What you get as far as "swamp" depends on your definition of swamp, really. The classic swamp is a drowned forest, usually with wet-adapted trees such as cypress that can tolerate having their feet wet. Another popular view of "swamp" is more of a marsh; that is, it's a wetland that doesn't support much in the way of trees, but still has lots of water. In both cases, the land is nearly flat, meaning that the river will slow down and spread out when it gets to that area. The water moves sluggishly. Thus, the river gets indecisive because all paths are equally good on a flat landscape and lazy because it's moving very slowly.