Davy, if you're comfortable with a distance of 20 yards ( 60 feet) then you could get a pretty close approximation of speed in mph by dividing the number of seconds it takes for an object to float the 20 yards into 40. (It actually works out to 40.8, but we're not doing rocketry science here). Assuming your 10 seconds to float the 20 yards, the water would be flowing about 4 miles per hour. (40 divided by 10 = 4).
There's an article on river currents and its effect on divers, etc at this website. They also have a table in there giving currents in mph based on the time it takes an object to float 100 feet.
Happy Floats! LOL