The St. Johns River, the longest river in Florida, has a depth that varies significantly along its 310-mile course. In its natural, un-dredged state, much of the river is surprisingly shallow, averaging only 7 to 10 feet deep. However, because it serves as a major commercial shipping artery, the lower section of the river (from Jacksonville to the Atlantic Ocean) is heavily managed by the U.S. Army Corps of Engineers. In the Jacksonville shipping channel, the river is maintained at a depth of 40 to 47 feet to accommodate massive container ships and military vessels. In contrast, the upper reaches of the river in Central Florida consist of wide, shallow lakes where the depth may only be 3 to 5 feet outside of the main channel. The river is also unique because it has an extremely low gradient, dropping less than 30 feet in elevation from its source to the mouth; this causes the water to move very slowly and allows the "tide" from the Atlantic to push saltwater up-river for dozens of miles, affecting the ecosystem and the navigable depth during high tide cycles.