Flood pulse dynamics of an unregulated river floodplain in the southeastern US coastal plain

Annual flooding in low-gradient rivers is recognized as an important subsid y between the river and its broad adjoining floodplains. Unfortunately, rel atively few low-gradient rivers are unregulated and retain their natural "f lood pulse" behavior in most developed regions of the world. Furthermore, a ttempts to quantify flood inundation dynamics of any river floodplain are s carce. We used aerial photography to delineate the degree of floodplain inu ndation and GIS to quantify flooded areas on the forested floodplain of a 6 .3-km reach of the Ogeechee River, an unregulated sixth-order river in the southeastern USA. A regression was used to quantify the relationship betwee n discharge and percentage floodplain inundation. Using 58 years of daily d ischarge data obtained from a U.S. Geological Survey gaging station, we con verted daily discharge into daily percentage inundation and produced an inu ndation-duration curve, which describes the percentage of time that a parti cular inundation level is exceeded. This showed, for example, that >50% of the floodplain was inundated 15% of the time (54 d/yr) and that 100% was in undated 3.6% of the time (13 d/yr) for the average year. At 50% inundation, system width exceeded channel width by 19 times. In a relatively wet year, we showed that 50-100%, of the floodplain was inundated for several months during the winter-spring. Even in a relatively dry year, >20% of the flood plain (seven times the river width) was inundated for several months. The l ong term pattern over a period of 58 years (1938-1995) showed considerable fluctuation in inundation and recession occurring throughout most years, wi th the highest peaks found during winter and spring. The floodplain failed to reach 50% inundation in only four of 58 years. However, in six years, >5 0% of the floodplain was inundated for at least 30% of the time (i.e., four months of the year). Floods of 50% inundation typically had a duration of at least 30 d. Thus, although inundation may fluctuate considerably within a year, much of the floodplain can be inundated for a relatively long durat ion. Description of such longterm patterns is essential for understanding n atural hydrodynamics of unregulated rivers, particularly as attempts are ma de to restore previously altered systems. The flood pulse for this forested floodplain river is less predictable and floods are of shorter duration th an the large tropical rivers for which the hood pulse concept was originall y conceived. Unlike tropical rivers where seasonal patterns of flooding are driven by precipitation, flooding in the Ogeechee River is primarily contr olled by seasonal differences in evapotranspiration. Description of inundat ion dynamics is critical to understanding how plants and animals adapt to a habitat that shifts from dry to lentic to lotic, and in quantifying produc tion of aquatic organisms and ecosystem processes.