FLOOD PULSE DYNAMICS OF AN UNREGULATED RIVER FLOODPLAIN IN THE SOUTHEASTERN U.S. COASTAL PLAIN

Abstract

Annual flooding in low-gradient rivers is recognized as an important subsidy between the river and its broad adjoining floodplains. Unfortunately, relatively few low- gradient rivers are unregulated and retain their natural behavior in most developed regions of the world. Furthermore, attempts to quantify flood inundation dy- namics of any river floodplain are scarce. We used aerial photography to delineate the degree of floodplain inundation 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 between discharge and percentage floodplain inundation. Using 58 years of daily discharge data obtained from a U.S. Geo- logical Survey gaging station, we converted daily discharge into daily percentage inundation and produced an inundation-duration curve, which describes the percentage of time that a particular inundation level is exceeded. This showed, for example, that >50% of the flood- plain was inundated 15% of the time (54 d/yr) and that 100% was inundated 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 floodplain (seven times the river width) was inundated for several months. The long- term pattern over a period of 58 years (1938-1995) showed considerable fluctuation in inundation and recession occurring throughout most years, with 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, >50% 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 duration. Description of such long- term patterns is essential for understanding natural hydrodynamics of unregulated rivers, particularly as attempts are made to restore previously altered systems. The flood pulse for this forested floodplain river is less predictable and floods are of shorter duration than the large tropical rivers for which the flood pulse concept was originally conceived. Unlike tropical rivers where seasonal patterns of flooding are driven by precipitation, flooding in the Ogeechee River is primarily controlled by seasonal differences in evapotranspiration. Description of inundation dynamics is critical to understanding how plants and animals adapt to a habitat that shifts from dry to lentic to lotic, and in quantifying production of aquatic organisms and ecosystem processes.