Finding the Logjam Fortnite location is needed to complete one of the weekly challenges, but it's only one of three locations. You need to land at The Rig, Hydro 16 and Logjam Woods to complete one of the weekly challenges.
Two of the three are not named on the map, so they can be hard to find if you don't know where to look. Luckily, all three are fairly close to one another, but you'll still need to land and play three different matches to complete the challenge.
Logjam Fortnite Location: How to Complete Weekly Challenge
All three locations are located toward the southwest of the map. The Rig is listed on the map, so it's probably favorable to go there first. It also is the perfect place to finish the propane tank challenge. TNTina is the one handing out the challenges this week which will allow you to double down.
Once you finish heading to the Rig and completing both parts, head either to the Damn which is directly east toward the river, or Logjam which is a little bit to the north of the Rig.
Both are fairly close to Slurpy Swamp. Once you land at both locations, you'll finish the challenge.