In winter they traded with Indians for furs, which are at their highest quality in the cold season, and the furs were sent downriver in the spring.
(they; traded; for furs; L:In winter) (furs; are; at their highest quality in the cold season) (the furs; were sent downriver; T:in the spring)
they furs the furs
traded are were sent downriver
for furs L:In winter
at their highest quality in the cold season
T:in the spring