Delta is a geographical term that describes the formation of wetlands. It is formed from the slow movement of water and deposit from a river into a larger water body. For example lake, sea and ocean. A vivid example is a place where the River Nile moves at a slower pace into the Mediterranean Sea.
Hence, in this case, as a river reaches its mouth it is often affected by incoming tides resulting in a formation called a DELTA.
I'm assuming you meant subsistence farming? If so then yes, it would be a large problem because they would then not have enough food to feed themselves.