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

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

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.