PIRATARIA FLUVIAL NA SERRA DO ESPIGÃO: DIVISÓRIA DOS RIOS URUGUAI E IGUAÇU-AMÉRICA DO SUL

Stream capture is a drainage rearrangement where a flux transference occurs and a contribution area from a drainage basin is incorporated to another. It is about expanding a river system over another one, caused by erosive advantage earned by conditioning factors such as lithostructure, pluviometric regime, topographic gradient, and base level. However, in the southern region of Brazil, a rare dynamic of stream piracy was verified between the drainage basins of the Uruguay and Iguazu (Paraná) rivers. Stream captures were observed along the Serra of Espigão, part of the drainage divide between the two basins. Still, it was not possible to identify which basin was advancing over the other. This paper investigated the occurrence of stream captures, identifying which factors are responsible for this atypical stream dynamic. Mapping the stream captures by remote sensing and further validation with fieldwork, it was verified that there is stream piracy for both sides of the drainage divide. Still, it is not clear which basin is behaving more aggressively. A longitudinal profile analysis of the channels involved in the stream captures showed a local control in the drainage network. This control sets a local base level to the Iguazu river tributaries. It indicates the lithostructural limit between two different geological units: the Serra Geral group basalts and the Botucatu formation sandstones. Depending on the geographic location of this base level, channels that drain to the Iguazu (Paraná) river become more aggressive or less aggressive than the ones that drain to the Uruguai basin, and so they capture or lose area for the other basin. Therefore, it is impossible to identify a river basin that exclusively pirates the other, prevailing, in this case, a mutual competition between the Uruguai and Iguazu (Paraná) rivers basins. This fact highlights the significance of local base levels to promote stream capture processes.

Topographic controls on divide migration, stream capture, and diversification in riverine life

Drainages reorganise in landscapes under diverse conditions and process dynamics that impact biotic distributions and evolution. We first investigated the relative control that Earth surface process parameters have on divide migration and stream capture in scenarios of base-level fall and heterogeneous uplift. A model built with the Landlab toolkit was run 51 200 times in sensitivity analyses that used globally observed values. Large-scale drainage reorganisation occurred only in the model runs within a limited combination of parameters and conditions. Uplift rate, rock erodibility, and the magnitude of perturbation (base-level fall or fault displacement) had the greatest influence on drainage reorganisation. The relative magnitudes of perturbation and topographic relief limited landscape susceptibility to reorganisation. Stream captures occurred more often when the channel head distance to divide was low. Stream topology set by initial conditions strongly affected capture occurrence when the imposed uplift was spatially heterogeneous. We also integrated simulations of geomorphic and biologic processes to investigate relationships among topographic relief, drainage reorganisation, and riverine species diversification in the two scenarios described above. We used a new Landlab component called SpeciesEvolver that models species at landscape scale following macroevolutionary process rules. More frequent stream capture and less frequent stream network disappearance due to divide migration increased speciation and decreased extinction, respectively, especially in the heterogeneous uplift scenario in which final species diversity was often greater than the base-level fall scenario. Under both scenarios, the landscape conditions that led to drainage reorganisation also controlled diversification. Across the model trials, the climatic or tectonic perturbation was more likely in low-relief landscapes to drive more extensive drainage reorganisation that in turn increased the diversity of riverine species lineages, especially for the species that evolved more rapidly. This model result supports recent research on natural systems that implicates drainage reorganisation as a mechanism of riverine species diversification in lowland basins. Future research applications of SpeciesEvolver software can incorporate complex climatic and tectonic forcings as they relate to macroevolution and surface processes, as well as region- and taxon-specific organisms based in rivers and those on continents at large.

Topographic controls on divide migration, stream capture, and diversification in riverine life

Drainages reorganise in landscapes under diverse conditions and process dynamics. We investigated the relative control that Earth surface process parameters have on divide migration and stream capture in scenarios of base level fall and heterogeneous uplift. A model built with the Landlab toolkit was run 51,200 times in a sensitivity analysis that used globally observed values. Large-scale drainage reorganisation occurred only in the model runs within a limited combination of parameters and conditions. Uplift rate, rock erodibility, and the magnitude of perturbation (base level fall or fault displacement) had the greatest influence on drainage reorganisation. The relative magnitudes of perturbation and topographic relief limited landscape susceptibility to reorganisation. Stream captures occurred more often when the channel head distance to divide was low. Stream topology set by initial conditions strongly affected capture occurrence when the imposed uplift was spatially heterogeneous. We also modelled riverine species lineages as they developed in response to the single topographic perturbation. We used a new Landlab component called SpeciesEvolver that models species lineages at landscape scale. Simulated species populated to the modelled landscape were tracked and evolved using macroevolutionary process rules. More frequent stream capture and less frequent stream network disappearance due to divide migration increased speciation and decreased extinction, respectively, in the heterogeneous uplift scenario where final species diversity was often greater than the base level fall scenario. Under both scenarios, the landscape conditions that led to drainage reorganisation following a single perturbation also controlled diversification, especially for the species that evolved more rapidly in some model trials. These results illustrate the utility of SpeciesEvolver to explore how life evolves alongside landscapes. Future research applications of SpeciesEvolver can incorporate more complex climatic and tectonic forcings as they relate to macroevolution and surface processes, as well as region- and taxon-specific organisms based in rivers as well as those on continents at large.