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Exchanges of water between the Upper Floridan aquifer and the Lower Suwannee River were evaluated using historic and current hydrologic data from the Lower Suwannee River Basin and adjacent areas that contribute groundwater flow to the lowest 76 miles of the Suwannee River and the lowest 28 miles of the Santa Fe River. These and other data were also used to develop a computer model that simulated the movement of water in the aquifer and river, and surface-and groundwater exchanges between these systems over a range of hydrologic conditions and a set of hypothetical water-use scenarios. Long-term data indicate that at least 15 percent of the average annual flow in the Suwannee River near Wilcox (at river mile 36) is derived from groundwater discharge to the Lower Suwannee and Lower Santa Fe Rivers. Model simulations of groundwater flow to this reach during water years 1998 and 1999 were similar to these model-independent estimates and indicated that groundwater discharge accounted for about 12 percent of the flow in the Lower Suwannee River during this time period. The simulated average groundwater discharge to the Lower Suwannee River downstream from the mouth of the Santa Fe River was about 2,000 cubic feet per second during water years 1998 and 1999. Simulated monthly average groundwater discharge rates to this reach ranged from about 1,500 to 3,200 cubic feet per second. These temporal variations in groundwater discharge were associated with climatic phenomena, including periods of strong influence by El Niñoassociated flooding, and La Niña-associated drought. These variations showed a relatively consistent pattern in which the lowest rates of groundwater inflow occurred during periods of peak flood levels (when river levels rose faster than groundwater levels) and after periods of extended droughts (when groundwater storage was depleted). Conversely, the highest rates of groundwater inflow typically occurred during periods of receding levels that followed peak river levels.

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This report describes progress and interim results of the second year of a 4-year study. The purpose of the 4-year study is to describe aquatic habitat types in the Apalachicola River floodplain and quantify the amount of habitat inundated by the river at various stages. Final results will be used to determine possible effects of altered flows on floodplain habitats and their associated fish communities. The study is being conducted by the U.S. Geological Survey in cooperation with the Northwest Florida Water Management District as part of a comprehensive study of water needs throughout two large river basins in Florida, Georgia, and Alabama. forests for habitat, food, protective cover, spawning sites and nursery grounds. Floodplain sloughs, ponds, tributaries and backwater lakes provide a refuge for fish and macroinvertebrate assemblages during drought conditions. The relation between biological diversity and productivity and the hydrologic conditions of the system are complex, with floral and faunal distributions varying spatially, seasonally, and annually.

Base flow.-Sustained low flow of a stream. In most places, base flow is groundwater inflow to the stream channel. Continuous-record gaging station.-A site on a stream used to systematically record river stages for determining daily mean discharge. Correlation.-A process by which the degree of association between two or more variables is defined. Cubic feet per second (ft 3 /s).-A unit expressing volume per unit time. One cubic foot per second is equivalent to the discharge of a stream whose channel is one square foot in cross sectional area and whose average velocity is one foot per second. Index station.-A long-term continuous-record gaging station that is used to evaluate regional flow conditions. Low-flow characteristic.-A statistic that describes the annual minumum average discharge for a selected consecutive-day period for a given recurrence interval in years. For example, a 7-day, 10-year low-flow characteristic (Q 7,10) of 18 ft 3 /s for a site indicates that the annual minimum average discharge for 7-consecutive days is equal to or less than 18 ft 3 /s once in 10 years on average; or, that there is a 10 percent chance in any year that the minimum average flow for a 7-consecutive-day period will be equal to or less than 18 ft 3 /s.