El Niño La Niña Events, Precipitation, Flood-Drought Events, and Their Environmental Impacts in the Suwannee River Watershed, Florida

90 ENVIRONMENTAL GEOSCIENCES El Niño La Niña Events, Precipitation, Flood-Drought Events, and Their Environmental Impacts in the Suwannee River Watershed, Florida HONGSHENG CAO Department of Geology, Florida State University, Tallahassee, FL 32306 ABSTRACT The Suwannee River watershed is extremely vulnerable to pollution because of its particular hydrological characteristics. The water interaction between surface water and groundwater exists to balance the river and the Upper Floridan aquifer via springs. El Niño and La Niña events are responsible for the heavy precipitation in the watershed. Some El Niño events resulted in severe flood events, whereas some La Niña events resulted in drought events in the past several decades. Those extreme flood-drought events caused serious damages and losses of property and life in the watershed and constituted threats on the hydrological environments. In El Niño years, groundwater can be endangered by the polluted flood water runoff, which drains development areas and phosphate deposit mines into sinkholes and springs. In La Niña years, the quality of river waters will be influenced by the groundwater with high nitrate concentrations. Coastal saltwater intrusion and insufficient nutrients near shore are possibly related to La Niña events. However, La Niña and drought events can help clean up the river itself. River sediments are hardened and compacted by sunlight and exposure to the atmosphere when the river drains dry. Pollutants are trapped in the river bottom or released to the atmosphere and excess nutrients are oxidized by the air. Key Words: drought, El Niño, flood, La Niña, precipitation, Suwannee River. INTRODUCTION Although El Niño/La Niña events warm/cool the waters off the coasts of Peru and Ecuador and in the equatorial central Pacific Ocean slightly and produce a warm or cold ocean current that flows southward along the coast of northern Peru in El Niño/La Niña years, their effects are not limited to Peru locally or South America regionally. The global changes in precipitation, streamflow patterns, and the related flood-drought events associated with El 2000, AAPG/DEG, 1075-9565/00/$15.00/0 Environmental Geosciences, Volume 7, Number 2, 2000 90 98 Niño events have been documented. Quinn et al. (1987) discussed the El Niño occurrences and the relative strengths of the El Niño events over the past four and a half centuries. Annual natural discharge of the River Murray Darling River system of Southeastern Australia is often inversely related to sea surface temperature (SST) anomalies in the eastern tropical Pacific Ocean (Simpson et al., 1993). Dry conditions in Australia tend to be associated with El Niño, and below-normal rainfall and streamflow are consistently identified in the El Niño years (Chiew et al., 1998). Eltahir (1996) realized that 25% of the natural variability in the annual flow of the Nile River is associated with El Niño events. By analyzing the SST of the Pacific Ocean and the streamflow of the Nile River, he developed a flood prediction procedure to improve the prediction of Nile floods. Waylen and Caviedes (1990) investigated the properties of annual and winter precipitation totals and streamflow characteristics in the Aconcagua River basin of Chile to identify flood and drought-generating processes and their possible linkage to the El Niño Southern Oscillation phenomenon. Amarasekera et al. (1997) examined the relationship between the annual discharges of the Amazon, Congo, Paraná, and Nile rivers and the SST anomalies of the eastern and central equatorial Pacific Ocean, an index of El Niño La Niña Southern Oscillation (ENLNSO). The strongest relationship between El Niño events and extreme drought years is found in the northwest United States. A strong relationship is also noticed between dry conditions and La Niña events in the southern United States (Piechota and Dracup, 1996). Sun and Furbish (1997) showed that El Niño and La Niña events are responsible for up to 40% of annual precipitation variations and up to 30% of river discharge variations in Florida. The cold La Niña events have received less attention than the warm El Niño events because the cold La Niña phase is less distinct and causes less catastrophe than the warm El Niño phase (Dracup and Kahya, 1994). The precipitation anomalies associated with La Niña events are opposite to the precipitation anomalies during the El Niño events. A strong relationship between streamflow and the cold La

