2. Factors affecting streamflow

The factors affecting streamflow—particularly the relationship between the water surface elevation (stage) and the discharge—are primarily driven by changes in the channel's "control" section. When this relationship changes over time, it is referred to as a shifting control.

The major factors that alter and affect streamflow at a given cross-section include:

• Physical Channel Changes: The physical characteristics of the river channel can change due to natural or artificial interventions. Factors such as weed growth, dredging operations, or channel encroachment can significantly alter how water flows through a section.
• Aggradation and Degradation: In alluvial rivers (rivers with movable beds), the flow is heavily affected by sediment transport. Aggradation (the deposition of sediment) or degradation (the scouring or erosion of the riverbed) actively changes the cross-sectional area and the stream's flow dynamics.
• Variable Backwater Effects: Streamflow at a specific location can be influenced by conditions downstream, such as a dam, a intersecting river, or a tidal reach. These downstream elements can create a backwater effect that alters the water level independent of the actual discharge. To accurately measure flow under these conditions, an auxiliary "fall gauge" must be installed downstream to calculate the water surface slope.
• Unsteady Flow (Flood Waves): Streamflow behaves very differently under highly unsteady conditions, such as during a flood.
  • During the advancing (rising) phase of a flood wave, approach velocities are higher than they would be for a steady flow at the same stage, resulting in a higher discharge.
  • Conversely, during the retreating (falling) phase, approach velocities are reduced, giving a lower discharge than a steady flow at an equivalent stage.
  • Because of this, the stage-discharge relationship during a flood does not follow a single line but rather forms a looped curve (hysteresis).

Additionally, the physical properties of the streamwater itself can affect how flow is measured depending on the technology used. For example, if flow is being monitored using ultrasonic meters, factors like temperature changes, salinity, air entrainment, and high loads of suspended solids can interfere with the signal velocity and the accuracy of the flow measurement.