A defining characteristic of chalk streams is the winterbourne section – the part of the river that only flows when groundwater is high. On the Kennet they reach all the way across the Marlborough Downs to Avebury, Winterbourne Bassett and beyond. The upper reaches of the Kennet’s tributaries including the Lambourn and the Og are classic winterbournes and their place names reflect the ephemeral pattern of flow (eg Lambourn – from ‘lamb’ and ‘bourne’, because the river mainly flows in the Spring during lambing season).
Although the cycle is predictable there is natural variability; in a dry year some winterbourne sections might not flow at all, and in a wet year they may flow almost all year round. Dry winterbournes look more like a path or a track than a river, but don’t be deceived; they are a hive of ecological activity and are some of the most biodiverse zones in our river system.
Research shows that parts of the river which flow intermittently have a greater biodiversity than constantly flowing reaches. Even the ‘scum’ on the bottom of a dry river is a diverse algal community. The winterbournes naturally transition between flowing, pool and dry states, creating aquatic–terrestrial habitat mosaics that change in space and time. They support a diversity of species depending on how much water is present – effectively treating the river as a ‘time-share’. For example during the pool phase specialists like caddisfly larvae, usually restricted to floodplain ponds, move into the temporary pools in the river. As those pools dry up mud beetles arrive to graze on the rotting algal material left behind.
Brown Trout have adapted to swim to the upper reaches of the winterbournes during high winter flows, where they can find clean gravel and less competition for spawning sites. As the waters recede during the summer, the adults and newly hatched juveniles move downstream. Other species have adapted to living in the same part of the river regardless of the state of flow. For instance, some specialist stonefly and mayfly nymphs are only found in the winterbournes and some beetles have evolved to inhabit river sediments which experience repeated inundation.
There is a clear succession of plant communities growing in these ‘sometimes wet’ rivers. Terrestrial grass gives way to marsh foxtail, followed by rapidly growing annuals like water speedwell and watercress, until the perennial river begins and different plant communities, dominated by stream water crowfoot and starwort, take over. The terrestrial plants and herbs that dominate the river in its dry phase provide habitat for pollinating insects, enhancing the productivity of nearby arable land. The winterbournes are the powerhouse of our chalkstreams. The moment they start to flow water species colonise and drift downstream to supply the whole river system with new organisms; the moment they dry they become an entirely new habitat. They are an essential part of the landscape we live in, so let’s cherish those dry stream beds and treat them with the respect they deserve.