Recently, decreases and changes in aquatic plant habits in irrigation canals are becoming an important problem. The purpose of this study was to determine the relationship between aquatic plant distributions and environmental conditions in irrigation canals in order to develop appropriate conservation plans. The range of aquatic plant distribution, water quality, the structure of the irrigation canal, and surrounding land use were surveyed for 91 plots covering a total distance of 20783.4 meters in Kamiina District, Nagano Prefecture. Thirteen aquatic plants including the threatened species, Sparganium japonicum, were identified, and eight types of plant communities were distinguished at the third level of the TWINSPAN classification. Three main plant community types were identified : Ranunculus nipponicus var. submersus community type, paddy weed - Sparganium japonicum community type and naturalized plant species Elodea nuttallii community type. The R. nipponicus var. submersus community type was found in irrigation canals with in lower water temperature and electric conductivity values than observation points for other community types.The paddy weed - S. japonicum community type was typified by lower oxygen saturation and higher electric conductivity than for other community types. As the naturalized plant species E.nuttallii community type was located between other types and also widely distributed, this poses an important problem for conservation of these habitats.
