Retrogression of the bed level of a river downstream a weir, occurs due to

Silt carried by the river tends to deposit on the river bed.
Downstream of a weir, the water becomes relatively clear (less silt) because much of the sediment settles upstream.
This clear water has a higher sediment-carrying capacity ("hungry water"), so it starts eroding (scouring) the river bed and banks downstream.
As erosion continues, the river bed level gradually lowers, which is called retrogression (degradation) of the bed.