We are currently experiencing very low flows in the upper reaches of the Hogsmill River which is causing concern, especially with respect to the ecology of the river. From the source at Bourne Hall Lake to the confluence with the Green Lanes Stream the Hogsmill River is designated as chalk stream which is an internationally scarce habitat. For several years we have been an active partner in the Hogsmill Catchment Partnership hosted by the South East Rivers Trust. Partners include, the Environment Agency, Thames Water, SES Water, Surrey County Council, Royal Borough of Kingston, Kingston University, Kingston Environment Trust, the Lower Mole Partnership and several individuals with expertise in ecology and river management.
Whilst ground water levels are low at present and low flows are to be expected, there is also evidence to suggest that a significant amount of water is not reaching the river. Consequently, with partners we are investigating from Bourne Hall Lake and downstream to try and identify where water is being lost and there is a search for possible solutions, which is being greatly assisted by the partnership.
In recent weeks our staff have been working with the Environment Agency (EA) as the overseeing authority, Sutton & East Surrey (SES) and the South East Rivers Trust. The investigation has involved SES obtaining a licence from the EA to divert some of the flow to the fountain in Bourne Hall Lake directly in to the Upper Mill Pond. This is possible as the bore hole and pump are located next to the Upper Mill Pond. This action effectively bypasses the Horse Pond which is suspected as the most likely location for the loss of water. During monitoring/further investigations it was discovered that water was indeed flowing out of the Horse Pond but via a surface water drainage inlet. To prevent that sand bags have been used to create a temporary barrier and that confirmed the loss of water via the inlet. However, whilst the Horse Pond level has recovered it is still not flowing over its weir in to the Upper Mill Pond, despite the continuing flow received from Bourne Hall Lake and the fountain, which is currently the main input, with low ground water meaning the springs are not currently flowing. This week the next stage has been to use florescent dye to investigate flows at the Horse Pond Weir and that showed that whilst a flow is present it is very sluggish taking 15 mins to flow under Chessington Road in to the Upper Mill Pond. The result also indicated that the flow present was either through or under the weir and not over it. The investigation has also revealed a significant leak in the Upper Mill Pond and we are now investigating a solution, however, the focus of attention remains on the Horse Pond as it receives the flow from Bourne Hall lake. The measuring station at Bourne Hall Lake shows that the flow from the fountain is being received by the Horse Pond.
As a result of this week’s investigations the EA are recommending that there is merit in temporarily turning off the Bourne Hall Park lake fountain next week for several days to further investigate the Horse Pond and what happens if there is no input from Bourne Hall Lake. The fountain has since been switched back on today, Friday 1 March.