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Tampa Bay Watershed Hydrology

The Tampa Bay Basinís hydrology is the outcome of the climate and physical geography. Climatic forces shape the land surface and affect the quantity of surface and ground water.

Physical Geography

The Florida peninsula is divided into the northern, central, and southern physiographic zones. The Tampa Bay Basin lies in the central zone, where the land surface is relatively fl at, and subtle changes in elevation delineate landform boundaries. Dominant landforms include the higher areas of the Brooksville Ridge, Polk Upland, and Lakeland Ridge. Lower areas include the Western Valley, which transects the uplands, and the Gulf Coastal Lowlands.

Elevations range from sea level at Hillsborough Bay to slightly greater than 240 feet above sea level in the eastern part of the Hillsborough Basin. Generally, elevations of 75 feet or more above sea level are associated with the regionís uplands and ridges.

The widely branching tributaries of the Peace, Manatee, Little Manatee, and Alafia Rivers all originate in the Polk Upland. Elevations of 175 feet or more above sea level are associated with the Brooksville Ridge, north of Zephyrhills, whose high, sandy hills have little surface drainage and provide important recharge for the Floridan aquifer.

The Western Valley separates the Brooksville Ridge from the Polk Upland. This broad, flat lowland extends from north of the Green Swamp (not shown on map) to a low ridge north of the city of Tampa and merges into the Gulf Coastal Lowlands with no significant topographic break. Elevations range from about 40 feet up to 100 feet above sea level (Campbell, 1989). Important surface water features include the western extent of the Green Swamp and parts of the Hillsborough and Withlacoochee Rivers.

The Gulf Coastal Lowlands, the dominant landform in the western area of the basin, adjoin Tampa Bay. These relict marine terraces (ancient shorelines) have low relief over broad plains bordered by slopes. Major municipalities such as the cities of Tampa and St. Petersburg are located in the Lowlands.

Karst features exist throughout the Tampa Bay area, particularly along the northern limit of the Polk Upland, within the low ridge immediately north of the city of Tampa (within the Gulf Coastal Lowlands), and within the Brooksville Ridge. The sinkholes that develop in this porous limestone terrain typically result in shallow, bowl-shaped depressions and a generally rolling topography (Sinclair and Stewart, 1985).

The physiography of the flat Gulf Coastal Lowlands region strongly influences the relationship between ground water and surface water. It also creates numerous wetlands, an important habitat in the Tampa Bay Basin. Ground water moves upward into low-lying areas on the land surface. Surface water and surface runoff also drain to these low areas, but ground water pressure often prevents the surface water from draining underground.

Surface Water

The basin contains numerous surface waterbodies, including lakes, springs, rivers, and Tampa Bay itself. At least three second-magnitude springs (with discharges of 6.46 to 64.6 million gallons per day) are found in the basin: Crystal and Sulphur Springs on the Hillsborough River and Lithia Springs on the Alafi a River. Lower magnitude springs are also present. The rivers and streams that receive these spring discharges are usually not darkened by a high tannic content from organic matter. Other streams in the area that receive ground water from seepage usually have clear water as well.