Final answer:
An example of an input to a river system is rainfall, contributing through surface runoff and groundwater seepage.
Step-by-step explanation:
An example of an input to a river system is rainfall, which contributes to the river through surface runoff and groundwater seepage. Factors such as precipitation patterns, vegetation, topography, land use, and soil characteristics influence the relative contributions of these inputs to the overall river discharge. During a heavy rainstorm, for instance, the river discharge increases significantly due to the rapid surface runoff while, under normal conditions, groundwater provides a steady flow into the river system.