late 15c., "floating platform of timber lashed or fastened together," from earlier meaning "rafter, beam" (c. 1300), from a Scandinavian source such as Old Norse raptr "log" (Old Norse -pt- pronounced as -ft-), Old Danish raft, related to Middle Low German rafter, rachter "rafter" (see rafter (n.1)).
In North America, rafts are constructed of immense size, and comprise timber, boards, staves, etc. They are floated down from the interior to the tide-waters, being propelled by the force of the current, assisted by large oars and sails, to their place of destination. The men employed on these rafts construct rude huts upon them, in which they often dwell for several weeks before arriving at the places where they are taken to pieces for shipping to foreign parts. [Bartlett, "Dictionary of Americanisms," 2nd ed., 1859]
"large miscellaneous collection," by 1830, said to be a variant of raff "heap, large amount," a dialectal survival from Middle English raf (compare raffish, riffraff), with form and sense associated with raft (n.1). But this use of the word emerged early in U.S., where raft (n.1) had meant "large floating mass or accumulation of fallen trees, logs, etc." by 1718.