How do I calculate water velocity?

Measure out a stretch (or reach) of a stream and starting upstream, drop a plastic ball into the water. Then, using a stopwatch, time how long it takes for the ball to travel the distance you previously measured. Do this at least 3 times, then add up the times and divide them by 3 (or the total number of times you timed the ball) to get the average velocity. From there, take the average time and divide it by the distance you measured to find out how many meters the water travels per second.