On the north side of the river at Guntersville Lock, a commercial problem at the turn of the 20th century was moving automobiles that arrived in Guntersville by rail car across the river to markets where they could be sold. A small lock was built in 1937 during dam construction to solve that problem. In 1943, 206 million tons of freight, much of it war materials, traveled the full length of the Tennessee River. After the lock was completed and industrialists could move loads freely down the river, loads grew larger, so TVA built a larger lock in 1965. The main lock now can lift and lower boats or barges up to 600 feet long and raise vessels up to 45 feet to handle the elevation difference between Lake Guntersville and Wheeler Lake. Guntersville Lock is maintained by TVA but operated by the U.S. Corps of Engineers, a relationship shared there and at nine other dams on the Tennessee River. TVA ensures enough water is in the river to operate the lock, while the Corps tells TVA when more or less water is needed in other waterways that use these reservoirs for storage, including the Cumberland, Ohio, and Mississippi Rivers. Nine main-river dams form a staircase of quiet, pooled water and controlled current along the 652-mile Tennessee River system. From just above Knoxville, the Tennessee River drops a total of 513 feet before emptying into the Ohio River.