HEP dams in Africa can stop the migration of prawns that eat molluscs.
The molluscs are the intermediate hosts of the parasite that causes the human disease schistosomiasis (bilharzia)
The incidence of the disease has increased around many rivers where HEP schemes have been built.
Sticking with crayfish in Africa:
The Louisiana Crayfish or Redclaw Crayfish has been introduced into rivers and lakes where bilharzia is present. This can control the disease, but the crayfish can have many damaging ecological effects.
For a good case study of the harm caused by introduced species, including crayfish, look for information on Lake Naivasha, Kenya.