The Rock River is a tributary of the Big Sioux River, about 144 miles long, in southwestern Minnesota and northwestern Iowa in the United States. Via the Big Sioux and Missouri rivers, it is part of the watershed of the Mississippi River. The river's name comes from a prominent rocky outcrop about 175 feet high of reddish-gray Sioux Quartzite, about 3 miles no…The Rock River is a tributary of the Big Sioux River, about 144 miles long, in southwestern Minnesota and northwestern Iowa in the United States. Via the Big Sioux and Missouri rivers, it is part of the watershed of the Mississippi River. The river's name comes from a prominent rocky outcrop about 175 feet high of reddish-gray Sioux Quartzite, about 3 miles north of Luverne. The outcrop of quartzite is contained in Minnesota's Blue Mounds State Park.