closed down. Ash Aqueduct The Basingstoke Canal runs from the Greywell Tunnel at Greywell in the west to the River Wey in the east, and crosses the Blackwater Valley on an embankment in the village of Ash. Thk Blackwater used to flow through a brick culvert under the embankment, but major reconstruction took place over the winter of 1994-95 as part of a project to route the A331 road along the river valley. Because bats were known to roost in the culvert, a new home for them was built on a nearby island. The central section of the embankment was then removed and replaced with a three-span aqueduct, with a centre span of and two side spans of each. The structure was built of post-tensioned concrete, using four cables on each side of the navigation channel, which were jacked apart until each was exerting a force of 820 tonnes to keep the concrete in compression. In order to avoid the aqueduct piers, the river channel was diverted to run through the western side span, and a new course was constructed until the road had risen sufficiently for a box-section culvert to carry the river under the new road to rejoin its original route. The new Ash Aqueduct affords views of the surrounding countryside. In the distance to the south is the Hog's Back, which runs between Farnham and Guildford, and is where the North Downs narrows. Water Quality The Environment Agency measure water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish, and chemical status, which compares the concentrations of various chemicals against known safe concentrations. Chemical status is rated good or fail. The water quality of the Blackwater system was as follows in 2019. Reasons for the quality being less than good include discharge from sewage treatment works; drainage from roads and transport infrastructure; contaminated land adjacent to the river; physical barriers such as weirs and impounding of the water which prevent the free migration of fish and other species; and the presence of the North American signal crayfish, an invasive species. See also List of rivers in England Bibliography References External links Blackwater Valley runners