The company is planning the work to start in August, subject to planning permission being granted by Durham County Council.
The information event, scheduled for Wednesday, June 19, will take place at the reservoir′s fishing lodge from 3pm to 7pm. It will also offer a chance to find out about leisure activities available at the site.
When the plans are approved, the first phase of the project will be preparatory work to the east of the reservoir, including the creation of a site compound, and will be carried out by Northumbrian Water′s partners, Interserve Construction.
This will be followed by the main part of the work, starting in Spring 2020, which will reshape and adjust the levels of the reservoir spillways and upgrade leisure amenities on the south shore, including improvements to facilities for anglers with disabilities.
A second phase of work, anticipated for 2021 to 2022, is currently being investigated.
Mike Ciaraldi, Northumbrian Water′s project manager, said: "We are very keen to meet with any local residents or users of the reservoir who would like to find out more about the planned work, so our customer information event on June 19th is a great opportunity to do that.
"The work is something we need to carry out, following our ten-yearly assessment by government inspectors, so we want to make sure we do it in a way that keeps disruption to a minimum and ensures we do not unduly inconvenience residents and visitors.
"As well as the event, we have created a space on our online community portal, where we will be posting regular updates and where people can ask questions directly to the team throughout the work."
People wanting to keep updated on the project can visit the portal at www.nwlcommunityportal.co.uk and follow the link to the Grassholme Reservoir page. Alternatively, they can telephone Northumbrian Water′s customer contact centre on 0345 7171100.