Ría Lagartos is found in the northwestern corner of the peninsula of Yucatán State. The Gulf of Mexico borders the reserve to the north. The municipalities Tizimin, Río Lagartos and San Felipe border it to the south and the state of Quintana Roo makes up its eastern border. The reserve is 270 km from Mérida and 50 km from Tizimin. Ría Lagartos Biosphere Reserve spans 60,348 ha (Andrews et al. 1998). The Yucatan peninsula is characterized by its limestone platform, formed by cretaceous sedimentary rocks, which lacks surface level water flows. Instead, the water filters through the limestone forming a shallow water table and a series of caves, subterranean streams, and natural fresh-water wells (Arriaga et al. 2000).
Ría Lagartos estuary is connected to the Gulf of Mexico in three places, including the natural mouth San Felipe and two artificial canals called San Felipe and Río Lagartos. The estuary varies between 25 meters and 3.5 kilometers wide and is about 74 kilometers long. Ría Lagartos' system includes the Río Lagartos Basin, Las Coloradas Basin and El Cuyo Basin that converge at El Puente and La Angostura. Infrequent water exchanges between the estuary and the Gulf, high evaporation rates relative to the precipitation rates, and stagnant water augment the estuary's salinity over time (Chávez et al. 1988; Batllori et al. 1990). In some portions of the reserve, salt content reaches 100g/kg of water (100 parts per million).
There are two climate types in the reserve. The Río Lagartos region is arid with median annual temperatures reaching above 22°C and El Cuyo region is the driest of the hot-humid climate, with median high annual temperature of 22°C. The median temperature in the reserve's borders is 26°C (INE 1999). Total annual precipitation in Río Lagartos is 550 mm and in El Cuyo it is 696 mm. The dominant winds are northeast to southwest trade winds and they bring high humidity. The reserve is considered to be in a high-risk zone because it is found within the trajectory of hurricanes originating in the Caribbean and Atlantic. Tropical hurricanes and strong winds known as the "nortes" have seriously impacted the peninsula and on occasion have brought strong rains and rough waves that have caused floods and coastal dune erosion, thereby creating new openings to the sea (CONANP 2000, INE 1999).
Ten different vegetation types have been described within the reserve. Canopy heights range widely, from only a few centimeters to more than 12 meters. The vegetative communities in the coastal dune are made up of mostly creeping species and xerophytic plants 1 meter tall. Thick mangroves are found in high salinity areas and usually only reach 1 to 2 meters in height. The dense dry tropical forest's canopy can reach between 8 to 12 meters (CICY 1990; Rzendowski 1986). The petenes, describes as small islands of tree groupings associated with springs or natural wells, have the highest floristic composition and reach between 18 and 25 meters.
Ría Lagartos Biosphere Reserve has two ecoregions: Mexican mangroves and Yucatan dry forests. The protected area's habitat is known as a wetlands system (Sullivan y Bustamante 1999; Dinerstein et al. 1995; Contreras 1993).
The National Institute of Anthropology and History (INAH) has determined the location of the reserve's archeological sites and has classified them into four categories according to their presence, quantity, and total monument area (INE 1999). Thus far, there are plans to restore and conserve the zones within the coastal dunes where a Maya Route ("sacbé" or white trail) began.
Another characteristic of the reserve, and the entire Yucatan Peninsula, is the lack of permanent, surface-level fresh water systems. The peninsula does have subterranean water flows because precipitation filters quickly through the limestone rock (INE 1999; Chávez et al. 1988).