The Apalachicola River in northwest Florida is part of a three-state drainage basin encompassing 50,800 square kilometers in Alabama, Georgia, and Florida. The river is formed by the confluence of the Chattahoochee and Flint Rivers at Jim Woodruff Dam from which it flows 171 kilometers to Apalachicola Bay in the Gulf of Mexico. Its average annual discharge at Chattahoochee, Florida, is 690 cubic meters per second (1958-80) with annual high flows averaging nearly 3,000 cubic meters per second. Its flood plain supports 450 square kilometers of bottom-land hardwood and tupelo-cypress forests.

The Apalachicola River Quality Assessment focuses on the hydrology and productivity of the flood-plain forest. The purpose of this part of the assessment is to address river and flood-plain hydrology, floodplain tree species and forest types, and water and tree relations. Seasonal stage fluctuations in the upper river are three times greater than in the lower river. Analysis of long-term streamflow record revealed that 1958-79 average annual and monthly flows and flow durations were significantly greater than those of 1929-57, probably because of climatic changes. However, stage durations for the later period were equal to or less than those of the earlier period. Height of natural riverbank levees and the size and distribution of breaks in the levees have a major controlling effect on flood-plain hydrology. Thirty-two kilometers upstream of the bay, a flood-plain stream called the Brothers River was commonly under tidal influence during times of low flow in the 1980 water year. At the same distance upstream of the bay, the Apalachicola River was not under tidal influence during the 1980 water year.

Of the 47 species of trees sampled, the five most common were wet-site species constituting 62 percent of the total basal area. In order of abundance, they were water tupelo, Ogeechee tupelo, baldcypress, Carolina ash, and swamp tupelo. Other very common species were sweet-gum, overcup oak, planertree, green ash, water hickory, sugarberry, and diamond-leaf oak. Five forest types were defined based on species predominance by basal area. Biomass increased downstream and was greatest in forests growing on permanently saturated soils.

Depth of water, duration of inundation and saturation, and water-level fluctuation, but not water velocity, were very highly correlated with forest types. Most forest types dominated by tupelo and bald-cypress grew on permanently saturated soils with inundation by flood waters 50 to 90 per cent of the time, or an average of 75 to 225 consecutive days during the growing season from 1958 to 1980. Most forest types dominated by other species grew in areas that were saturated or inundated 5 to 25 percent of the time, or an average of 5 to 40 consecutive days during the growing season from 1958 to 1980. Water and tree relations varied with river location because range in water-level fluctuation and topographic relief in the flood plain diminished downstream.