Natural flood plains are among the most biologically productive and diverse ecosystems on earth. Globally, riverine flood plains cover > 2 × 106 km2, however, they are among the most threatened ecosystems. Floodplain degradation is closely linked to the rapid decline in freshwater biodiversity; the main reasons for the latter being habitat alteration, flow and flood control, species invasion and pollution. In Europe and North America, up to 90% of flood plains are already ‘cultivated’ and therefore functionally extinct. In the developing world, the remaining natural flood plains are disappearing at an accelerating rate, primarily as a result of changing hydrology. Up to the 2025 time horizon, the future increase of human population will lead to further degradation of riparian areas, intensification of the hydrological cycle, increase in the discharge of pollutants, and further proliferation of species invasions. In the near future, the most threatened flood plains will be those in south-east Asia, Sahelian Africa and North America. There is an urgent need to preserve existing, intact flood plain rivers as strategic global resources and to begin to restore hydrologic dynamics, sediment transport and riparian vegetation to those rivers that retain some level of ecological integrity. Otherwise, dramatic extinctions of aquatic and riparian species and of ecosystem services are faced within the next few decades.

The dynamic nature of alluvial floodplain rivers is a function of flow and sediment regimes interacting with the physiographic features and vegetation cover of the landscape. During seasonal inundation, the flood pulse forms a 'moving littoral' that traverses the plain, increasing productivity and enhancing connectivity. The range of spatio-temporal connectivity between different biotopes, coupled with variable levels of natural disturbance, determine successional patterns and habitat heterogeneity that are responsible for maintaining the ecological integrity of floodplain river systems. Flow regulation by dams, often compounded by other modifications such as levee construction, normally results in reduced connectivity and altered successional trajectories in downstream reaches. Flood peaks are typically reduced by river regulation, which reduces the frequency and extent of floodplain inundation. A reduction in channel-forming flows reduces channel migration, an important phenomenon in maintaining high levels of habitat diversity across floodplains. The seasonal timing of floods may be shifted by flow regulation, with major ramifications for aquatic and terrestrial biota. Truncation of sediment transport may result in channel degradation for many kilometres downstream from a dam. Deepening of the channel lowers the water-table, which affects riparian vegetation dynamics and reduces the effective base level of tributaries, which results in rejuvenation and erosion. Ecological integrity in floodplain rivers is based in part on a diversity of water bodies with differing degrees of connectivity with the main river channel. Collectively, these water bodies occupy a wide range of successional stages, thereby forming a mosaic of habitat patches across the floodplain, This diversity is maintained by a balance between the trend toward terrestrialization and flow disturbances that renew connectivity and reset successional sequences. To counter the influence of river regulation, restoration efforts should focus on reestablishing dynamic connectivity between the channel and floodplain water bodies.

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ResumenEl Concepto de Discontinuidad Serial afi rma que la formación de embalses en cadena dentro de los ríos genera cambios en la estructura de la biota acuática que se acumulan a lo largo del cauce. En el río Porce (Cuenca del río Magdalena-Cauca, Colombia) se formaron dos embalses en cadena, los embalses Porce II y Porce III. Para conocer la respuesta del ensamblaje de peces dentro del sector del río que fue embalsado se tomaron muestras de cuatro periodos de lluvias abundantes y cuatro de menores lluvias entre los años 2011 y 2013, en sitios dentro de los dos embalses seleccionados según su posición en el eje longitudinal (cola, transición y presa); para eliminar el efecto de la profundidad en cada sitio se tomaron muestras a dos profundidades (superfi cial-fondo). Se capturaron 15 especies; 12 en Porce II y 10 en Porce III. Se observaron diferencias espaciales en cuanto a la composición y estructura de especies de peces, tanto en el eje longitudinal (cola-presa), como en el vertical (superfi cie-fon do). En el embalse Porce II, la riqueza y la abundancia de individuos disminuyeron en dirección cola-presa y fue mayor en la superfi cie que en el fondo; en el embalse Porce III, aunque la riqueza presentó la misma tendencia en el eje longitudinal, la abundancia fue mayor en el fondo que en la superfi cie. Las características del ensamblaje no estuvieron asociadas a los periodos pluviométricos en ninguno de los dos embalses. Se concluye que la composición y estructura del ensamblaje está defi nida por las características morfológicas del embalse, así como la edad y tiempo de retención hidráulica.Palabras claves: ensamblaje de peces, diversidad, peces de agua dulce, discontinuidad serial, río andino AbstractThe Serial Discontinuity Concept affi rms that the formation of reservoirs in cascade within rivers generates changes in the structure of the aquatic biota that accumulate along the basin. In the Porce River (MagdalenaCauca River Basin, Colombia) two reservoirs were formed in cascade, Porce II and Porce III. To know the response of the fi sh assemblage in the sector of the river that was dammed, we collected samples during four periods of abundant rains and four of lesser rains from 2011 to 2013 from sites inside both reservoirs selected according to position on the longitudinal axis (tail, transition and dam). To eliminate the effect of depth, at every site, we took samples from two depths, surface and bottom. 15 species were captured; 12 in Porce II and 10 in Porce III. Differences in the composition and structure of the species of fi sh were found to vary depending on the location along the river fl ow (from tail to dam), and also by the water depth (surface to river bottom). In Porce II reservoir, the richness and the abundance of fi sh diminished in direction tail to dam and the abundance decreased from surface to bottom. In Porce III though the species richness presented the same trend along the longitudinal axis, the abundance was greater on the bottom that at the surface. The characteristics of the ass...

/ Habitats or environmental factors that convey spatial and temporal resistance and/or resilience to biotic communities that have been impacted by biophysical disturbances may be called refugia. Most refugia in rivers are characterized by extensive coupling of the main channel with adjacent streamside forests, floodplain features, and groundwater. These habitats operate at different spatial scales, from localized particles, to channel units such as pools and riffles, to reaches and longer sections, and at the basin LeveL. A spatial hierarchy of different physical components of a drainage network is proposed to provide a context for different refugia. Examples of refugia operating at different spatial scales, such as pools, large woody debris, floodplains, below dams, and catchment basins are discussed. We hope that the geomorphic context proposed for examining refugia habitats will assist in the conservation of pristine areas and attributes of river systems and also allow a better understanding of rehabilitation needs in rivers that have been extensiveLy altered.

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