Explanation:

Excessive in-stream sand-and-gravel mining causes the degradation of rivers. In-stream mining lowers the stream bottom, which may lead to bank erosion.  The removal of sand from the river bed increases the velocity of the flowing water, with the distorted flow-regime eventually eroding the river banks.  

It also deepens the rivers and estuaries which may lead to saline-water intrusion from the nearby sea which increases the salinity in the river. Statement 1 is incorrect.

Sand acts as an efficient filter for various pollutants and thus maintains the quality of water in rivers and other aquatic ecosystems. Statement 2 is correct.

Sand mining also affects the adjoining groundwater system. Sand aquifer helps in recharging the water table and sand mining causes sinking of water tables in the nearby areas. Statement 3 is correct.