The line on the map indicates the route of an occasional stream.  It is actually termed "an occasional" stream in Repton's Red Book of his landscape design for the site.  Water there has never been a permanent feature there as there is no water source in Warley Woods.  The stream forms when there is sufficient heavy rain, that runs down both sides of the valley and takes a while to drain away. 

There used to be a permanent (but artificially created) pond on site, (thanks to Repton) and this stayed as a feature in the early years of the public park.  It was at the Abbey Road gates, and is probably the only piece of flat land in the park.   But in the 1920s, when Abbey Road was created, land was used from Warley Woods to widen the lane (Slatchhouse Lane) to a road (Abbey Road) and a culvert for water was taken under this new road.  It flows down to Thimblemill Brook.  So now we only get a pool after exceedingly heavy rain, that has to take its time to go through the culvert.  We are told that it was created by using herringbone bricks as its base and in the drought of the 1970s, the grass cracked around the bricks and the shape of them can be seen.

It used to be a very rare pool - just once every few years, but now we see it several times a year.