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On September 27, the median dissolved oxygen concentra- tion at 10 sites in the Lumber River Basin was 5.2 mg/L with a minimum of 1.3 mglL; the median concentration in the Waccamaw River Basin was 2.55 mg/L with concentra- tions at all 6 sites less than or equal to 2.7 mg/L. Also on September 27, the median dissolved oxygen concentration measured at 16 sites in the Tar River Basin was 4.5 mg/L with a minimum concentration of O. I 7 mg/L, and a maxi- mum concentration of 8,07 mglL at Rocky Mount, which is where the USGS measured the maximum dissolved oxygen concentration in the Tar River. On September 28, at 1 I sites in the Chowan River Basin, the median dissolved oxygen concentration was 2.5 mglL and the minimum concentration was 1.2 mglL. Although dissolved oxygen concentrations were quite low, the sustained hypoxic conditions that occurred in floodwaters from Hurricane Fran did not occur after Hurri- cane Floyd (table 7). These somewhat higher dissolved oxygen concentrations that occurred in Hurricane Floyd floodwaters may have been the result of one or a combination of the following factors: ~ w " " z Farm surroundt'd by floodwaters Air temperatures following Hurricane Floyd were somewhat cooler than after Hurricane Fran. Higher and more sustained flows after Hurricane Floyd provided greater dilution of oxygen-consuming materials. Slower recession of Hurricane Floyd floodwaters resulted in a more gradual delivcry of organic matter from the floodplain to the river than during Hurricane Fran flooding. - - - - --- - - - --.. -"""'Ili"IiII Table 7. Dissolved oxygen concentrations, in milligrams per liter, at selected sites in the Neuse and Cape Fear River Basins, N.C., approximateiy 5 days following the passages of Hurricanes Floyd (1999) r and Fran (1996) [-, no data] Site no. (fig. 13) Hurricane Floyd Sepl. 16--17. 1999 (sampled St"pf. 20-21,1999) 7.5 4.9 Stalionname 22 23 27 28 ~ Neuse River near Clayton Neuse River at Smithfield Ncuse River near Goldsburo Neuse River at Kinston Contcntnea Creek at Hookertan Cape Fear River at Lock 1 2.8 2.9 4.4 ~- HurrlcllDefo'raD Sept. 5-4i, 1996 (sampled Sept. 10, 1996) 5.0 3.8 0.4 0.9 1.0 2.5 - Specific conductance values were low comparcd to previous records (table 8) indicating the effects of dilution from the floodwaters of Hurricane Floyd. Likewise, pH values were about 0.5 to 1.0 standard units lower than typical values measured during the previous 9-year record. The lower pH likely was the result of the large volume of rainfall during September and October; rainfall typically has a pH of about 4.5 in eastern North Carolina (National Atmospheric Deposition Program, 1999), Measure- ments of pH ranged from 4.4 in the Lum- ber River on September 21 and 23 to about 6.5 at several locations in the Tar River on October 5. Nitrogen, Phosphorus, and Organic Carbon For three of the five sites for which comparisons could be made (table 8), maximum ammonia concen- trations measured in floodwaters were approximately equal to the median con- centrations rneasured from January 1990 to August 1999, and certainly Two Months of Flooding in Eastern i\orth Carolina. September-October ]999