Complex sentence: The previous night, organizers dumped sacks of sawdust into the River Inn as a sign to start the rebellion; floating through the town and down the Inn Valley, it alerted the rebels.
Split into simpler sentences: The previous night organizers dumped sacks of sawdust into the River Inn as a sign to start the rebellion. When the sawdust floated through the town and down the Inn Valley, it alerted the rebels.