Retrogression of the bed level of a river d/s of a weir is basically caused due to:

Explanation:

Retrogression:

• It is a temporary phenomenon that occurs after the construction of a weir/barrage in the river flowing through alluvial soil.
• As a result of the backwater effect and increase in the depth, the velocity of water decreases resulting in the deposition of sedimentation load.
• The water flowing through the weir/barrage has less silt, so water picks up silt from the downstream bed. This results in lowering the d/s river bed, this is known as retrogression.
• It may occur for the first few years and bed levels often recover their previous level.
• Within a few years, water flowing over the weir has a normal silt load and this cycle reverses.
• Then due to greater depth, silt is deposited and the d/s bed recovers to equilibrium.
• The retrogression value is minimum for flood discharge and maximum for low discharge.

​Hence option (3) is correct.