Which aspect of a stream is primarily responsible for determining the size of particles it can transport?

The competence of a stream refers to its ability to transport sediment based on the size of the particles it can carry. This concept is crucial for understanding sediment transport dynamics in fluvial systems. Competence is influenced by several factors, including the stream’s velocity and the energy available within the flow. As a stream's velocity increases, its competence also increases, allowing it to transport larger particles.

In this context, competence is distinct from capacity, which describes the total amount of sediment a stream can transport. While discharge, which is the volume of water flowing through the stream at any given time, affects both velocity and overall sediment load, it is the competence that specifically addresses the size of the particles in transport.

Thus, competence is the defining factor when considering what size of materials a stream can effectively move, making it the correct answer in this question. Understanding this concept is vital for analyzing erosional processes and sediment dynamics within river systems.