Complex sentence: They returned in 1844 with James and Charles Rossman to build a dam across rapids in the Rubicon River and the Rossmans built a sawmill that harnassed the power of the water to make lumber for the growth of the area.
Split into simpler sentences: They returned in 1844 with James and Charles Rossman to build a dam across rapids in the Rubicon River. The Rossmans built a sawmill that harnassed the power of the water to make lumber for the growth of the area.