That's a good question, because often, using active voice makes a sentence shorter and easier to understand. In this case, I would leave it as-is. " or focussing on the OXEN: "Oxen on the opposite bank of the river pulled the rafts, which were chained together", but the focus of the sentence is the RAFTS, so they should be the subject of the sentence, and the rafts are not AGENTS, they are inanimate, so you can't use active voice.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.