Not nearly as large as that, but when we used to fish up in Canada, we used to have to go through a narrow channel that had lots of little floating "islands" of vegetation to get to a smaller lake. We would sometimes take two boats, with my buddy and me in the front one, with our dads following behind. We would get ahead of them and then pull one of the islands into the channel so that when they got there, it looked like they had reached a dead end.
Physics. Since it was floating they only needed to overcome the inertia and any wind resistance to move it. Even a human can push on large floating objects and get them to move if the water is still and wind isn't blowing. Pushing lightly but steadily will also do the job.
Kenny Rogers and Dolly Parton taught us about moveable islands long ago... nobody was paying attention.
"Islands in the stream
That is what we are
No one in between
How can we be wrong? Sail away with me
To another world
And we rely on each other, ah ha
From one lover to another, ah ha"