The crossing at St Saviour's reservoir will be fenced off for the next few weeks while repairs to the wall are carried out.
The dam walkway takes pedestrians across the 1930s reservoir wall and provides easy access to the popular Millennium Walk.
Guernsey Water says both car parks will remain open, as will the walk.
The repairs were recommended after an independent inspection of the dam wall.
The walkway will be used to anchor scaffolding to allow for repairs on the water side of the structure.
Carl Falla is the capital deliver manager at Guernsey Water:
“Before water levels in the reservoir increase from the winter rains, we’re taking this chance to carry out some minor works to the dam wall before we lose the opportunity until next summer.”
Work was started on the reservoir in the late 1930s but was halted by the war. It became operational in 1947.
Guernsey Post introduce new services from early July
Guernsey Waste improves the quality of its free garden compost
Guernsey cat lover to ride the length of Britain in nine days
Unclaimed CI Co-op cash funds community projects
Work to commence on 70 affordable flats and homes in Guernsey
A new tax, more 'proportionate' to wealth, to be put to Guernsey's States
Guernsey gives £75k for Islands Unlimited inter-islands sailings
Guernsey tax package to raise more States income but protect low earners
