Riparian vegetation is vulnerable to human impact worldwide, and this is especially so in arid areas, yet there have been few quantitative studies and this is especially so in Spain. The state of the riparian vegetation along three major rivers and seasonal watercourses of south-eastern Spain was evaluated during 1992–93, using the species composition and community structure in watercourses of different sizes under different management. Reaches of the watercourses were classified using five vegetation indices, namely percentage cover, species richness, degree of connectivity between patches of the plant communities, number of exotic species, and evidence of natural regeneration. With the integration of these into one index, the degradation state of the riparian vegetation in each reach was quantified. In addition, types of human activities exerting the greatest impact were noted, and a scale to evaluate the intensity of each impact was established. The indications are that agriculture has very substantially altered the natural vegetation, and this index has served to highlight the most altered zones, and thus those in most urgent need of restoration. Less-degraded zones could serve as models and sources of plant species for future restoration. The degradation index made it possible to establish quickly, easily, and with a high degree of accuracy, the state of conservation of the riparian vegetation in the study area.