IOWA CITY, Iowa (AP) – The United States Postal Service says it is backing off of proposals to close mail processing plants in Cedar Rapids and Waterloo. The agency announced today (Thursday) it would close smaller plants in Carroll and Creston and move their work to Des Moines.  The postal service had been studying whether to close the plants since last fall as part of a nationwide downsizing of its distribution network.  Bruce Clark, president of the Iowa Postal Workers Union, says the closures of Carroll and Creston were probably inevitable since they did not have automated machinery. But he said the decisions to keep Cedar Rapids and Waterloo open were “fantastic news.”   In a statement, the postal service said it found “no significant opportunity to improve efficiency or service” by closing Waterloo or Cedar Rapids.