The Kushiro Marsh in eastern Hokkaido is the largest wetland in Japan. Monitoring changes in environmental conditions is important to support preservation of the wetland. In the summer of 2016, Hokkaido was affected by several typhoons and suffered record-breaking heavy rainfall. ∼50 ALOS-2 SAR interferograms were constructed covering the Kushiro Marsh over the period 2014 to 2018, including the summer of 2016. A time series of vertical displacements of the wetland detected by the interferograms corresponded to water level changes in rivers of the wetland. This implies that the SAR data successfully detected height changes of the water surface in the wetland. Between August 6, 2016 and September 5, 2016, an area of the wetland (∼1 km) to the southeast of Akanuma shifted ∼2.7 m horizontally in the downstream direction, which coincided with a large and rapid increase in the water level caused by the heavy rains. This large displacement remained after the water level fell. Prior to this large horizontal shift, an uplift of ∼10 cm was identified in almost the same region. This uplift might have been caused by groundwater leaking from the basement of the Akanuma pond, and implies that the peat layer, which had a thickness of several meters had floated slightly, like a floating island, possibly causing the large horizontal shift.