In this paper, we describe a grand vision of our project, which is to develop an autonomous water monitoring system by operating an unmanned aerial and surface vehicle collaboration team with the focus on large and long rivers. Key backgrounds and challenges in realizing this vision are briefly outlined, and we show a simple path planning for an unmanned surface vehicle for the effective water sampling in the Wabash River, the natural river in the US, as one of our envisioned scenarios. Lastly, some future works are described.