CAO: SUWANNEE RIVER WATERSHED 91 Niña events has been recognized in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest (Dracup and Kahya, 1994). They concluded that the seasonal streamflow anomaly associated with the La Niña events is the opposite of that associated with the El Niño events. No special effort has been made previously to provide information on the correlation between the flood-drought events and El Niño/La Niña events and their environmental effects on watersheds, although watersheds are known to experience frequent flood-drought with a varied duration and intensity. This article describes the precipitation and flood-drought events in the Suwannee River watershed with the El Niño/La Niña events in the past 50 years and discusses their environmental effects on the watershed. EL NIÑO AND LA NIÑA EVENTS The interaction between atmosphere and sea surface in the equatorial Pacific Ocean causes an oscillation of temperature and pressure between the eastern equatorial Pacific Ocean and the western equatorial Pacific Ocean. This phenomenon is collectively referred to as the El Niño/La Niña Southern Oscillation (ENLNSO) in this article, where El Niño/La Niña refers to the temperature oscillation in the eastern equatorial Pacific Ocean, and Southern Oscillation refers to the pressure fluctuation between the east equatorial Pacific Ocean and west equatorial Pacific Ocean. During El Niño events, unusually high atmospheric sea level pressures develop in the western equatorial Pacific Ocean regions, and unusually low sea level pressures develop in the southeastern equatorial Pacific Ocean. Weakening trade winds allow the warmer water from the western Pacific Ocean to flow toward the eastern Pacific Ocean. El Niño refers to the irregular increase in SSTs from the coasts of Peru and Ecuador to the equatorial central Pacific Ocean. It is an ENLNSO warm event or the warm phase of ENLNSO, in which warm eastern and central equatorial Pacific SST positive anomalies exist. El Niño is just a part of the result of an equatorial Pacific Ocean oriented cycle that occurs irregularly at intervals of 3 5 years. The El Niño event itself typically lasts 12 18 months. The cold La Niña events sometimes (but not always) follow El Niño events. During La Niña events, tendencies for unusually low atmospheric sea level pressures in the west Pacific Ocean and high atmospheric sea level pressures in the east Pacific Ocean are linked to periods of anomalously cold equatorial Pacific SSTs, which is referred to as La Niña. The term La Niña is used to describe those times of cold eastern and central equatorial Pacific SST negative anomalies, which sometimes are referred to as ENLNSO cold event or cold phase of ENLNSO. Not all El Niño/La Niña events have the same strength because the atmosphere cannot always react in the same way from one El Niño/La Niña event to another. In the past, there have been some strong, moderate, and weak El Niño/La Niña occurrences. A list of El Niño and La Niña years is provided by the National Centers for Environmental Prediction (Tables 1 and 2). El Niño/La Niña may be getting stronger as time goes on. The El Niño event over 1990 1995 backs up this assertation. The El Niño events have been coming more frequently in recent years (every 3 years in the 1990s) than they ever did in the past. Most of the events in the 1990s have been strong moderate compared with former occurrences. Since March of 1997, SSTs in the central and eastern equatorial Pacific have been higher than normal. The SST for September 1997 was the highest in the last 50 years. HYDROLOGICAL SETTING The Suwannee River originates in the Okefenokee Swamp of south Georgia. The swamp