Riparian forests are critical ecotones linking aquatic and terrestrial
habitats, providing important ecosystem services such as sediment
control and nutrient regulation. The function of riparian forest is
intimately linked to river hydrology and floodplain dynamics. The
Tocantins River in the eastern Amazon currently has 7 mega-dams along
its course with 2 more proposed. As these dams alter the hydrology of
the river, it is expected that the riparian vegetation will respond to
these hydrological alterations. To understand large-scale and cumulative
impacts of multiple dams on floodplains of the Tocantins, we quantify
the landscape scale changes in floodplain extent, flood timing, and
hydroperiod along an approximately 1400 km stretch of the extensively
dammed Tocantins River. Because riparian forests are also impacted by
other anthropogenic changes, namely climate change and land use change,
we developed linear models to examine the impacts of these drivers in
addition to dams. We use water level data collected by the Agencia
Nacional das Aguas from the 1930s to the present to map floodplains
throughout the Tocantins basin. Several years of floodplain data were
validated against SAR data. We then compared floodplain maps before and
after damming to determine whether significant changes occurred.
Preliminary analysis shows that after installation of the Estreito dam,
the hydroperiod downstream of the dam increased 7.5 days on average with
a standard deviation of 36.5 days. The floodplain also expanded by 177
sq m. throughout the study area between the two time periods analyzed.
Expansions of floodplain area and hydroperiod have implications for
riparian forest ecology. Riparian trees are adapted to seasonal flooding
and exhibit traits which allow them to be waterlogged for long periods
of time, while upland vegetation may lack these characteristics. As dams
change hydrology in the rivers and surrounding floodplains, less-adapted
trees may be impacted by hydrologic alterations.