The Fox River flows from north to south in its first section. The Upper Fox River begins as a small stream northeast of Pardeeville. It flows southwest towards Portage and comes within 2 miles  of the Wisconsin River before turning north. After flowing past Montello, the river goes northeast until reaching Lake Butte des Morts. Here it is joined by the tributary Wolf River before entering the west side of Lake Winnebago at Oshkosh. The Upper Fox flows for a total of 142 miles . The Lower Fox begins at the north end of Lake Winnebago, where it flows north past Neenah, Menasha, and Appleton as it begins its 40-mile  course northeast towards Lake Michigan. The river drops around 164 feet  over this short stretch. Prior to the construction of European-style dams after 1850, the river had many sizable rapids. The Lower Fox ends after flowing through the city of Green Bay and into Lake Michigan through Green Bay. Altogether, the Fox-Wolf watershed drains an area of about 6,429 square miles , giving the Fox an average discharge rate of 4132 ft3/s  into the bay.
Question: The river drops 164 feet over how many miles?
Answer: 40