Its because the rain stimulates life outside the water, there’s more food everywhere.
The moist soil / sand means grubs & worms come to the surface. Insects are on the move looking for food / evading rain. Birds are snapping them up and discarding bits and pooping. Rain is washing all this stuff from river banks into the water.
Et cetera.
Same thing happens on the rising tide. High tide is the climax. Then everyone takes a break on the ebbing tide.
Its because the rain stimulates life outside the water, there’s more food everywhere.
The moist soil / sand means grubs & worms come to the surface. Insects are on the move looking for food / evading rain. Birds are snapping them up and discarding bits and pooping. Rain is washing all this stuff from river banks into the water.
Et cetera.
Same thing happens on the rising tide. High tide is the climax. Then everyone takes a break on the ebbing tide.