PALSAR full polarimetry data were used to detect thaw/freeze conditions on the ground to try to determine the cause of a spring flood on a northern river in Russia. Full polarimetric data was used to identify the forest area, and the radar backscattering coefficient, a0HV, was examined. The a0HV over a frozen forest area was -19.9 dB, while that of a thawing forest area was -11.9 dB, and freeze/thaw events over the forest area were well represented by this parameter. Using this characteristic, together with optical sensor (AVNIR-2) data and microwave radiometer (AMSR-E) data, freeze/thaw conditions were examined for an area including Markha, where a large ice jam flood occurred in 2007, and compared it to 2009, when no flooding occurred. The results indicate that the thawing event proceeded more rapidly between March 29 and April 10, 2007 for the forest, and this rapid thawing may be one of the possible causes for the large ice jam flood in Markha in 2007.