The Bill Williams River is a 46.3-mile-long (74.5 km)river in western-central Arizona in Mohave County, United States. The river proper is the northern border of La Paz County, which it drains in the north, as well as areas of far western Yavapai County. It is a major drainage westwards into the Colorado River of the Lower Colorado River Valley south of Hoover Dam and Lake Mead, and the drainage basin covers portions of northwest, and west-central Arizona. The equivalent drainage system paralleling the east-west lower reaches of the Bill Williams is the Gila River, which flows east-to-west across central Arizona, joining the Colorado River in the southwest at Yuma. The Bill Williams River's confluence with the Colorado is north of Parker, and south of Lake Havasu City.