The floating traffic jams off ports. The multiplying costs of moving freight. The resulting shortages of goods. All of this had seemed like an unpleasant memory confined to the Covid-19 pandemic.

No such luck.

As head of ocean freight for the Americas at Rhenus Logistics, a company based in Germany, Ms. Loomis spends her days negotiating with international shipping carriers on behalf of clients moving products and parts around the globe. Over the last few months, she has watched cargo prices soar as a series of disturbances have roiled the seas.

Late last year, Houthi rebels in Yemen began firing on ships entering the Red Sea en route to the Suez Canal, a vital artery for vessels moving between Asia, Europe and the East Coast of the United States. That prompted ships to avoid the waterway, instead moving the long way around Africa, lengthening their journeys by as much as two weeks.

Then, a severe drought in Central America dropped water levels in the Panama Canal, forcing authorities to limit the number of ships passing through that crucial conduit for international trade.

In recent weeks, dockworkers have threatened to strike on the East and Gulf coasts of the United States, while longshore workers at German ports have halted shifts in pursuit of better pay. Rail workers in Canada are poised to walk off the job, imperiling cargo moving across North America, and threatening backups at major ports like Vancouver.

The intensifying upheaval in shipping is prompting carriers to lift rates while raising the specter of waterborne gridlock that could again threaten retailers with product shortages during the make-or-break holiday shopping season. The disruption could also exacerbate inflation, a source of economic anxiety animating the American presidential election.