The rainy season lasts from November until April in the Pantanal of southwest Brazil. This gives rise to the seasonal flooding that defines the region and is responsible for its biological bonanza. The Paraguay River channel becomes filled and begins to act like a giant dam. All of its tributaries subsequently fill followed by inundation of the region itself to form a giant, patchy lagoon consisting of myriad ponds and bays. For all of this to work there has to be a "bottom to the bowl", an aquiclude, or barrier, to prevent the water from rapidly sinking. This is provided by a layer of clay which helps retain the flood for nearly half a year. To further support this whole wetland mechanism the land slope is almost imperceptible toward the Paraguay River. Thus in March and April, when the river does begin to draw the water down, it does so in infinitesimally small increments.