Channel morphology and sediment origin in streams draining the Georgia Piedmont

SummaryUrbanization is common across much of the US However, the Southeast, including the Georgia Piedmont, is developing much faster than other regions (USDA, NRCS, 2004). Consequently, water resources in the Middle Chattahoochee Watershed of western Georgia are threatened by increased sedimentation from extensive urban development as well as from other land covers such as livestock grazing and silviculture. A 2-yr study was developed to assess sediment transport and origin across 16 watersheds draining urban, developing, pastoral, managed forest and unmanaged forest landscapes. Total suspended solids (TSS) and total dissolved solids (TDS) yields and sediment rating curves were measured concomitantly with channel morphometry measurements in each stream. Urban streams featured the lowest baseflow concentrations, but sediment concentrations rose rapidly during stormflow in urban streams. Detailed cross-sections assessed channel stability and showed that pasture streams were the most unstable streams during stormflows. Finally, sediment source tracking was performed in a subset of intermittent streams using amorphous to crystalline ratios of iron to estimate the fraction of sediment coming from instream vs. landscape sources. Artificial stormflows were generated to mobilize bed sediment for the development of an instream sediment signature. If these ratios differed during natural events, it was inferred the differences were due to sediments mobilized from the terrestrial landscape. Results indicated that higher ratios of amorphous:crystalline Fe occurred during artificial floods (urban = 0.60 and unmanaged forest = 0.14) than natural stormflows (urban = 0.08 and unmanaged forest = 0.03) in watersheds dominated by urban and unmanaged forest land cover, suggesting that crystalline (i.e., terrestrial) sources of Fe were transported to the stream during rainfall events.