covers 1,761.2 km 2 (680 mi 2 ), three-fourths of which drains into the Suwannee River (Ceryak et al., 1983). The Suwannee River itself is 380 km (235.98 mi) long and ultimately discharges into the Gulf of Mexico. It TABLE 1. The season-by-season list of warm El Niño conditions in the tropical Pacific with different SST intensity: weak periods are designated as weak, moderate periods as moderate, and strong periods as strong. The unlisted years are normal years (data from database of the National Centers for Environmental Prediction/Climate Prediction Center, 1999). Year Jan Mar Apr Jun Jul Sep Oct Dec 1951 weak 1953 weak weak 1957 weak weak moderate 1958 strong moderate weak weak 1959 weak 1963 weak moderate 1965 moderate strong 1966 moderate weak weak 1968 weak 1969 moderate weak weak weak 1970 weak 1972 weak moderate strong 1973 moderate 1976 weak 1977 weak 1978 weak 1980 weak 1982 weak moderate strong 1983 strong moderate 1986 weak moderate 1987 moderate moderate strong moderate 1988 weak 1990 weak weak 1991 weak weak moderate moderate 1992 strong strong weak weak 1993 weak moderate moderate weak 1994 moderate moderate 1995 moderate 1997 moderate strong strong 1998 strong moderate

92 ENVIRONMENTAL GEOSCIENCES TABLE 2. The season-by-season list of cold La Niña conditions in the tropical Pacific with different SST intensity: weak periods are designated as weak, moderate periods as moderate, and strong periods as strong. The unlisted years are normal years (data from database of the National Centers for Environmental Prediction/Climate Prediction Center, 1999). Year Jan Mar Apr Jun Jul Sep Oct Dec 1950 moderate moderate moderate moderate 1951 moderate 1954 weak moderate 1955 moderate weak weak strong 1956 moderate moderate moderate weak 1964 weak moderate 1965 weak 1970 moderate 1971 moderate weak weak weak 1973 weak strong 1974 strong moderate weak weak 1975 weak weak moderate strong 1976 moderate 1983 weak 1984 weak weak weak 1985 weak weak 1988 weak strong 1989 strong weak 1995 weak 1996 weak drains an 28,500 km 2 (11,001 mi 2 ) watershed along with its three largest tributaries: the Alapaha, Withlacoochee, and Santa Fe rivers. Approximately 11,033.4 km 2 (4,260 mi 2 ) of the watershed are located in northwestern Florida; the remainder of the watershed drains parts of southcentral Georgia. The Suwannee River watershed is composed of the Upper Suwannee River watershed and the Lower Suwannee River watershed (Crane, 1986). The Upper Suwannee River watershed is above the gage station at Ellaville, Florida. The Lower Suwannee River begins at the confluence of the Withlacoochee River where the Suwannee River resumes its southerly course. The study area generally encompasses the Lower Suwannee River and a small portion of the Upper Suwannee River (Figure 1). To the north of Ellaville-White Springs, the Upper Floridan aquifer in the Upper Suwannee River watershed is overlain by as much as 91.5 m (300 ft) of sandy clays, clayey sands, sandstone, and limestone. Flow in this part of the river is essentially dependent on surface runoff from tributaries and seepage from the surficial aquifer. To the south of Ellaville-White Springs, the carbonate rocks of the Upper Floridan aquifer are exposed and are incised by the Suwannee River. The surface waters have a close relationship with the groundwater in the watershed. The absence of any significant overburden over the carbonate rocks of the Upper Floridan aquifer allows rainfall to infiltrate into the aquifer easily. During low-flow conditions, groundwater discharge FIGURE 1: Location of the Suwannee River watershed, Florida. from the Upper Floridan aquifer supplements surface runoff through springs and seeps in the river corridor in the reach downstream from White Springs. Therefore, the Suwannee River s discharge is greatly increased by the substantial groundwater contributions of the numerous springs along its banks and from aquifer baseflow. The Upper Floridan aquifer, the primary drinking water resource, is poorly confined and vulnerable to contamination from the surface in much of the study area. Groundwater/ surface water interaction is an important factor affecting water quality in the Suwannee River watershed. This area contains numerous springs and other groundwater inputs that supply base flow to the Suwannee River during low flow. The quality of spring water also reflects the drinking water quality in the Upper Floridan aquifer in areas where it is most vulnerable to contamination from land uses. PRECIPITATION The climate of the Suwannee River watershed is humid subtropical. The Suwannee River watershed experiences

