Hydrology, Water Quality, and Channel Morphology Across an Urban-rural Land Use Gradient in the Georgia Piedmont, USA
Type of DegreeDissertation
Forestry and Wildlife Sciences
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The southeastern United States is experiencing rapid urban development. Consequently, Georgia’s streams have been threatened by hydrologic alteration, nutrient and bacteriological impairment, and channel morphometry changes from extensive development and from other land use activities such as livestock grazing and silvicultural practices. A study was performed to assess the above activities across an urban to rural land use gradient within 24 west Georgia watersheds ranging in size from 500-2500 ha that were drained by 1st, 2nd, and 3rd order streams. Dominant land covers in the study watersheds included: urban, developing, pasture, managed forest, and unmanaged forest. Stream hydrology was continuously monitored in 18 watersheds from 29 July 2003 to 23 September 2004 using InSitu pressure transducers. Dependent variables were estimated from the discharge data and placed into four categories, including flow frequency (i.e., the number of times a predetermined discharge threshold is exceeded), flow magnitude (i.e., maximum and minimum flows), flow duration (i.e., the amount of time discharge was above or below a predetermined threshold), and flow predictability and flashiness. Fine resolution data (i.e., 15-min interval) was also compared to daily discharge data to determine if resolution affected how streams were classified hydrologically. Urban watersheds experienced flashy discharges during storm events, whereas pastoral and forested watersheds showed more stable hydrographs. Flow frequency variables were most tightly correlated to land cover. Further, stream hydrology response variables were explained similarly with both the 15-minute and daily data resolutions. A two-phase study approach was used to investigate differences in stream water nutrient and bacteriological loading across the land use gradient. During phase 1, nutrient and biological data were collected within 18 watersheds, and data were used to generate regression models between land cover and the nutrient/biological parameters. Results from the phase 1 suggested nutrient and fecal coliform concentrations within watersheds having >5% impervious surface often exceeded those levels in nonurban watersheds during both base flow and storm flow. During phase 2 of the study, regression models were tested based on data from 6 new watersheds with representative land use/cover patterns for the area. To assess sediment movement and channel morphometry, 18 study watersheds were monitored. Biweekly grab samples and stacked-pole samplers were used to determine instream concentrations of total suspended solids (TSS) and total dissolved solids (TDS) during both base flow and storm flow. Multiple headwater cross-sections and sediment grids were measured routinely and following storm events to assess streambed stability. Higher TSS loads were present in nonurban watersheds during baseflow conditions. However, during stormflow, TSS loads dramatically increased within watersheds having >5% impervious surface cover and watersheds with intensive silviculture. Stream cross-sections and grids suggest that urban and pastoral streams had unstable stream channels, where fill and scour were common. To further explore sediment movement within study watersheds, sediment source tracking techniques were performed in 8 of the west Georgia streams. Fe in TSS samples from 2 flooding regimes was used to track sediment origin. Artificial flooding was used to develop a signature of Fe concentrations for in channel sources of sediment, and natural flooding was characterized and compared to in channel signatures. Urban and unmanaged forest streams received sediment inputs from terrestrial sources, whereas developing, pasture, and managed forests were dominated by instream sources of sediment.