Sentence:  Excessively high floods may be caused by obstructions which were not noted by the observer subsequently removed. Information of this kind is necessary, but it is equally necessary that something of the local <WSD>hydrology</WSD> should be also known and applied. 
Meaning of the word marked with <WSD> tags:  The properties, distribution, and flows of water in a specific locale; the hydrological characteristics of a particular place or region.