They told Shanghai Daily the latest tests indicated water quality was normal and so far there is no pollution.
The city is working to ensure its water quality, including removing the pigs while they are further upstream, setting up aquatic plant barriers and increasing the frequency of quality checks, they said.
Shanghai's agriculture committee has found the pigs died of porcine circovirus, which is not transmittable to humans.
Tags on the ears of the pigs indicate they were from Jiaxing but the tags only show place of birth, China Daily reported Wednesday.
A spokesman for Jiaxing's veterinary department told a news conference there has been no abnormal animal epidemic in the city.
Jiang Ho said the city has 600 non-hazardous treatment stations for dead pigs and farmers who hand them over receive a government subsidy for each carcass.
Jiang acknowledged some farmers dispose of pig carcasses directly into the rivers because they believe "dead pigs are very unlucky."