The refugees just sailed into wall Maria. There isn't much more to it than that. As far as I know the river gates were never explained, but they have been shown. In chapter 1 you can see the river flow through the walls and in chapter 73 you can see in more detail how the river flows freely in and out the walls through bars. I assume them to be able to open similarly to the regular gates. They have not been attacked and it has not been explained why. At this point it makes more sense to attack the regular gates than the bars, seeing we don't even known whether Titans can even swim.

The refugees just sailed into wall Maria. There isn't much more to it than that. As far as I know the river gates were never explained, but they have been shown. In chapter 1 you can see the river flow through the walls and in chapter 73 you can see in more detail how the river flows freely in and out the walls through bars. I assume them to be able to open similarly to the regular gates. They have not been attacked and it has not been explained why. At this point it makes more sense to attack the regular gates than the bars, seeing we don't even known whether Titans can even swim.