Cross plots, or crest-to-crest relationships, are developed to help the hydrologists with their forecasts. The cross plots provide additional information as to how high a river may crest and the timing of the crest. Cross plots are developed by plotting the crest flow or stage at one location against the flow or stage at another location. The two locations must be somewhat related, they are usually on the same river, or on nearby rivers that react similarly. One location usually crests before the other. Points are plotted over many years for different events, and a relationship can be formulated by drawing a "best fit" line through the points. The difference in crest time can also be noted on the plot.
Cross plot confidence levels vary: The less scatter around the best fit line, the more confidence you can have in the cross plot. If the two points being compared are separated by a short distance, the confidence is increased, but the value of the cross plot is reduced because the lead time is so small. If a large tributary flows into the river between the two points being compared, the confidence level is reduced.
During an event, once the first river point has crested, the hydrologist can use the cross plot to get an idea of when and how high the second river point might crest. This can be a good way to check the output from a model to see if it is reasonable or not. This can also be a way to estimate a forecast in the event that the RFC is unavailable (i.e. due to a power outage, emergency evacuation, etc..)