The temporal dynamics in the lake water mixture derived from river flooding, local rainfall and runoff, exchange with an adjacent lake, groundwater exchange, and evaporation was computed from detailed measurements over an annual cycle for a permanent lake on the central Amazon floodplain. River water invaded the lake at the start of rising water, but by mid-rising water, lake water steadily flowed out from the lake and into the river, while river levels continued to rise. Despite a relatively low ratio of catchment area to water surface area, the lake exported to the river three times the amount of water originally received from the river. The lake water mixture was dominated by river water early during the water year, increasing to 70% of the mixture.

When lake water began flowing out from the lake, the river water fraction was steadily diluted as runoff became dominant. By the end of the water year, runoff contributed 57% of the total water input, river inflow 21%, rainfall 11%, inflow from an adjacent lake 6%, and seepage inflow 4%. However, local runoff, river inflow, and lake water carried over between water years have the potential for considerable interannual variation. The finding that runoff from relatively small local catchments can be sufficiently large to prevent flood waters from entering the lakes during periods of rising water may critically limit our ability to characterize the long-term frequency and duration of flooding in lakes on the floodplains of large rivers in the wet tropics. ¿American Geophysical Union 1994