River plants from an Iberian basin and environmental factors influencing their distribution

River plants were studied at 119 sites from a southern Iberian river basin. A site length of 50-100 m was found to be adequate for characterising loca l plant richness in these rivers. Average number of river plant species per site was 48 (range 21-76). Species richness showed a maximum at mid-course , with considerable variation in the upper course and a decreasing trend to wards the floodplain. Sixteen environmental variables were initially consid ered to explain the biological variability. Nine, retained by the forward s election procedure, were significant predictors of the species distribution . Canonical correspondence analysis was used to directly relate plant speci es composition to these environmental variables. Altitude, conductivity, ri ver width, pH, percentage of hard substrates and fine particulate organic m atter on the river bed, average rainfall and temperature, and human-related disturbance were significantly related to the species distribution. Specie s positioned at the axes extremes could be arranged into species groups rel ated to different geographical areas of the river system.