Read the passage. from Geography of the Blackstone Valley 1 The average dam in the Blackstone Valley was only about nine feet tall. Even small dams were important for mill owners. Bullding a dam causes the water in the river to rise until it flows over the dam as a waterfall. The rising water created reservoirs or mill ponds that allowed mills to operate their water wheels almost all year long. They provided a steady source of water even in times of drought. 2 The Blackstone River drops about 440 feet over its 46-mile length. That's an average drop of about 91/2 feet per mile. This drop is important. Water weighs 8.34 pounds per gallon It is the weight. not the speed of the water that pushed the water wheels that powered the mills. The series of drops along the Blackstone over multiple rapids and waterfalls made it perfect for the construction of water-powered mills. 3 The geography and geology of the Valley made this relatively small and seemingly ordinary New England River the birthplace of the American Industrial Revolution Eventually it won the nickname of "America's Hardest Working River." (from "Geography of the Blackstone Valley" by National Park Service) What was the relationship between the Blackstone River's drop of 440-feet over its 46-mile length and the river's usefulness for industry? 1. The drop created reservoirs of water that helped power the mills all year long. 2. The drop increased the speed of the water that gave the mills power to operate. 3. The drop balanced the weight of the water with the speed of the water to power the mills. 4. The drop produced the water weight nee o turn the wheels that powered the mills.