Seasonal variations in water quality of an oxbow lake in response to multiple short-time pulses of flooding (Jataí Ecological Station - Mogi-Guaçú River, Luiz Antonio, SP-Brazil)

ABSTRACT

Mogi-Guaçu River is a six-order floodplain river in the upper Paraná River Basin, Southern Brazil. Its yearly discharge varies from a minimum of 100 m(3).s(-1) to a maximum of 600 m(3).s(-1). Diogo Lake is a shalow lake located at its floodplain within the Jataí Ecological Station (Luiz Antonio, São Paulo State) and is connected throughout the year to the river through a narrow and shallow channel. The main finding of this study is that the river hidrology controls the annual variations in lake hydrochemistry through a series of hydraulic effects related to oscillations in river discharge. Lake water quality is a resultant of differential contribution from local and regional watersheds. During the low water period, lake water quality is determined by inputs from Cafundó Creek, which drains the local watershed into the lake. Raising the river level during the rain season results in the damming of lake and culminates with the entrance of river waters into the plain. The geochemistry of waters in this system is determined by weathering of sandstones with basalt intrusions. Waters are acidic (river pH = 6.00 to 7.02 and stream-lake pH = 5.15 to 6.7) and dominant cations are Na+ and K+. Major anions are almost exclusively represented by bicarbonate and an unknown concentration of organic acid anions. The overall ionic load of these soft waters in the system is therefore very low.