Sub-aquatic sediment is covered with capping material by a capping system comprising a template barge and a spreader barge. While the spreader barge is distributing capping material, the template barge guides the spreader barge as it systematically moves over a pre-defined sediment capping region. The spreader barge comprises a spreader pool in which a broadcast spreader accurately and evenly distributes capping material within the pool, which then sinks to the sediment. The capping material is distributed with minimal disturbance to the sediment.