We document the outstanding diversity of fluvial ecosystems in eastern Andalusia, mostly attributable to the high environmental heterogeneity of the region. The area's altitudinal and climatic gradients are among the most pronounced in the Iberian Peninsula, and together with a concomitant high variability in geological characteristics and human impacts, result in a noticeable heterogeneity of the rivers' thermal regime, discharge regime and chemical properties. Fluvial communities respond to this spatial heterogeneity with marked qualitative and quantitative changes among rivers and along the upstream-downstream continuum, generally exhibiting a great decrease in taxonomic and functional diversity as human impacts increase towards the lower reaches. Discharge fluctuations add heterogeneity on the temporal scale and are an additional essential determinant of biological diversity. Climatic, geological and hydrological characteristics profoundly affect the structure of the riparian vegetation, which in turn strongly conditions the community structure of benthic macroinvertebrates and organic matter turnover in fluvial ecosystems. © Asociación Española de Limnología.

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