Where do all the fish come from?

In Cambodia there is a big lake called Tonle Sap Lake. It flows
into the Mekong River. During the dry season the lake could get
very low. And those who live on the lake by means of house
boats would literally get stuck in the mud.

A natural phenomenon happens every year during spring
time. The Mekong River has it beginning in China.
As the snow melts off the mountain tops in the colder climate
areas, it feeds into the Mekong River as it flows down south
toward the China Sea. As it reaches the Delta area in South
Vietnam, the water becomes congested (bottled up).
It starts flowing backwards up the Mekong River and into
the Tonle Sap River into the Tonle Sap Lake. The lake would
expand 6-10 times its size and flood all the low-lying areas.

This is what brought the freshwater fish up the river into the
Tonle Sap Lake, making it the only lake in the
world teaming with freshwater fish from South China Sea.

Since I was not too far from the lake, we would have been
part of the low lying area covered by water during this
natural occurance. All the rivers that empty into the lake
during this time will receive her blessing of water and fish
as it makes its course up the river. Most rivers will be dry until
this happens. I assume this; I could be wrong. I lived near a river
both at my father's place and my aunt's place.
I know they go dry, except for ponds in the middle of the river.
And it is no fun to ford a raging river.