An underwater trenching system is mountable on a side of a barge to be propelled by the barge along a waterway, the bed of which contains a trench with a laid pipeline. To remove the excess sediment from the trench the trenching unit delivers pressurized water and air to the trench. A sparge assembly with jet nozzles directs jets of water, breaking up the formation that has built up around the pipeline. The airlift assembly creates a turbulent flow to lift the disturbed sediment and remove it from the created trench.