorset's winterbourne streams are chalk. Chalk is porous and soaks up rain like a sponge, storing it in the rock as groundwater. Winter rain percolates through the chalk and tops the groundwater levels up - usually at it's highest in spring after the winter rain.

As the groundwater levels rise, the water emerges as springs that feed the winterbournes. The springs can pop up anywhere along the riverbed or can be obvious such as at Wherry Pit at the head of the South Winterborne River.

As groundwater levels fall through the late spring and summer, the springs dry up and the stream will stop flowing. The groundwater doesn't get much of a top up in the summer, even if it rains a lot, as vegetation and water evaporation mean that little rainfall soaks through to the chalk.

These streams are called winterbournes as they are reliant on winter rain to flow - both the River Winterborne and South Winterborne have dry mid sections.

Flows can be further complicated by water abstraction from boreholes that lowers the groundwater; at Winterbourne Abbas, Wessex Water compensate for water abstraction by adding water to the stream when it is at its lowest flow. Flows can also be affected by water entering the stream system from sewage treatment works and septic tank seepage - although extension of mains sewerage has almost eliminated this problem.

Use the links below to view animations of how a winterbourne stream's water flows during the four seasons - the light blue at the bottom of the image signifies the change in groundwater levels with the dark blue being the springwater which makes the winterbourne flow.

Winterbourne stream animation (very fast internet connection)

Winterbourne stream animation (fast internet connection)

Winterbourne stream animation (standard internet connection)

Stream signature

The Environment Agency carries out flow surveys on all winterbourne streams in Dorset, and they display this data in graphs showing the changes from year to year - this flow data is known as a winterbourne signature.

To record these signatures, Environment Agency staff visit a number of sites along the winterbourne streams once a month, recording whether the stream is dry or flowing, and also taking a photographic record.

Dorset contains a significant number of winterbournes, so the monitoring work is boosted with help from a small group of volunteers who record data on a number of streams within the project area.

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