Experimental researches were conducted on flood carrying capacity of compound open channels with wide and narrow floodplains as well as hydrodynamic behavior of overbank flow across vegetated floodoplains. Results show that hydrodynamic behavior of compound channels with narrow floodplains is different from that of the wide floodplains. In such compound channels with narrow vegetated floodplains as in the Pearl River delta network, it has been found that vegetated domains become really impenetrable when the length of vegetated domain is larger than Lm, the length of flow in floodplain coming into the main channel, and that the influence of vegetated floodplains upon flood stages of a channel is not very significant.