Answer:
The negative effects of blocking the flow of river water are as follows-
(1) Due to the blockage of the river water flow, it leads to an increase in the amount of water over a particular area, as a result of which flood occurs. This flood causes various damages including the household properties and the soil loses its fertility.
(2) Blocking or river water flow also causes destruction to the freshwater habitat. The rate of evaporation also increases when river water is blocked, due to which the algae in the water increases and oxygen level decreases, affecting the marine plants and animals of freshwater bodies.
(3) The water quality degrades due to the blocking of river water, as various types of minerals including beneficial and harmful minerals are accumulated in the blocked areas.