IMPACTS OF EROSION AND SILTATION ON THE LOWER ETOWAH RIVER, GEORGIA

Within the last ten-years a thin layer (<2 cm) of silt has covered the natural cobble bottom of the Etowah River. The diminished quality of the Etowah River due to this siltation has been a growing concern among local non-profit groups and community leaders. This study investigates the origin of the increased sedimentation in the lower Etowah River using various methods: 1.Determining the sedimentation rate of the Etowah River through the installation of an erosion pin, 2. Investigation of current siltation being “legacy silt” from Civil War Era milled ponds adjacent to the river banks, and 3. Analysis of recent (over the past ten years) changes in discharge rates, times, occurrences from the Allatoona Dam.

The erosion pin provided 12 weeks of data that established the average sedimentation rate to be less than 2 mm (measured from September through November). At five random field sites we collected grab samples that were sent to the Center for Applied Isotope Studies at University of Georgia for analysis. In order to establish the age of the sediment, or at minimum, constrain the age of sedimentation to recent, we analyzed for Cs-137, Pb-210, and Be-7. The analysis of the isotopic dating was inconclusive for Cs-137 and Pb-210, perhaps, because the overall grain size (d50=0.3 mm) was too large to hold the isotopic signatures. Berillum-7 was detected in one sample; confirming modern sedimentation at that location. Although the isotopic dates did not confirm or deny the input of legacy silt into the Etowah River, we did establish from Census data that there are only 17 known milled ponds along the entire reach of the Etowah River in 1850; making legacy silt less likely as a prominent sediment source. During this study water was released from the Allatoona Dam daily with a maximum discharge in Rome of 2,000 cfs. Although the discharge rate changes seasonally, and as hydroelectricity is needed, the discharge permitting of the dam has not changed since its construction in 1946.

The increased siltation that has been of concern to the community is likely influenced by dam release. The pulses of sedimentation that can visually be seen correlate to increases in discharge. However, the origin of the sediment may not be locally derived. The next step in this project is to investigate sedimentation rates near new developments adjacent to the lower Etowah River and its tributaries.