Question

How can using a dam as a source of freshwater also harm agricultural resources?

Answer 1

The dam acts as a barrier between upstream and downstream.  Dams are also built for irrigation, but it is also deliberately destroyed in favor of irrigated farming. Though it provides freshwater, and makes people be protected by droughts, but it can also damage the agricultural resources by limiting the water in some areas. 

Answer 2

Dams can collect topsoil during floods.