Final answer:
The flood stage is when a stream's water level rises high enough to overtop its banks, and the recurrence interval helps to estimate the frequency of such floods occurring.
Step-by-step explanation:
The amount of discharge needed to overtop a stream's banks and spill onto the surrounding land is the flood stage. Flooding occurs when the water level in a stream exceeds its banks. The concept of a recurrence interval is related to the flood stage, which represents the average interval of time within which a flood of a specific size can be expected to occur.
For example, a 1 in 100 year flood has a 1% chance of occurring in any given year, not that it will happen exactly every 100 years. It's also important to understand that river discharge is the volume of water moving through a river channel over time and can be influenced by surface runoff and groundwater seepage, varying due to factors like precipitation patterns and soil characteristics.