The work, part of a £12m programme of upgrades at the site, will temporarily lower the water level by up to 4 metres in the reservoir to allow construction works to be completed, before being set at their new level.
The work to lower the levels will increase the resilience of the reservoir and help to protect the Victorian dam wall during extreme flooding, leaving a lasting legacy for future generations.
Leisure upgrades are also planned subject to planning and will include improved disabled facilities for fishing, a new beach for anglers on the south shore, and, for the first time, sailing and angling will be separated.
Improvements will also be made to facilities currently used by the Teesdale Sailing and Watersports Club, with new boat launch and parking facilities.
The work complements other planned developments, including a planned observatory, which will help turn Grassholme Reservoir into a key multi-purpose leisure destination.
During the course of the work, which is scheduled to run from March to December 2020, heavy plant movements and fluctuating water levels will require restrictions of access for the public throughout that period. Walking routes will be restricted, in particular access to the dam crest and these will be clearly signposted and advertised. Many other routes will still be available to use safely. The project customer portal will provide updates for those who wish to come and take in some of Teesdale's beautiful countryside. See link:
While the Grassholme Reservoir Visitor Centre will remain open and manned by Northumbrian Water's Waterside Park Rangers for information and to sell fishing equipment and permits for use on other sites, angling will be moved to nearby Hury Reservoir and the site will have some limitations for walkers using the reservoir circular route.
All of Grassholme's fish stock for 2020 will transfer to Hury Reservoir, normally only used for fly fishing, which will become a site for all method angling throughout the year. While self-service permits will be available at Hury, normal permits can be bought at Grassholme, where anglers will also be able to record visits as part of Northumbrian Water's loyalty scheme.
Don Coe, Northumbrian Water's Waterside Parks Operations Manager, said: "The upgrades being planned for Grassholme Reservoir have created a fantastic opportunity to deliver some wonderful improvements for our leisure visitors.
"While we will need to move fishing away from the site for the 2020 season, it's a great opportunity for anglers to try out other sites across Teesdale and for us to add an extra dimension to the sport at Hury Reservoir, which is just a short drive away.
"People who turn up at Grassholme will still be able to access the visitor centre from the top car park, where they can enjoy a drink and a snack, buy equipment and also get information about the other sites they can visit while the main reservoir area is closed to visitors."
"We're sure that the changes and restrictions we put in place for 2020, for the safety of all visitors, will be worthwhile because of the improvements that people will see when it re-opens in 2021 and we thank visitors for their patience while the work takes place."
The work planned for 2020 is part of ongoing investment in this area in line with the site's most recent ten-yearly assessment by government inspectors.
The company has worked with statutory bodies, investing considerable time and effort, including surveys and consultations, into developing the plans, which meet requirements outlined in the site's latest Government reservoir inspection.
Plans for further work, anticipated for 2021 to 2022, are currently being finalised and will be the subject of a planning application, which is expected to be submitted to Durham County Council in the Spring. A special event for customers to visit the site and learn more about the work is being planned for April and full details will be issued closer to the time.