CAO: SUWANNEE RIVER WATERSHED 93 long, warm, humid summers and mild winters with an annual mean temperature of 19 C (66.2 F). During the summer months, the mean maximum temperature is 33 C (91.4 F) and the mean minimum temperature is 22 C (71.6 F). During the winter months, the mean maximum temperature ranges from 19 C (66.2 F) in the Upper Suwannee River watershed to 21 C (69.8 F) in the Lower Suwannee River watershed. The mean minimum temperature ranges from 5 C (41 F) in the Upper Suwannee River watershed to 9 C (48.2 F) in the Lower Suwannee River watershed. In winter, low temperatures of 5 to 7 C (23 19.4 F) associated with Canadian air masses may occur. However, cold spells are generally of short duration. Normally, rainfall is greatest from June through September and least from November through January. The Suwannee River watershed has an annual mean rainfall of 132.1 cm with 50% of this amount falling during the summer months (Raulston et al., 1998). Precipitation patterns within the Suwannee River watershed are associated with the position of the transition zone between the continental weather pattern of the southeastern United States and the tropical weather pattern of the Caribbean Sea. Summer rainfall is associated with localized thunderstorm activity. Rainfall during spring and summer, although unevenly distributed, is normally sufficient for plant growth. In winter, fronts bring sweeping bands of rain and cooler temperatures. Frontal rains are usually more evenly distributed areally and are of longer duration than is summer rainfall. Because evaporation and plant transpiration are significantly lower during the winter, these frontal rains and summer rainfall are important for recharging groundwater. Upper Floridan monitor wells show large annual increases in water levels at the end of the summer rain season. The mean seasonal rainfall for spring, summer, fall, and winter is 25.4 cm (10 in.), 50.8 cm (20 in.), 30.48 cm (12 in.), and 22.86 cm (9 in.), respectively. Approximately half of the average annual rainfall falls from June through September. A shorter rainy season occurs from late February to late April. Most summer rain comes from short duration afternoon or early evening local showers and thunderstorms. These rainstorms occasionally produce 5.08 7.62 cm (2 3 in.) of rain in 1 2 hr. The winter and early spring rains are generally associated with largescale weather frontal developments and are occasionally of long duration, from 12 36 hr. Although rainfall averages 132.08 cm (52 in.)/yr in the watershed, wide variations occur between locations and from year to year (Figures 2 and 3). FLOODS AND DROUGHT EVENTS The most severe floods in the watershed are associated with storms, which produce widespread distribution of rainfall for several days. Although flooding can occur in all seasons, maximum annual surface water stages occur most frequently from February through April as a result of a series of frontal-type rainfall events over the watershed. The area is also subject to summer and fall tropical storms and occasional hurricanes, which may occur from June through November. They are the major causes of widespread, excessive rainfall and associated flooding. Generally, rainfall is distributed equally throughout a watershed, causing increases in flooding downstream. Below White Springs, the average annual flow increases from 52.612 m 3 / sec (1879 ft 3 /sec) at White Springs to 184.24 m 3 /sec (6580 ft 3 /sec) at Ellaville to 195.832 m 3 /sec (6994 ft 3 /sec) at Bran- FIGURE 2: Variation of the annual precipitation from 1940 1995 and the annual mean precipitation at Madison, Florida (modified from Raulston et al., 1998).

