What was lost when China criminalized online rumor-mongering in September? Even a casual user of Sina Weibo, China’s leading social-media platform, and other online services would have noticed a drop-off in the volume of posts since the new regulations, and a subsequent crackdown on prominent users, were announced. But hard data was, as usual, hard to come by.

That changed last week in the aftermath of Typhoon Fitow, a massive storm that flooded -- by most accounts –- up to 70 percent of Yuyao, a prosperous city of about 850,000 located roughly 60 miles south of Shanghai. Over the last several years, natural disasters such as this one have raced to the top of Sina Weibo’s trending topics lists, as users rallied to share information about what precisely was happening in the disaster zone. In many cases, the information was both more current and more comprehensive than what China’s state-run media outlets were able to provide. (This should come as no surprise: A thousand bloggers stranded in a flood zone with smartphones can relay more information, more quickly, than any professionally trained camera crew.)

Not all of the information is accurate. In disaster zones, mistakes are made, incorrect information is sometimes relayed, and yes, rumors are spread. But still, few in China would have chosen China’s state media over Sina Weibo for coverage of April’s tragic Lushan earthquake in Sichuan, or the notorious -- and fatal -- July 2012 floods in Beijing.

By some accounts, Yuyao's flooding has been as bad, if not worse, than Beijing's last year. But coverage on social media has been nowhere near as intense. According to data collated and reported by a researcher at the Communist Party's People’s Daily Public Opinion Monitoring Center, during the most intense period of flooding:

Monitoring found that there were 170,000 posts with news of the Yuyao floods on Sina Weibo, far lower than the 4.99 million tweets following the Lushan earthquake, and the 610,000 tweets following Beijing’s July 21 rainstorms and flooding of last year. Similarly, on further examination, there was lively discussion about the Lushan earthquake among the accounts belonging to the top fifty opinion leaders, yet in regard to the Yuyao floods, 27 opinion leaders forwarded news, and only 16 people published commentary.

Some of the decline in posts to Sina Weibo might be attributed to the increasing popularity of the Weixin -- or WeChat -- messaging service, which incorporates social-media features. But even the monitoring report makes reference to the government’s ongoing “opinion management” activities. China’s online citizens, especially the most high-profile among them, have clearly begun censoring themselves for fear of arrest. They're right to be worried: On Wednesday, the Yuyao government announced -- via its official Sina Weibo account, natch -- that it had arrested two women for spreading what they characterized as flood-related rumors. (The brief history of such prosecutions has not been encouraging.)

Such arrests cost China far more than valuable information during a storm. They strike at the very development of Chinese civil society, which has in recent years provided a rare check on government ineptitude and malfeasance, particularly in the state's response to major disasters. The tough new rules don't really increase the government's control over public opinion; rather they fuel resentment at Beijing's monopoly over information. Indeed, several reports indicate that sugar-coated news stories out of the Yuyao floods have catalyzed anti-government protests in Yuyao in recent days -- exactly the kind of unrest the Weibo crackdown was meant to prevent.

(Adam Minter is the Shanghai correspondent for the World View blog and a contributor to the Ticker. Follow him on Twitter.)