"Negan won't send his people down into this stretch of swamp, not if he doesn't have to. (…) Negan wanted to map the best routes with cover from the Sanctuary to the Hilltop. He -- he decided the swamp was too dangerous. Didn't think it was worth the risk."
—Dwight explains the swamp to the Alexandria survivors.[src]
Nothing is known of the swamp's history before the outbreak began.
At some point after discovering Hilltop Colony, Negan had the Saviors map out all the routes with cover between the Hilltop and the Sanctuary. One such route the Saviors examined was the swamp. However, it was deemed to be too dangerous by Negan to send people through and he chose not to risk it.
As the survivors of Alexandria Safe-Zone try to find a safe route to the Hilltop Colony with the Saviors undoubtedly covering every pass, Dwight suggests the swamp as an alternative as Negan found it too dangerous to send men into. Though the Alexandria survivors found the idea dubious due to Dwight being a Savior and Negan thinking it was too dangerous for his own people, Dwight was able to convince them to take the risk.
After arriving at the edge of the water, Daryl began leading a team made up of himself, Rosita, Siddiq and Scott through the water to clear out walkers and create a safe path for everyone else. Armed with long sticks, the group gingerly makes their way through, prodding with the sticks every few feet to find walkers who are hiding underwater. Along the way, Daryl and Siddiq come across an area with around a dozen walkers blocking the path. With it being the only way across, the two men clear out the walkers despite the danger. The team ultimately succeeds in clearing a path through the swamp for everyone else.
As Daryl's team clears out the water, a couple of walkers wander into the area where the other survivors are waiting. Tara takes a reluctant Dwight to take care of the walkers before trying to kill him in revenge for Dwight killing Denise leading a brief chase to the edge of the swamp that is interrupted by the appearance of a group of Saviors who decided to search the area anyway despite the danger. With the Saviors in danger of finding the others, Dwight reveals himself and spins a tale of how he survived the fight at Alexandria, learning that Laura hasn't been seen and thus the Saviors don't know of his betrayal. Dwight claims to have already checked the swamp and seen no sign of anyone and misdirects the Saviors away, witnessed by Tara and Rosita.
When Tara and Rosita return with the news, Daryl is enraged, but the two women assure him that Dwight won't betray them. With the Saviors and walkers gone, Daryl leads the Alexandrians safely through the swamp and to the Hilltop.