Here's something useful
http://www.britarch.ac.uk/ba/ba96/feat3.shtml
and it links to this
http://www.york.ac.uk/depts/arch/Projects/StarCarrWebsite/index.htm
I wonder if the surrounding area (or the actual area?) is being drained (for farming reasons) and that's sucking all the water out of the carr. They don't seem to come out and say that directly, I'm not sure.