94 ENVIRONMENTAL GEOSCIENCES FIGURE 3: Variation of the annual precipitation from 1940 1995 and the annual mean precipitation at Lake City, Florida (modified from Raulston et al., 1998). ford and to 297.472 m 3 /sec (10,624 ft 3 /sec) at Wilcox. From White Springs to Branford, the Suwannee River flow increases primarily due to inflow from the Withlacoochee River and groundwater discharge from the Upper Floridan aquifer. Since 1948, three major floods and several minor floods have occurred in the watershed. Three extreme floods have occurred in 1948, 1959, and 1973 in the last 55 years. At Ellaville, the 16.5-m-above mean sea level (MSL) flood stage has been exceeded several times since 1948. The 68.1 ft above MSL ( 95,300 cubic feet per second [cfs]) of 1948 is the highest peak for the period of record. The 1973 flood peak of 27 m (88.56 ft) above MSL ( 38,100 cfs) is the highest peak for the period of record (Ceryak et al., 1983). During the flood of March April, 1948, 500 square miles were inundated along the Suwannee River and its principal tributaries. The Suwannee River was out of its banks from north of the Florida Georgia state line to the Gulf of Mexico (Raulston et al., 1998). Maximum discharges in most of the watershed occurred in the 1948 flood event. The flood of 1948 resulted from a sequence of storms during March and April, which produced abnormally heavy and prolonged rainfall over most of the watershed. The most intense storm occurred in the 3-day period from 31 March through 2 April, 1948. During peak stages, the Suwannee River was out of its banks from the Gulf to north of the Georgia Florida state line and its width varied from 0.5 6 miles. Floodwaters remained for 30 days over the lowlands and for longer periods in depressions that drain by percolation and seepage. Water was 2.44 m deep in parts of Ellaville and 0.61 1.23 m deep in Branford and Suwannee Springs. In March 1959, flood waters covered an estimated 350 square miles along the Suwannee River and its tributaries with some ponding up to 5 miles wide. Compared with the flood of 1948, peak stages in 1959 were 0.61 1.23 m lower on the major tributaries and the Upper Middle Suwannee River. Peak stages in the Lower Suwannee River were 1.83 2.74 m below those of the 1948 flood. During the first and third weeks of March, intense frontal-type storms produced 15.2 20.3 cm of rainfall over most of the watershed during the 1948 flood. The flood of April 1973 resulted from abnormally heavy and prolonged rainfall during March and April over most of the watershed. The April 1973 flood was the maximum flood of record and exceeded the 1948 flood by 0.91 m at White Springs. Flood stage at White Springs is 24 m above MSL. The 1973 flood was 0.91 m lower than the 1948 flood at Ellaville. Floodwaters remained for 30 days over the lowlands and for longer periods in depressions that drain by percolation and seepage. Two recent minor floods characterize the variability of stream flow in the watershed. Following 3 years of near-record low surface water and groundwater levels (1988 1990) in the Suwannee River watershed, a prolonged winter frontal system in early 1991 caused a 10-year flood event in the Upper Middle Suwannee River watershed. During 1990, river levels at Ellaville (at the confluence with the Withlacoochee River) were very low, including the lowest discharge, 835 cfs, recorded at the station on November 8, 1990. By March 12, 1991, the river was discharging 53,100 cfs and the river had risen over 7.6 m. During the normally low rainfall month of October, in 1992, 27.9 33 cm of rain fell in one weekend on the Upper Santa Fe River watershed, causing the river to approach flood stage. The river actually flowed over the land bridge at O Leno State Park for the first time in recent history (Raulston et al., 1998). Shortly afterwards, the river returned to normal stage. Drought conditions, as ordinarily defined in humid areas, exist when there is insufficient moisture in the soil to maintain plant life or when precipitation is insufficient to meet

