Hydrology, vegetation, and soils of riverine and tidal floodplain forests of the lower Suwannee River, Florida, and potential impacts of flow reductions

Abstract

A study relating hydrologic conditions, soils, and vegetation of floodplain forests to river flow was conducted in the lower Suwannee River, Florida, from 1996 to 2000. The study was done by the U.S. Geological Survey in cooperation with the Suwannee River Water Management District to help determine the minimum flows and levels required for wetlands protection. The study area included forests within the 10-year floodplain of the Suwannee River from its confluence with the Santa Fe River to the tree line (lower limit of forests) near the Gulf of Mexico, and covered 18,600 hectares (ha) of forests, 75 percent of which were wetlands and 25 percent uplands. The floodplain was divided into three reaches, riverine, upper tidal, and lower tidal, based on changes in hydrology, vegetation, and soils with proximity to the coast. The Suwannee River is the second largest river in Florida in terms of average discharge. Median flow at the confluence of the Suwannee and Santa Fe Rivers is approximately 181 cubic meters per second (m3/s) or 6,480 cubic feet per second (ft3/s) (1933-99). At the upper end of the riverine reach, river stages are unaffected by tides and have a typical annual range of 4.1 meters (m). Tides affect river stages at low and medium flows in the upper tidal reach, and at all flows in the lower tidal reach. Median tidal range at the mouth of the Suwannee River is about 1 m. Salinity of river water in the lower tidal reach increases with decreasing flow and proximity to the Gulf of Mexico. Vertically averaged salinity in the river near the tree line is typically about 5 parts per thousand at medium flow. Land-surface elevation and topographic relief in the floodplain decrease with proximity to the coast. Elevations range from 4.1 to 7.3 m above sea level at the most upstream riverine transect and from 0.3 to 1.3 m above sea level on lower tidal transects. Surface soils in the riverine reach are predominantly mineral and dry soon after floods recede except in swamps. Surface soils in upper and lower tidal reaches are predominantly organic, saturated mucks. In the downstream part of the lower tidal reach, conductivities of surface soils are high enough (greater than 4 milli-mhos per centimeter) to exclude many tree species that are intolerant of salinity. Species richness of canopy and subcanopy plants in wetland forests in the lower Suwannee River is high compared to other river floodplains in North America. A total of 77 tree, shrub, and woody vine species were identified in the canopy and subcanopy of floodplain wetland forests (n = 8,376). Fourteen specific forest types were mapped using digitized aerial photographs, defined from vegetative sampling, and described in terms of plant species composition. For discussion purposes, some specific wetland types were combined, resulting in three general wetland forest types for each reach. Riverine high bottomland hardwoods have higher canopy species richness than all other forest types (40-42 species), with Quercus virginiana the most important canopy tree by basal area. The canopy composition of riverine low bottomland hardwoods is dominated by five species with Quercus laurifolia the most important by basal area. Riverine swamps occur in the lowest and wettest areas with Taxodium distichum the most important canopy species by basal area. Upper tidal bottomland hardwoods are differentiated from riverine forests by the presence of Sabal palmetto in the canopy. Upper tidal mixed forests and swamps are differentiated from riverine forests, in part, by the presence of Fraxinus profunda in the canopy. Nyssa aquatica, the most important canopy species by basal area in upper tidal swamps, is absent from most forests in the lower tidal reach where its distribution is probably restricted by salinity. Hydric hammocks, a wetland type that is rare outside of Florida, are found in the lower tidal reach and are flooded every 1-2 years by either storm surge or river floods. Lowe