A scientist wants to determine how the speed of flowing water affects the size of...

The scientist is determining how the speed of flowing water affects the size of sediment particles it can carry by varying the water speed and measuring the sediment sizes. This involves manipulating one variable (water speed) and observing the effect on another variable (sediment size), which is the essence of a controlled experiment. The setting and control of conditions ensure that the experimenter can establish a cause-and-effect relationship. References:

Integrated Physical Sciences, Chapter 2: Methods of Scientific Investigation