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.
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
Changes to make Guernsey's buses more reliable
Aurigny plane makes emergency landing after leaving Guernsey
New festival part of Guernsey's Seafront Sunday
Guernsey firefighters tackle round-island walk in full kit
