After dropping to a two-year low, retail cargo traffic at major U.S. ports began climbing in April, according to the National Retail Federation.

The Washington-based trade group reported an 8.9 percent jump in container traffic over March, though overall volume was 4.7 percent below the same period during last year.

Cargo traffic is expected to remain slow through the summer and early fall before it picks up in October and returns to year-over-year growth by Halloween, the report said.

"That's in line with forecasts that the economy could begin its slow recovery this fall and reflects retailers' sales expectations for the holiday season," Jonathan Gold, the group's vice president for Supply Chain and Customs Policy, said in a statement.