In the present study, we automatically estimated a water spectrum in the MODIS images using the techniques developed by Sawada et al., and in an effort to construct a monitoring system for assessing flooding over a wide area, we attempted to assess the flood zone created by the flood that occurred from August to December 2011. Specifically, we calculated the beginning dates of flooding and flooding period in the Chao Phraya River Basin using the flooded area estimated from 10-day composite images using MODIS data and then estimated the flooded area attributed to the Chao Phraya River.