CAO: SUWANNEE RIVER WATERSHED 95 the needs of established human activities (Pride and Crooks, 1962). Spring and summer droughts of varying severity occur but not in any predictable patterns. Severe droughts usually occur during the fall and late spring. November is normally the driest month of the year. The duration for the period of rainfall deficiency is one of the more important factors that influence the severity of a drought and is probably the most significant cause of extreme drought in the watershed. Variation in rainfall has an important influence on droughts. Rainfall is not uniformly distributed areally nor is it properly timed for optimum utilization. Drought could occur even though the rainfall for a given period was higher than the average when the distribution is such that most of the rain falls during a short period. Records of rainfall and streamflow provide data for studying droughts by defining the area covered and the severity, the frequency, and the duration of drought. The most severe drought event recorded occurred in the watershed during 1954 1956. The drought resulted from rainfall deficiencies in amounts ranging from 7 11 in. during each of the 3 years. Annual rainfall totals in the watershed for 1954, 1955, and 1956 were 45.89, 42.33, and 42.46 in., respectively. The statewide runoff during 1955 was estimated to be 6 in., compared with 14 in. for an average year. The 1954 1956 drought caused critical shortages of surface water in the watershed, because its 3-year duration was an event of unusual occurrence. Zero streamflow conditions were recorded at gaging stations in the Upper Suwannee River watershed in 1954 and 1955 (Ceryak et al., 1983). The rainfall pattern shows that the 1954 1956 drought in the watershed ranks as the most severe in the past 55 years. The drought of 1973 1977 also was widespread; the recurrence interval for that drought ranged from 15 35 years, depending on location. Dry conditions occurred in 1938 1943 (except 1941), 1961 1963, 1967 1968, 1977 1978, and 1989 1990, although the watershed did not experience drought in some of these periods. A period of record-low groundwater levels in 1989 1990 throughout most of the watershed was observed (Raulston et al., 1998). ENVIRONMENTAL IMPACTS El Niño and La Niña events cause billions of dollars in damage around the world in floods and droughts. The losses were caused by droughts and fires in Australia, Southern Africa, Central America, Indonesia, the Philippines, South America, and India during those events. There were floods in the United States, Gulf of Mexico, Peru, Ecuador, Bolivia, and Cuba, and more hurricanes than usual affected Hawaii and Tahiti in El Niño years. The consequences of El Niño La Niña events in the Suwannee River watershed included increased rainfall, destructive flooding and drought, devastating forest fires, and other related negative impacts. The Upper Floridan aquifer of the watershed can be endangered by polluted flood waters or by mine (phosphate deposit) runoff into sinkholes and springs. As mining, urbanization, and other development takes place, which clears or drains land or covers it in nonpermeable surfaces, flooding is likely to increase in both frequency and volume. Any mining operation in the floodplain can endanger water resources. Such damage will significantly reduce the natural value of the river to the state and its citizens and decrease the potential for recreation and tourism in the surrounding areas of the Suwannee River watershed. Large areas of cleared land, overburden piles, and exposed subsurface soils would allow thousands of tons of soils, clays, dissolved metals, phosphate, and other pollutants to be carried downstream when they are inundated. The impact on the Suwannee River watershed would be severe. Therefore, land clearing and dragline operations are a major threat to water quality during periods of peak flows and floods. In La Niña years, rare rainfall, hot days, drought, and forest fires are expected. During drought years, wildland fire is a serious and growing hazard over much of the watershed, posing a great threat to life and property, particularly when fire moves from forest or range land into developed areas. Nitrate concentrations (as nitrogen) in the Suwannee River watershed are affected by the water interaction between the river and the adjoining aquifer. Groundwater and spring samples have a higher nitrate concentration than do stream samples (Pittman et al., 1995). Possible sources of nitrate in the groundwater in the Suwannee River watershed include fertilizer, animal wastes from dairy and poultry operations, and septic tank effluent. During low flow in the river or drought, groundwater discharge increases nitrate concentrations and loads in the Suwannee River. During high flow or flood, groundwater recharge occurs from rainfall and river water enters the aquifer, thus decreasing nitrate concentrations in groundwater. The dissolved organic carbon concentration of groundwater increased from 1.0 mg/l during low flow or drought to 28 mg/l during high flow or flood because stream water recharges groundwater during high flow or flood. The dissolved solids concentrations in