Yes, they can be, but it does depend on the specific design. As you can guess about rivers, your path is going to be much narrower. When you kayak in lakes and oceans, you will have all the room in the world. But, this will not be the case going down rivers.
So, if you want to be able to maneuver with ease, you will probably want to look for shorter kayaks. The improved speed you get from more extended models will not do much good when you need to dodge obstacles left and right.