Opponents of the dam point out that there are three different kinds of floods on the Yangtze River: floods which originate in the upper reaches, floods which originate in the lower reaches, and floods along the entire length of the river. They argue that the Three Gorges dam will actually make flooding in the upper reaches worse and have little or no impact on floods which originate in the lower reaches. Twelve hundred years of low water marks on the river were recorded in the inscriptions and the carvings of carp at Baiheliang , now submerged.
Pathogens, heavy metals and organic toxins are the main public health concerns related to duckweed farming. Pond workers (who come into direct contact with the waste materials used to fertilize the ponds), populations (particularly children) living in the vicinity and the consumers (humans and animals) of products from duckweed pond systems may be at risk. Those risks can be alleviated by adopting work routines, pond designs, such as two-pond systems where duckweeds and fish are grown separately, and food preparation guidelines that limit, if not eliminate, the transfer of pathogens from the wastewater to animals and humans. In the case of duckweeds grown on industrial wastewaters containing heavy metals and organic toxins, it is strongly advised that they are not used for feed and food production, but rather disposed of as safely as possible, for example in bottom-sealed landfills ( Iqbal, 1999 ).