most streams increased as the flows declined during drought events (Berndt et al., 1998). Saltwater intrusion will occur along the coastal area if the drought events occurred during La Niña years. In coastal localities, drawdown of the water table or piezometric surface often allows seawater to enter wells (Dunne and Leopold, 1978). The process is called saltwater intrusion. Owners of summer homes on the coast will experience increasing degradation of the quality of water in their wells during the drought events because wells along the coast will suffer contamination by saltwater. As the second largest river in the State of Florida with the second largest draining area, the Suwannee River must sup-

96 ENVIRONMENTAL GEOSCIENCES ply a substantial portion of the nutrients responsible for the Florida Big Bend s near-shore productivity in the Gulf of Mexico. Therefore, during drought events, the near-shore productivity decreases dramatically due to insufficient nutrients, thus impacting fishing. The drought events can be healthy for the river itself. Over time, pollutants and excess nutrients build up in the water and in the sediment in the bottom of the river. When the river drains almost dry, the sediment is hardened and compacted by sunlight and air. Pollutants are trapped in the river bottom. Excess nutrients are oxidized by the air. The river refills when normal rainfall occurs and thus its water quality is improved. DISCUSSION Since 1970, El Niños have been occurring every 2.2 years, up from every 3.4 years around 1870, every 4.5 years around 1750, and every 6 years in the late 1600s (Dunbar et al., 1994). That means flooding frequency has the potential to increase in years to come. Typically, El Niño occurs more frequently than does La Niña. Although La Niña events occur after some (but not all) El Niño events, the probability of drought also will be increased. On the basis of the SST anomaly and the June November Southern Oscillation Index (SOI) selection criteria (SOI 0.50) in the eastern Pacific Ocean, the El Niño composite years should include 1957 1959, 1965 1966, 1972 1973, 1982 1983, 1986 1988, 1990 1993, 1994 1995, and 1997 1998 (Table 1). Water warmer than usual in the eastern Pacific during 1986 1988, 1991 1992, 1993, 1994, and 1997 1998 was well documented. There were floods in the United States, Gulf of Mexico, Peru, Ecuador, Bolivia, and Cuba during 1982 1983. More hurricanes than usual affected Hawaii and Tahiti during 1982 1983. During the 1986 1988 El Niño, the warm water penetrated eastward in the spring of 1987. The 1990 1995 El Niño event is believed to be the longest recorded incidence in history. The probability of this happening is one in every 2000 years. It is unusual for El Niño events to occur in such rapid succession. In the 1991 1992 El Niño year, the warm water penetrated toward the east in the northern hemisphere in the spring of 1992. Strong El Niño conditions occurred in December 1997 with warm water extending all along the equator. El Niño events vary in strength. The 1997 1998 El Niño was unusually strong. The strength of the 1997 El Niño event could equal or surpass that in 1982 1983, making it the strongest El Niño this century. The precipitation in the El Niño years is above the annual mean precipitation. In the El Niño years, such as 1953, 1957 1959, 1965 1966, 1972 1973, 1976, 1983, 1986, 1988, 1991 1992, and 1995, the precipitation in Madison and Lake City is above annual precipitation (Table 1, Figures 2 and 3). The high rainfall in 1964 and 1985 are the two cases that are not correlated with the El Niño event. The flood of 1959 resulted from high precipitation, which occurred in an El Niño year. The 1973 flood can be explained by the 1972 1973 El Niño event because the rainfall of 1973 in Lake City is higher than the annual mean precipitation. The storm of 1994 1995 is now believed to be nearly as strong as the 1982 1983 storm. The high rainfall and related flood in 1947 1948 is possibly correlated with an El Niño event. On the basis of the SST anomaly and the June November SOI selection criteria (SOI 0.50) in the eastern Pacific Ocean, La Niña composite years include 1950 1951, 1954 1956, 1964 1965, 1970 1971, 1973 1976, 1983 1985, 1988 1989, 1996 1997 (SOI +0.47), and 1998 1999 (Table 2). The very cool water (negative anomalies) in the eastern Pacific occurred in 1988 1989, which is a strong La Niña event. The somewhat less cool water in 1995 has been recorded, making 1995 1996 a weaker La Niña year. Strong La Niña conditions occurred during December 1998: the eastern Pacific was cooler than usual, and the cool water extended