How do we detect floods?
Floods are associated with considerable sediment transport and higher than normal water levels.
Therefore, the study of sediment layers in river banks, lake bottoms or the ocean floor can reveal large sediment deposition events, that could be associated with floods or landslides. The characteristics of the sediments can be used to identify the process that generated it, and the organic matter trapped within the sediments can be used to date the time of the flood.

Here is an example of how a sediment layer from a flood looks like in a sediment core



It is taken for a paper that might be interesting to you: Lake Sediments as Archives of Recurrence Rates and Intensities of Past Flood Events. The image in the left is a picture of the sediment core (with a overlay showing the changes in reflectance along it).