Streams and winterbournes
During a normal winter, when rainfall is heavy and able to percolate through the chalk, the aquifer will be topped-up. The head of the stream may move up the valley as the water table rises. In summer, little rainfall percolates into the chalk as it is mostly taken up by plants and lost through evaporation. The water table drops and the head of the stream moves down the valley, leaving the top section of the valley dry. This section is called a winterbourne because it usually flows after the winter rains. Many winterbourne streams have their own special wildlife which is adapted to cope with intermittent flows.
Lag-time
The time between a period of rainfall and a stream rising in response to the rain is called the lag-time. For most normal small rivers and streams, the lag-time is usually a matter of hours. However, for chalk streams, the long, slow journey of the rainwater through the chalk rock can extend the lag-time after a period of heavy winter rain to weeks and often months. This can lead to surprising patterns in which a stream remains dry during a wet winter, only to start flowing months later in the spring or summer.
Falling water table
If there is less winter rain than expected, the water table can drop further and the stream bed will remain dry for longer. The abstraction of groundwater from the chalk aquifer through boreholes, to meet the growing demand for domestic drinking water, has artificially lowered the water table further, leading to some chalk streams flowing less frequently, even in the winter months.