Peace River in Florida Loses as Much as 11 Million Gallons a Day to Sinkholes and is Vulnerable to Running Dry
The Peace River is one of the largest watersheds in Florida, extending from central Florida's phosphate mining district to the southwest coast, where it discharges into Charlotte Harbor. The river serves as a drinking-water supply for a growing population of downstream residents in southwest Florida.
A 5-year U.S. Geological Survey (USGS) study conducted by hydrologists Patricia Metz and Bill Lewelling reports that the upper Peace River near Bartow, Florida, can lose large quantities of water to underlying aquifers. On average, the upper Peace River loses water at a rate of 11 million gallons per day (17 cubic feet per second) to sinkholes, fractures, and crevices along a 2-mile section of the river south of Bartow. These losses make the river vulnerable to running dry during periods of low rainfall, and they limit its ability to support ecosystems and provide water to residents downstream.
The upper Peace River flows through karst, a type of topography formed in limestone that can be dissolved by rainwater, which is naturally slightly acidic (with a pH below 7.0). Such topography is characterized by sinkholes, caves, and underground drainage. The bedrock beneath the upper Peace River is mainly limestone, and the river and floodplain contain numerous karstic features, including sinkholes, fissures, cavities, and conduits.
Although the upper Peace River has long flowed through karst, it has not always been vulnerable to running dry. Historically, the upper Peace River flowed year round, and its floodplain contained artesian wells and a spring (Kissengen Spring) that discharged an average of 20 million gallons per day (31 cubic feet per second) into the river. Now, however, the upper Peace River goes dry during below-average rainfall years and when aquifer levels are low. During 7 of the past 10 years, the river has gone dry in this area, mostly during the end of the spring dry season. During 2000-2002, the dry area extended beyond this 2-mile section, and the USGS streamflow gage at Bartow recorded periods of no flow for the first time in its 68-year history. The river at this gage once again went dry for several days in May 2009.
About 10 prominent karst features in the low- and high-water channel of the upper Peace River provide the paths through which large amounts of streamflow drain to the underlying aquifers. The largest single-day loss measured during the 2002-2007 study period was 32 million gallons per day on June 28, 2002.
Streamflow losses varied throughout each year and were related to seasonal fluctuations in groundwater levels. During this study, the largest streamflow losses occurred at the beginning of the summer rainy season (May and June), when groundwater levels were low and large volumes of water were needed to replenish unfilled cavities and void spaces in the underlying aquifers.
"The greatest influence on streamflow declines in the upper Peace River is the lowering of the groundwater levels below the riverbed elevation," said Metz. "As groundwater levels decline below the riverbed elevation, the cavernous layers capture the river flow and divert it through holes and cracks into the groundwater system." This study shows that streamflow losses are exacerbated during drought years, which is also when groundwater use increases.
Streamflow declines have resulted in changing flow patterns, streamflow loss, and the drying up of the artesian wells and Kissengen Spring. The progressive, long-term decline of streamflow in the upper Peace River began as early as the 1950s, with intensive groundwater withdrawals for phosphate mining. In 1975, when groundwater use for phosphate-mining processes was at its maximum, groundwater levels were as much as 50 feet below the riverbed elevation along the upper Peace River. Since then, there has been a reduction in the use of groundwater for mining processing, and aquifer levels have risen. Currently, groundwater levels remain as much as 30 feet below the riverbed elevation, depending on climatic conditions and groundwater use. Although the aquifer levels have risen in this region, groundwater levels that are still below the riverbed elevation create the potential for streamflow losses through the karst features.
This USGS study was funded in cooperation with the Southwest Florida Water Management District (SWFWMD) to understand the relation between geology, groundwater levels, and streamflow losses. The study also helps explain other factors affecting historical and current streamflow trends of the upper Peace River. The report, "Hydrologic Conditions that Influence Streamflow Losses in a Karst Region of the Upper Peace River, Polk County, Florida" (USGS Scientific Investigations Report 2009-5140), will provide residents, businesses, regulatory agencies, and scientists with information that can help them make informed decisions about water-supply issues in central Florida.
For more information, including a photo gallery of the Peace River during various hydrological conditions, a video of the river flowing into a sinkhole, a bibliography of previous USGS publications on the Peace River, and a link to the new report, visit USGS Report Quantifies Streamflow Losses within the Upper Peace River Watershed.
in this issue:
Peace River Vulnerable to Running Dry