Competence in a stream is directly proportional to which of the following factors?

Competence in a stream refers to the maximum size of sediment that a stream can transport. This characteristic is fundamentally related to the physical dynamics of the stream, particularly its velocity. When the velocity of a stream increases, it has more energy and can therefore carry larger and heavier particles. Conversely, a slower-moving stream will struggle to transport larger sediment, leading to sediment deposition rather than transport.

Stream velocity is critical because it directly influences the energy exerted by the water flow. As streams encounter changes in gradient or discharge, their velocity can change, consequently affecting their competence. This relationship means that higher velocities allow for the transport of larger particles, making this option the most accurate reflection of the concept of stream competence.

In contrast, stream temperature, the time of year, and discharge rate also play roles in the dynamics of streams, but they do not have the direct and proportional relationship with competence that stream velocity does. For instance, while a higher discharge rate typically indicates more water flow and can correlate with increased velocity, it isn't the velocity itself that's responsible for competence. Thus, stream velocity stands out as the key factor directly proportional to a stream's competence.