BELLE VERNON, Pa.—Barges nearly two football fields long, piled high with coal, float smoothly down the lower Monongahela River here—until they reach the 75-year-old Charleroi Locks and Dam, where many get waylaid at the gate.

The lone functioning lock is set in crumbling concrete. Pieces of steel hang loose, threatening to gouge barges as they pass. Part of the river is blocked by construction. For barge operators, Charleroi is a choke point on the river.

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