Complex sentence: People have worked with logging and timber floating for more than 300 years in this valley, and numerous artifacts are left relating to this: dams, walls constructed by stones out in the rivers, "renner" to get the logs past troublesome spots.
Split into simpler sentences: People have worked with logging and timber floating for more than 300 years in this valley. Numerous artifacts are left including dams, walls constructed by stones out in the rivers, and runs to get the logs past troublesome spots.