Logging watersheds (A) means removing the vegetative cover. This cover normally helps to slow the flow of water and to bind the soil together. By removing it, this will dramatically increase the sudden discharge of water into the river during heavy rainstorms increasing the chance of flooding downstream.
Building on floodplains (B) can increase the chance of flooding if a significant portion of wetlands are developed upon but this would be even more critical if such development occurred further upstream.
Building dams (C) will dramatically reduce the chances of flooding downstream as significant quantities of water are kept behind the dam.
Developing downstream irrigation schemes (D) should also reduce the risk of flooding as the flow of water is directed in channels or canals and more water is consumed for agricultural purposes.