And throughout its length, the St. Johns occasionally widens into large lakes, including Lake George and Little Lake George, in the Georgetown and Welaka areas. The total drop of the river from its source in swamps South of Melbourne to its mouth in the Atlantic is less than 30 feet, a rate of about one inch per mile, which makes the St. Johns one of the slowest flowing rivers in the world. Because the river is so slow, it is difficult to flush pollutants. Pollution sources include discharges from wastewater treatment plants and runoff from urban and agricultural areas after it rains. Such runoff carries pesticides and other pollutants into streams that lead to the river.
Pollution is concentrated around urban areas.
Major tributes of the St. Johns are, among others, the Wekiva, Econlockhatchee and Oklawaha rivers.