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The pretty excellent Atlantic Cities does the math on the impact in cities that have gotten rid of single-use retail bags. Even though the industry that makes plastic bags won’t reveal how many they manufacture each year, some estimate more than 100 million bags went unused after cities taxed or outright banned them. In D.C., after a five-cent tax went into effect in Jan. of 2010, the change in the Anacostia River was swift:

2008 study by D.C.’s Department of the Environment found that plastic bags made up 21 percent of the trash in the Anacostia River, and more than 40 percent of the trash in its tributaries. According to [D.C. Council member Tommy] Wells, the program cut bag litter in the river by 60 percent after only five months. It also raised about $2.5 million in its first year, which is being used to build sewer grates to prevent trash from entering the river. Roughly $1 million of those funds have been granted to local cleanup organizations.

“It’s probably been one of the most successful initiatives like it in the country,” Wells says.