Abstract Aerial photographs taken in 1945, 1957, 1970, and 1985 were used to examine the invasion of red gum (Eucalyptus camaldulensis Dehn.) into an extensive, natural grassland. This grassland occupied a high flood frequency site adjacent to the major River Murray, Australia. Photogrammetric techniques were used to provide information on the successive states of 200 m × 200 m grid cells covering the wetland. The information collected was then assembled into a Markov Chain model of change in the wetland. Prior to 1945 the tree-plain boundary appears to have been stable. The model, extrapolated into the future, showed the almost complete extinction of extensive grass plains, although small-scale intimate mixtures of plain and red gum will exist as a stable form. Analysis of hydroperiods suggested that a major factor in this is the increased summertime levels and reduced winter-spring levels of the River Murray as a consequence of the need for downstream transmission of irrigation water. The affected areas tend to lie between the summertime heights of the river in the pre-regulation and post-regulation days. Although there are few water management options for this particular wetland, it is likely that other riverine wetlands have suffered similar changes but could be managed to avoid some of the adverse effects of such changes in hydroperiodicity.