Riparian forests and floodplains are critical ecotones that link aquatic and terrestrial habitats, providing ecosystem services including sediment control and nutrient regulation. Riparian forest function is intimately linked to riverine hydrology and can be considerably altered when flows are modified. The Tocantins River in the eastern Amazon currently has seven mega-dams (>30 MW), with two additional mega-dams proposed. To understand the cumulative impacts of multiple dams on floodplains, we quantified changes in floodplain extent, flood timing, and hydroperiod after the operation of the Serra da Mesa (1998) and Peixe Angical (2006) dams for a 500 m stretch of the Tocantins River upstream of the two dams (205 km upstream of Serra da Mesa, 28 km upstream of Peixe Angical). We used a digital elevation model and water level data collected by the Agencia Nacional das Aguas from 1986 to 2016 to create daily floodplain inundation maps, which we used to test whether there were significant changes in floodplain hydrology before and after damming. We also quantified precipitation dynamics and land cover change in the watershed during the same period to determine whether non-dam factors might have also played a role in altered floodplain hydrology.
Results/Conclusions
After installation of the Serra da Mesa dam, ~1.50 km2 (41.5%) of the pre-dam floodplain area (3.61 km2) no longer flooded and an additional ~1.30 km2 (35.9%) of the pre-dam floodplain had a shorter hydroperiod (mean±SD = -10.34±4.37 days). Vegetation in pre-dam floodplain included 46.4 % cerrado, 28.5% forest, 7.6% pasture; post damming, 18.9% of the no longer flooded forest and cerrado had been converted to pasture. The drying trend was limited to the wet season, as during the dry season, 0.2 km2 (4%) had a longer hydroperiod (mean±SD = 39.34±13.66 days) after the installation of the Serra da Mesa dam. This is an effect of higher low flows in the river after damming. There were no significant trends in annual precipitation, however the proportion of rainfall converted to discharge downstream of Serra da Mesa decreased significantly in the post-dam period, strongly suggesting that observed changes in floodplain hydrology were driven by the dam rather than climate dynamics. Future directions include exploring these trends further upstream where more dams have been installed and investigating how hydrologic alteration impacts riparian forests.