Because in the summer the trees take out all the moisture and the soil is bone dry. And ponds are hard work. The one at Rothersthorpe needed to be cared for and after 5 years I had to put a new liner in it.
I'll tolerate the winter flooding and who knows, the willow may improve the situation next winter.
PS the water has drained away in the lower part but it is still boggy towards the top of the drive.