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El Pescado (literally ‘the fish’) is a lowland stream in the pampean plain, entering the Rio de La Plata at 35˚55¢S,57˚45¢W, 70 km SW of the city of Buenos Aires. The stream develops as a potamon with variable flood frequency along its 36 km length. The water is slightly soft, with relatively high conductivity and dominance of HCO–3 , Na + and Cl – ions. Three fish families, Characidae (29%), Pimelodidae (14.6%) and Loricariidae (12.8%) are most abundant among 55 species. Differences in species composition among four stations with different environmental traits were observed. Flooding slightly affected the fish fauna. In open-water and rocky-bottom stations, both species number and number of individuals were correlated with conductivity, an indication of the decrease of fish density with floods. Species richness was well over worldwide means. Diversity values, higher than in small tropical streams in Venezuela and southern Brazil, ranged from 0.32 to 2.59, being higher during autumn and spring. The stream has a high persistence, with the composition of the fish community maintained or recovering as soon as flooding ends. One Synbranchus marmoratus specimen was found among rocks, the first record of this behaviour for the species.

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An investigation was performed in a pond (San Miguel) directly associated with the Salado River (Pampean grasslands). The aim of the study was to analyze river interactions. Physical and chemical conditions and the plankton community were studied in order to determine the effects of changes in hydrological conditions. The main factors that influenced the physical–chemical characteristics of the pond have been identified by means of cluster analysis. Conductivity was the main factor that determined groups in sampling periods. Changes in conductivity were clearly associated with the water‐level of the pond and depended on the flow of the Salado River and fluctuations of its conductivity, which is itself a function of evaporation and the inflow of underground water of high conductivity. Other factors that affected the physical–chemical conditions included concentrations of phosphorus and polyphenols, the main allocthonous sources. The concentration of these compounds was decreased in low water conditions. Principal Components Analysis suggested that there were four major regulatory factors in the pond, as follows. First, a dry season, with a prolonged isolation phase and a considerable increase in conductivity, turbidity and suspended solids, during which the abundance of plankton was greatly reduced, as were incorporated benthic species. A high and sudden increase of river flow determines the second regulatory factor, the flood season, when dissolved allocthonous material enters, conductivity decreases and there is a conspicuous dominance of cyanophytes, protozoans and crustaceans. The different degrees of hydrological connection with the Salado River produce changes in dissolved phosphorus forms and the composition of the plankton. The third regulatory factor is an increase in particulate material. This is associated with the highest total phosphorus values as a consequence of the phosphorus– particle relationship. The opposite situation, the fourth regulatory factor (a decrease in particulate material), produces clear water conditions with a dominance of chlorophytes, planktonic diatoms and rotifers.