To understand the mechanism for and effect of standing water depth on CH4 emissions from freshwater marshes, we measured CH4 emissions as well as labile organic C (LOC) and CH4 concentration in porewater in situ in Sanjiang Plain of Northeast China. Average fluxes of CH4 emissions from Carex lasiocarpa, Carex meyeriana and Deyeuxia angustifolia marshes were 35.6, 31.1 and 25.7mg CH4 m−2 h−1, respectively, during the 3-day measuring period in August 2001, increased as standing water depth incremented from 5 to 20cm. Standing water depth determined the type of marsh plants, which governed CH4 transport, and the amount of plant litters, which resulted in the difference in LOC for methanogenesis among marshes. LOC greatly affected CH4 concentration and total amount of CH4 in porewater, whereas the latter was significantly related with CH4 emissions. The more aboveground plant biomass especially stem density accelerated CH4 emission but was of less importance compared with the standing water among marshes.