University of California, Berkeley researchers have created a floating capsule that holds an Android based GPS-enabled smartphone.  They created a custom Android app that constantly transmits the location of the sensor back to a central hub for monitoring water flow.  They also have a custom sensor base (pictured above) that monitors water quality, salinity, and other characteristics.  They use about one hundred sensors at a time, throw them in the water, let them float downstream for awhile, then recover them and analyze the data.  Some of the units contain propellers, and can navigate freely on their own.

Each bot currently costs a few thousand dollars a piece, but they are hoping to bring the costs down significantly.

Read more and check out videos of the bots in action.