This sort of sudden memory loss is sometimes attributed to Gotpissedagainintheredlion....

What you might have been trying to recall is that because all the sources of water (upstream of Marlborough) near Avebury are Winterbournes, and when they dry-up the ditch surrounding Silbury remains full of water, as Silbury collects holds and syphons water up into the moat much the same as a water-feeder designed for poultry; then perhaps it was intended to keep a supply of water for people in Avebury for whom it evidently remains in short supply as those that live there to this day (Mr G)aren't adding enough to their drink....and as a result keep forgetting what......

I can't remember what I was about to say....