Geographical features:
All lagoons are shallow coastal bodies of water separated from the ocean by a series of barrier islands.   Inlets, either natural or man-made, cut through barrier islands and permit tidal currents to transport water into and out of lagoons. 

The Indian River Lagoon (IRL) is part of the  longest barrier island complex in the United States, spanning 156 miles of Florida's east coast, from Ponce de Leon Inlet in the Mosquito Lagoon to Jupiter Inlet near West Palm Beach.  The IRL system actually consists of 3 lagoons:  the Mosquito Lagoon which originates in Volusia County, the Banana River in Brevard County, and the Indian River Lagoon which spans nearly the entire coastal extent of Brevard, Indian River, St. Lucie and Martin Counties.

Physical Features:
The IRL is characteristically shallow, and as such, is strongly influenced by precipitation and evaporation.  Heavy rains, stormwater runoff, freshwater releases, and periods of drought all contribute to fluctuations in water temperature and salinity.  In terms of salinity, the IRL is a well mixed estuary because it is also heavily influenced by wind patterns.  Wind not only has the effect of  vertically mixing the water column, it also influences surface currents that increase mixing.  This results in a vertical profile of the water column where virtually no change in salinity is observed from the surface to the bottom.  Additionally, because tidal effects decrease as distance from an inlet increases, wind are also the primary mechanism of volume transport in the interior of the IRL. 

Different regions of the IRL experience differences in tidal amplitude, current speed, and tidal excursion (the horizontal transport distance associated with either ebb or flood tide).  Each of these factors is greatest in the southern portion of the IRL and decreases to the north.  One important exception to this pattern occurs around inlets where current speed during maximum ebb or flood tide can exceed 1 m/sec^-1 (3.3 feet per second) due to the constricting effects of narrow inlet channels.

Ecological features:
The IRL is a complex mosaic of a variety of ecosystems ranging from terrestrial to freshwater, brackish and marine.  These systems interact with one another to create an environmental complex having incredible habitat diversity.  Because the IRL straddles the transition zone between colder temperate and warmer sub-tropical biological provinces, tropical and temperate species coexist and thrive. This unique feature of the IRL accounts for much of its high biological diversity and helps distinguish the IRL from other estuarine systems.