The Winterbourne stream flow through the north end of the nature reserve and is an important part of the patchwork of interconnecting habitats. In the summer the winterbourne is a slow meandering stream.
In the winter months, a much larger volume of water flows along the winterbourne only flows in winter which normally flows direct into the River Ouse. However at high tide, there is a large flap at the point at which the winterbourne stream enters the river and so as the tidal water rises the weight of water closes the flap.
This means that the Winterbourne cannot escape to the river and so it backs up getting deeper and deeper until it tips into the woodland and into heart of reeds. The Winterbourne stream water level may rise by two and a half metres.
The water that floods into the woodland will eventually reach the Leighside pond which is temporarily engulfed. It is also thought that water the Leighside pond fills from below the pond floor.
By allowing the Winterbourne to naturally flood the reserve, which is on a flood plain, this saves about 8 cellars in Lewes town from being flooded on every high tide.
As the tide level in the River Ouse falls, the Winterbourne stream can again flow back into the river.