Water quality, levels and understanding impoundments

In a natural river, water is not dammed behind gates or sluices and the level varies primarily with the amount of flow coming down the catchment. The Scheme sluice structures were built, and the Ember channel was made larger than the natural river to retain a higher water level for aesthetic purposes and to manage flood risk.

Sluice structures impound water resulting in a deep, lake-like appearance. The river is largely silent apart from at the structures. Many people are used to the sight and sounds of impounded rivers and value their appearance. The negatives of sluice structures are:

  • Impounding water usually results in poorer water quality because it is static and less well oxygenated

  • In the summer months, algal blooms can thrive in the slow-moving water and take away further oxygen

  • Fish kills (because of low oxygen) are a much greater risk in impounded rivers. Some pollution does not get washed away so easily and can bind to fine sediment which gets deposited on the bed

  • This fine sediment gets moved on mass during higher flows when the gates are opened, releasing contaminants back into the water

  • River gravels, which fish use to spawn on and invertebrates live in get smothered in this silt and the gravel disappears

  • All the above issues become worse with climate change

  • They are a barrier to fish and eel passage, which can be partially mitigated by options described on our 'understanding fish passes page'

Healthy river habitats include exposed gravels, flow diversity, marginal and in-channel vegetation and clearer water. See our 'understanding our structures page' to find out more about the sluices, including how Island Barn ensures water is diverted into the Old Mole and Ember Loop. 

  An impounded river controlled by structures. Source: Environment Agency 

A naturalised stream. Source: Environment Agency 

Further information can be found via the important links section on the right side of the page.

Water Quality

In Spring 2021, we engaged with the community and other stakeholders where we shared a shortlist of options for how the Scheme could look in the future. We appreciate your feedback during this process, especially your concerns about water level lowering in the Scheme. In response to the feedback received, we conducted a study in 2022 to review technical solutions that could mitigate the impacts of water level lowering that would result from the removal of sluice structures along the Lower Mole. 

We acknowledge the concerns you raised about potential impacts to water quality due to changes in water levels. In response to this concern, we conducted an assessment at the beginning of 2022 to evaluate the impacts of the shortlisted options on water quality. We have kept this assessment up to date as the shortlisted options have evolved and changed and have considered the potential impacts to water quality associated with the revised shortlisted options. 

What did we consider to state that water quality won’t deteriorate? 

There are now no alterations proposed in the area between Viaduct Sluice and Island Barn Sluice. As a result, the hydrology and hydrodynamics of the river system at Esher Sewage Treatment Works (STW) will not be altered, meaning that the volume and flow of water to this location will remain the same as it currently is. 

The proposed changes will not alter the location or the potential load of pollutants from Esher STW. 

It is expected that if there are intermittent discharges from Esher STW that the pollutants will be adequately mixed by the time, they reach the lowered water levels downstream of Island Barn Sluice. This would occur with the pink and orange options. The volume of the water body also remains the same for a particular flow scenario, so this means that it is unlikely that the impact of intermittent discharges would be changed. 

The amount of flow down most parts of the river will remain the same. Therefore, it is unlikely that the pollutant load from Esher STW will change. 

If water levels are lowered at Mole Ember sluice, the Thames will occasionally control (raise) the levels in the channel upstream, but this will be controlled by structures at Teddington. It will not impact on water quality. 

Will there be any further investigation into water quality? 

Although it is possible that a water quality model could be developed to further test options, it is not currently recommended by our technical specialists who are certain of the same or improved water quality. We will review this as the Scheme progresses.

See our 'improving environment on the Lower Mole page' for the impacts of the water levels and impoundments on biodiversity.

In a natural river, water is not dammed behind gates or sluices and the level varies primarily with the amount of flow coming down the catchment. The Scheme sluice structures were built, and the Ember channel was made larger than the natural river to retain a higher water level for aesthetic purposes and to manage flood risk.

Sluice structures impound water resulting in a deep, lake-like appearance. The river is largely silent apart from at the structures. Many people are used to the sight and sounds of impounded rivers and value their appearance. The negatives of sluice structures are:

  • Impounding water usually results in poorer water quality because it is static and less well oxygenated

  • In the summer months, algal blooms can thrive in the slow-moving water and take away further oxygen

  • Fish kills (because of low oxygen) are a much greater risk in impounded rivers. Some pollution does not get washed away so easily and can bind to fine sediment which gets deposited on the bed

  • This fine sediment gets moved on mass during higher flows when the gates are opened, releasing contaminants back into the water

  • River gravels, which fish use to spawn on and invertebrates live in get smothered in this silt and the gravel disappears

  • All the above issues become worse with climate change

  • They are a barrier to fish and eel passage, which can be partially mitigated by options described on our 'understanding fish passes page'

Healthy river habitats include exposed gravels, flow diversity, marginal and in-channel vegetation and clearer water. See our 'understanding our structures page' to find out more about the sluices, including how Island Barn ensures water is diverted into the Old Mole and Ember Loop. 

  An impounded river controlled by structures. Source: Environment Agency 

A naturalised stream. Source: Environment Agency 

Further information can be found via the important links section on the right side of the page.

Water Quality

In Spring 2021, we engaged with the community and other stakeholders where we shared a shortlist of options for how the Scheme could look in the future. We appreciate your feedback during this process, especially your concerns about water level lowering in the Scheme. In response to the feedback received, we conducted a study in 2022 to review technical solutions that could mitigate the impacts of water level lowering that would result from the removal of sluice structures along the Lower Mole. 

We acknowledge the concerns you raised about potential impacts to water quality due to changes in water levels. In response to this concern, we conducted an assessment at the beginning of 2022 to evaluate the impacts of the shortlisted options on water quality. We have kept this assessment up to date as the shortlisted options have evolved and changed and have considered the potential impacts to water quality associated with the revised shortlisted options. 

What did we consider to state that water quality won’t deteriorate? 

There are now no alterations proposed in the area between Viaduct Sluice and Island Barn Sluice. As a result, the hydrology and hydrodynamics of the river system at Esher Sewage Treatment Works (STW) will not be altered, meaning that the volume and flow of water to this location will remain the same as it currently is. 

The proposed changes will not alter the location or the potential load of pollutants from Esher STW. 

It is expected that if there are intermittent discharges from Esher STW that the pollutants will be adequately mixed by the time, they reach the lowered water levels downstream of Island Barn Sluice. This would occur with the pink and orange options. The volume of the water body also remains the same for a particular flow scenario, so this means that it is unlikely that the impact of intermittent discharges would be changed. 

The amount of flow down most parts of the river will remain the same. Therefore, it is unlikely that the pollutant load from Esher STW will change. 

If water levels are lowered at Mole Ember sluice, the Thames will occasionally control (raise) the levels in the channel upstream, but this will be controlled by structures at Teddington. It will not impact on water quality. 

Will there be any further investigation into water quality? 

Although it is possible that a water quality model could be developed to further test options, it is not currently recommended by our technical specialists who are certain of the same or improved water quality. We will review this as the Scheme progresses.

See our 'improving environment on the Lower Mole page' for the impacts of the water levels and impoundments on biodiversity.

Page last updated: 07 Feb 2024, 10:15 AM