Complex sentence: The Mohave Valley is a valley, mostly on the east shore of the south-flowing Colorado River in northwest Arizona; the valley extends into California's San Bernardino County; also, the north of the valley extends into extreme southeast Clark County, Nevada.
Split into simpler sentences: The Mohave Valley is a valley located mostly on the east shore of the south-flowing Colorado River in northwest Arizona. The valley extends into California's San Bernardino County; the northern side of the valley extends into extreme southeast Clark County, Nevada.