Final answer:
Flood stage is reached when water level in a stream overflows its banks, necessitating measurement of discharge necessary to overtop the banks. Flood stages are critical for floodplain management. Flood frequency curves and recurrence intervals help in understanding and predicting floods.
Step-by-step explanation:
Flood stage is defined as the water level at which a stream overflows its banks and spills onto the floodplain. In more technical terms, flood stage is reached when the amount of discharge necessary to overtop a stream's banks and spill onto the floodplain is reached, answering the student's question with option b. It is not based on the slope of the plot of the change in discharge over time nor simply on a historical comparison of discharge levels.
Flood stages can cause significant damage to developed areas, particularly those within a floodplain which often are subject to development due to their attractive water views. However, these areas may be best used for non-essential structures or recreational areas like playgrounds and golf courses due to the inherent risk of flooding.