What characterizes uniform flow in hydraulic systems?

Uniform flow in hydraulic systems is characterized by flow that maintains a constant width and depth throughout a given reach of the flow. This means that the velocity distribution and the hydraulic parameters do not change with respect to the distance along the channel or pipe. In uniform flow, the flow properties such as velocity, depth, and cross-sectional area remain consistent, allowing for the application of various hydraulic equations without the need for adjustments for varying conditions along the flow path.

The other options represent different conditions that do not align with the definition of uniform flow. For instance, flow with varying width and depth would indicate a non-uniform flow condition where the hydraulic parameters fluctuate, which complicates analysis and calculations. Flow driven solely by gravitational forces can occur in both uniform and non-uniform conditions; therefore, it does not specifically characterize uniform flow. Similarly, flow influenced by external pressure may impact the flow characteristics but does not define whether the flow is uniform or non-uniform. Each of these scenarios does not align with the key characteristic of uniform flow being stable and unchanging across the flow path.