Why is there little plant life near a river/stream?

Most of the plant's roots get submerged under water. In this condition, they may not get oxygen for respiration; so the roots are forced to respire anaerobically. This results into accumulation of lactic acid which kills the plant.

That is why there may be little plant life near a river or stream; especially in water-logged areas.

The water may be polluted and the wetness of the dirt may not be suitible for numerous plants