Pella Strand is on her sixteenth notebook, and the end is in sight.

The municipal surveyor, who has spent seven weeks tracing the path of an unmapped underground waterway beneath the Docklands and Bramblegate districts, reported on Monday that she has now mapped a total of 930 metres of the Lower Conduit system. Her full survey report — commissioned by Chief Municipal Engineer Dorothea Kinnear — is expected by Friday.

“I can hear the spring,” Strand said, standing in the cistern chamber beneath the former Ashwater Brewery on Cooperage Lane, where three brick channels converge at a central stone distribution basin dating to 1782. “Through the collapsed section at the end of the western branch, you can hear running water. The source is there. I simply cannot get to it.”

The collapse — a section of approximately fifteen metres where the conduit’s brick arch has given way, likely in the mid-nineteenth century when the last of the Cooperage Lane breweries ceased operations — marks the effective terminus of the western branch. Beyond it, the spring that supplied the Ashwater, Chandler’s, and Bramblegate Ale Works breweries for over a century continues to flow, feeding water into the system at a rate that Strand estimates at several hundred litres per hour.

“The water in this cistern is cold, clean, and flowing,” she said. “It has been flowing since at least 1782. Possibly longer. The conduit did not create the spring. It captured it.”

The full survey will document 930 metres of brick-lined conduit across two branches: the eastern branch running from the cistern to the stone-arched outfall beneath the old commercial wharf at Bramblegate Steps, and the western branch extending from the cistern to the collapsed section. The junction lies approximately 340 metres from the Ashwater outfall. The cistern itself — roughly circular, four metres in diameter, with a vaulted ceiling of 1782 limestone — is, in Strand’s assessment, “the most remarkable piece of pre-Victorian engineering I have encountered in this city.”

Sixteen notebooks. Seven weeks. Fifteen hundred individual measurements. And a river beneath the city that no one had mapped in nearly two and a half centuries.

The Bobington Historical Preservation Society’s application for heritage listing has been formally submitted to the Municipal Heritage Committee. Arthur Penrose, publican of The Old Cooperage, whose flooded cellar first revealed the connection to the western branch, reports that the water level has stabilised at four inches and shows no sign of rising further. He has placed a board across the cellar stairs.

“She said the water was always there,” Penrose said. “The conduit just reminded it where to go.”

Strand expects to submit her report to Kinnear on Friday. It will include maps, structural assessments, flow measurements, and a recommendation for the cistern chamber’s preservation. She is already thinking about what the spring source might look like, when someone eventually clears the collapsed section and reaches it.

“Somewhere under Cooperage Lane,” she said, “there is a spring that has been running since before anyone built anything on this street. I would very much like to see it.”