'I have mentioned that the finished article must have looked spectacular on its completion and others have made mention of the ditch and bank having a gleaming white appearance at that point in time, but would that really have been the case?
The ditch alone must have taken so long to dig out that surely earlier digging would have ‘greened up’ as continued digging took place! Evidence of this is seen quite regularly when groups of people are seen carrying out ‘maintenance’ on many of the white horses and other such hill figures and designs throughout the countryside. If this remedial work was not carried out every few years many of these figures would have long since disappeared as Mother Nature reclaimed what was rightfully hers. To all intents and purposes, that is exactly what has happened at the Great Circle with both ditch and bank now completely covered in grass, save for the well trodden paths along the top of the banks.
Of course it is entirely possible that the ditch and bank that had already been dug out and heaped up were cleaned regularly as work continued so that the white surface remained. But would that have been necessary if the ‘look’ was not important? Probably of little importance but nevertheless an interesting point to at least consider'.
http://www.themodernantiquarian.com/site/4062/experimental_earthwork.html
This was dug/built half a century ago - the chalk still shows through in the ditch, which is a much shallower depth than the pre-silted ditch at Avebury. Don't know if this helps with the water question though :)