Final answer:
Q3 Flood Data provides Base Flood Elevations if available, which are critical for flood risk management. The 21,200 cfs discharge on 9/23/2009 would need to be plotted against historical data to determine its recurrence interval.
Step-by-step explanation:
Q3 Flood Data indeed provides Base Flood Elevations (BFEs) where they are available. BFEs are an important part of flood risk management and are used to help enforce building codes and insurance requirements. The discharge measured on 9/23/2009 at the gaging station was 21,200 cubic feet per second (cfs). When plotting this on a graph, one would plot the discharge against the corresponding water surface elevation to determine the flood level. As for the recurrence interval, which is a statistical measurement typically expressed in years, it represents the likelihood of a flood event reaching or exceeding a specific magnitude in any given year. To find the recurrence interval for this specific flood event, one would need a series of historical flood data to compare the discharge value against.