The upper reaches of the River Kennet start with the winterbournes west and north of Marlborough, flowing past Avebury and Silbury Hill during the winter and drying naturally in the summer.
The waterbody status of the Upper Kennet (2019) is moderate. Fish populations are of moderate quality, whereas invertebrates have sustained high biological quality since 2014. Despite increased dissolved oxygen concentrations since 2016 poor flow continues to limit water quality. The water body failed to meet chemical standards in 2019 due to the presence of Polybrominated diphenyl ethers (PBDE).
The farthest upstream reaches of the River Kennet begin at Berwick Bassett down to Silbury Hill and Swallowhead Springs. This is the winterbourne section, and in a normal year flows from mid-winter until early or mid-summer. In dry years it does not flow at all. The channel has been modified and is ditch-like and over deep, often overgrown with terrestrial vegetation for large sections. Regular dredging and vegetation clearance has caused a loss of habitat and disconnection with the flood plain.
Actions complete: Farm workshops to explain the importance of winterbournes and promote more sensitive channel management. Farm workshops on track management and catchment sensitive farming.
- Channel bed raising to replace dredged gravels, re-profiling of banks to allow re-connection with the floodplain and use of large woody material to encourage in-stream diversity and the recovery of natural processes.
- Clear information for landowners and Parish Councils to stop dredging.
The Swallowhead Springs, opposite Silbury Hill are thought of as the start of the River Kennet. The river usually flows year-round from Fyfield downstream to the market town of Marlborough, but in exceptionally dry years such as the 2011/12 drought the entire river upstream from Marlborough has been known to dry up.
Anecdotal evidence suggests that the upper reaches of the River Kennet dry more frequently since water abstraction began, and modelling by Action for the River Kennet (ARK) confirms that the river dries more frequently as a result of water abstraction. An investigation into impacts of abstraction on flows in the Upper Kennet will be undertaken by Thames Water as part of the 2020-2025 Water Industry National Environment Plan (WINEP).
The channel profile in the reach from Swallowhead Springs to Manton has a mix of healthy, more natural sections and over-deep incised sections.
The sewage pumping station at East Kennet regularly fails during high ground-water events. Sewage backs up and surcharges from manholes and spills out from the pumping station into the water course. Despite investment from Thames Water the problem continues to persist into 2020 and requires resolution.
Tracks, including the Ridgeway national trail, are an active conduit for sediment into the river.
Actions complete: During 2012/13 drought the Upper Kennet dried up completely and many fish died. To help fish populations to recover, the main barriers to fish migration at Mildenhall (Durnsford Mill), Marlborough (Town Mill and Coopers Meadow), Manton (Preshute House Weir) and Marlborough Gauging Station have been removed or modified by projects run by Action for the River Kennet. At West Overton a project to realign the river to its natural channel, carried out by the landowner and Environment Agency, has been successful. There has been some habitat restoration work from Manton through to the Marlborough Gauging Station.
- Modifications to the Marlborough College Lakes and associated structures were identified in 2015 as an important potential improvement to benefit river water quality, fish migration and channel profile and dynamics. The project was reviewed by the Kennet Catchment Partnership in April 2018 and remains a priority for the Upper Kennet if funding can be found.
- The Mill at Manton is a barrier to fish migration at almost all states of flow and immediately downstream a stone weir impedes conveyance; removal of the weir is recommended.
- Catchment walkovers are needed to identify and address sources of sediment.
- Effective strategy needs to be devised to deal with sewage in high groundwater periods.
The River Og
The River Og flows into the Kennet just downstream of Marlborough. It is a small chalk stream with a healthy brown trout population.
The Og water body is at Good ecological status.
The upper reaches of the Og are winterbournes flowing from mid-winter until mid-summer in normal years. In dry years flow is reduced to the most downstream reach.
The river has been significantly re-aligned to follow straight field boundaries in large sections upstream from Ogbourne St Andrew, and in the upper reaches beyond Ogbourne St George the bed also contains dense scrub and hedgerow. There are good sections of natural river channel, e.g. the reach from Bay Bridges to Marlborough.
Localised dredging has disconnected the river from its floodplain and created steep incised channels with poor habitat. There are significant sediment pathways, mainly from equestrian fields into the river.
The villages of Ogbourne St George and Ogbourne St Andrew experience groundwater flooding and some surface water flooding after heavy rain, but there are no records of fluvial (river) flooding affecting properties.
The pumping station at Bay Bridge is overwhelmed in high groundwater periods and requires a 24/7 tanker operation to prevent sewage overflowing into the river.
Actions completed: Water abstraction from the Og borehole was reduced to zero from 2015 to protect river flows and an alternative water supply to Swindon has been completed by Thames Water. ‘Baffles’ to enable fish passage were installed on the Poulton Gauging Station in 2019 by Thames Water and the Environment Agency. Habitat restoration in the field upstream from the gauging station was completed by ARK in 2017.
- Channel modifications to restore natural channel profiles and dynamics, including river bed raising.
- Channel realignment to the paleo-channel, particularly upstream from Ogbourne St Andrew.
- Clear messaging to prevent further dredging or channel straightening, neither of which will reduce flood risk, and could make it worse.
- Better track management to reduce sediment input to the river.
- Opportunities for tree planting and wetland creation as part of natural flood management should be explored.
- A solution for the pumping station at Bay Bridges is required.