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La Miel River is a tributary of the Magdalena basin and the main tributary of the Amaní reservoir. In order to establish the spatial and temporal dynamics of periphytic algae genera downstream of the reservoir, data collected every two months between 2006 and 2009, at seven sampling sites (five in the main channel and two tributaries) were analyzed. 130 genera and 15 classes were recorded, Cyanophyceae and Bacillariophyceae being the most representative in density and frequency. Both the generic density and richness showed significant variations in the spatial dimension, but did not differentiate the sampling sites on the La Miel River from those in tributaries and did not show a specific pattern related to hydrological seasons; but changes of richness were observed between samples and a association between the algal community and parameters related to water dynamics was found. We conclude that although the variables analyzed are not limited by the discharge, other structural characteristics of the community such as the taxonomic composition and diversity are affected, especially at the site of direct influence of this process. [ABSTRACT FROM AUTHOR]

Changes in the biota inhabiting the streams downstream of dams have been the subject of numerous studies but few have focused on recovery gradients downstream of the dam or the influence of seasonality on the pulse of the flow in regulated streams. Since 2000 the hydro-power company Isagen SA E.S.P. has been monitoring some emerging characteristics of the fish community in the lower basin of La Miel River (Magdalena River Basin) downstream of the dam to document the response of the fish fauna to new environmental conditions. Based on this monitoring, we tested the hypothesis that there would be a gradient of recovery in the fish assemblage characteristics along the longitudinal axis, downstream from the turbine's output to its mouth in the Magdalena River. The highest values observed in the dominance index were a result of the arrival of migratory species to the site of the water discharge from the dam during low water period. Finally there is a need to weigh alternative locations of a dam taking into consideration the possibility that downstream tributaries can diminish the impact of the dam and hydropower plant operation. [ABSTRACT FROM AUTHOR]

One of the most obvious impacts of damming a river is the disruption of migratory routes, however, the characteristics of the dam, the river and its fish fauna make impossible the generalizations about the possible effects. Since the beginning of the operation of La Miel I hydropower plant, it was detected the entrance of fish into the hydropower plant (through the surge chamber) from La Miel river. To determine the composition of the fish and evaluate the potential effects on migration, for one year (April 2010-April 2011) adult individuals were tagged and released and ichthyoplankton collections were made, both in the surge chamber and in the discharge channel. Individuals of migratory and non-migratory species, mainly piscivorous and detritus-scrapers were found. Individuals who entered the surge chamber were able to find quickly the exit through the restriction orifice and returned to the main channel of La Miel River. The fish did not spawn in the central structures. We conclude that the surge chamber is not an absolute barrier to migration, but it is a staging habitat for migrant individuals. [ABSTRACT FROM AUTHOR]