farther westward. La Niña events also vary in strength. For example, the 1987 La Niña was stronger than was the 1995 La Niña. During most of the La Niña years, the rainfall in Madison and Lake City is below the annual precipitation (Table 2, Figures 2 and 3), for example, in 1950, 1954 1956, 1973 1975, and 1989 1990. The lowest precipitation event, 1954 1956, and the associated severe drought were matching the strong 1954 1956 La Niña event. Therefore, the annual precipitation and stream discharge amount can be predicted from the SST anomalies and the SOI. This can provide some guidance for the water management policy and planning. It is helpful to realize flood and drought potential and flood and drought hazards in land use planning and for management decisions concerning flood plain utilization when the future El Niño/La Niña years approach. This provides the basis for further study and planning and the solutions to minimize future flood and drought damages during the El Niño/La Niña years. The study of correlation between the Suwannee River flood/drought events and El Niño/La Niña events would improve flood routing and forecasting and also would be an important contribution to the judicious implementation of land use planning. CONCLUSIONS The precipitation in the Suwannee River watershed is considerably controlled by the El Niño/La Niña events. During El Niño years, strong rainfall is expected and flood events result; whereas in La Niña years, low rainfall and dry conditions occur. Three severe flood events (1948, 1959, and 1973) and one severe drought event (1954 1956) historically correlated with El Niño and La Niña events in the last 55 years. The El Niño/La Niña events in the Suwannee River watershed increase the chances of high rainfall, destructive flooding, drought, and devastating forest fires in that area.

CAO: SUWANNEE RIVER WATERSHED 97 The flood-drought events influence the water quality of the Suwannee River watershed because the water interaction exists between the rivers and the aquifers. Nitrate concentration, dissolved organic carbon concentration, dissolved solids concentration, and phosphorus concentration in both river water and groundwater increase or decrease in response to the flood-drought events related to El Niño La Niña events. Saltwater intrusion can occur in wells along coastal areas if the drought events occur during La Niña years. The drought events also can be healthy for the river itself because the sediment is hardened and compacted by sunlight and exposure to the atmosphere, causing pollutants to be trapped in the river bottom and surplus nutrients to be oxidized by the atmosphere. La Niña events produce an indirect impact on near-shore fishing in the Gulf of Mexico. During drought events, the near-shore productivity decreases dramatically due to the insufficient nutrients because the Suwannee River normally supplies a substantial portion of the nutrients responsible for the near-shore productivity. The results of this study contribute to a better understanding of the effects of El Niño/La Niña events on the environments of the Suwannee River watershed and may be used for future planning and forecasting in the area. ACKNOWLEDGMENTS I gratefully acknowledge the financial supports from the Florida Department of Environmental Protection and Department of Geological Sciences at the Florida State University. Thanks are due to J. B. Cowart and J. K. Osmond for their assistance. The author is indebted to Holly Williams for reviewing an earlier version of the manuscript. Two anonymous reviewers provided valuable critical reviews that led to significant improvements in the manuscript. REFERENCES Amarasekera, K. N., Lee, R. F., Williams, E. 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98 ENVIRONMENTAL GEOSCIENCES ABOUT THE AUTHOR Hongsheng Cao Hongsheng Cao is currently a Ph.D. candidate in hydrology at the Florida State University. He expects to defend his dissertation in Summer, 2000. He received his B.S. degree (1987) in tectonics from the Chengdu Institute of Technology (China) and his M.S. degree (1990) in stratigraphy and paleontology from the Chinese Academy of Geological Sciences, where he then worked as a research geologist for 5 years. He is broadly trained in the earth sciences. His research interests include hydrology, chemical hydrology, isotopic geochemistry, environmental hydrology, and Asian geology, with emphases on the Tarim Basin (China) geology, the Floridan aquifer (Florida), and the application of strontium and uranium isotopic tracers in hydrology. He has published more than 15 papers and research reports.