Main Street Bridge -- Traffic count average daily: 21,400

The Main Street Bridge is a bridge crossing the St. Johns River in Jacksonville, Florida. It was the second bridge built across the river. It carries four lanes of traffic, and is signed as U.S. Route 1/US 90 (SR 5/SR 10). The lift bridge opened in July 1941 at a cost of $1.5 million. In 1957 it was named after Mayor John T. Alsop, Jr., but continues to be known, even on road signs, as the Main Street Bridge.

The St. Johns River is the longest river in the U.S. state of Florida and its most significant for commercial and recreational use. At long, it winds through or borders twelve counties, three of which are the state's largest. The drop in elevation from the headwaters to the mouth is less than ; like most Florida waterways, the St. Johns has a very slow flow rate at a third of a mile an hour (0.2 km/h), and is often described as "lazy".Whitney, p. 215. It is one of the few rivers in the United States to run north. Numerous lakes are formed by the river or flow into it, but as a river its widest point is across, spanning several miles between Palatka and Jacksonville, the latter being the largest urban area on the river. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County. In all, 3.5 million people live within the various watersheds that feed into the St. Johns River.Belleville, p. xxi.

Comprising a drainage basin of , the St. Johns is one of the major interior wetlands of Florida.The St. Johns River: Nominated as an American Heritage River, Environmental Protection Agency. Retrieved on July 17, 2009.Whitney, p. 136. It is separated into three major basins and two associated watersheds for Lake George and the Ocklawaha River, all managed by the St. Johns River Water Management District. Like all waterways in Florida, the St. Johns is fed primarily by rainfall, most of which is immediately deposited into the river and its tributaries as stormwater runoff. Much of the water flowing into the St. Johns also comes from the Floridan Aquifer, a natural underground reservoir that serves as the fresh water source for populations living north of St. Petersburg. The St. Johns itself is being considered as a potential fresh water source for 2020 as populations continue to grow, taxing the Floridan Aquifer.

A vast variety of people have lived on or near the St. Johns, including Archaic people, Timucuan, French and Spanish settlers, Seminoles, slaves and freemen, Florida crackers, land developers, tourists, and retirees. It has been the subject of William Bartram's journals, Marjorie Kinnan Rawlings' books, and Harriet Beecher Stowe's letters home. Although Florida was the location of the first permanent European colony in what would become the United States, it was the last U.S. territory on the east coast to be developed, and it remained a wilderness frontier into the 20th century. When attention was turned to the state, however, much of the land was rapidly overdeveloped in a national zeal for progress. The St. Johns, like many Florida rivers, was altered to make way for agricultural and residential centers and suffered severe pollution and human interference that has diminished the natural order of life in and around the river. It was named one of 14 American Heritage Rivers in 1998 but was included on a list of America's Ten Most Endangered Rivers in 2008.Ball, David (April 14, 2008).St. Johns River Makes ?Endangered' List, Jacksonville Financial and Daily Record. Retrieved on July 17, 2009. Restoration efforts are underway for the basins around the St. Johns as Florida continues to deal with population increases in the river's vicinity.