Evaluating the retention capacity of a new subtropical run-of-river reservoir

In man-made reservoirs, the sedimentation and assimilation of elements usually prevail as a result of a decrease in the flow regime and an increase in the hydraulic retention time. Thus, the retention capacity derives from hydraulic flushing, as well as chemical and biological reactions. The aim of this study was to assess the element retention capacity of a new subtropical reservoir (Piraju Reservoir situated in São Paulo State, Brazil). Limnological monitoring was performed over four consecutive years (August 2003 to August 2007). We determined 19 variables (chemical, physical, and biological) every 3 months at the inlet (Paranapanema River) and outlet water of the Piraju Reservoir. For each variable, a mass balance was performed and the alpha parameter (i.e., retention capacity) was defined resulting in 323 determinations. From these results, only 10% led to the occurrence of element retention. Retention events were episodic; the fecal coliforms (seven times) and the N-NH₄ (six times) were the variables that presented the highest number of retentions. The results show that different variables can be linked to both the retention and release of elements from the reservoirs. The results show the great significance of the physical processes (in this case, hydraulic retention time and mixing regime) in determining the element retention and exportation from the Piraju Reservoir. The water temperature was a secondary variable for the processes related to retention (such as chemical reactions and biological assimilations).