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Rapid Risk Assessment of Watersheds and Dams using GIS and Modeling

Jacquelyn Duke, Joseph White, Peter Allen

The mechanisms of riparian development due to the presence of the PL-566 reservoir imposed flow and the dam have been initially described in this study based on observations of stream, soil, and vegetation water movement. Interactions between stream, soil, and plant water were assessed to better understand the influence of altered hydrologic regimes on terrestrial ecological communities. Initially, investigation of the development and growth of riparian vegetation associated with PL-566 reservoirs was chosen to assess the impacts that changes in the storage of the reservoirs have on peripheral groundwater and vegetation water uptake. Other considerations were assessed including vegetation succession and enhancement of biological diversity influenced by the presence and structure of the reservoir. While this analysis was originally intended for upstream communities, accessibility issues required establishing an intensive field study site downstream of a PL-566, namely SCS structure No. 25 in the Cow Bayou drainage. The shift in site also required and changes in research focus. The downstream hydrologic environment is unique in that a constant flow regime is created by the presence of the reservoir. Vegetation, namely various tree species including Ulmus crassfolia, Carya illinoiensis, Fraxinus texensis, Populus deltoides, and to a lesser extent Juniperus ashei now flourish in this environment, potentially due to near perennial stream-derived groundwater resources. In addition, the flow regime has also fostered the development of a beaver dam that until recently was inhabited and active. The beaver impoundment is located approximately 400 meters downstream of the reservoir dam and further restriction of water flow has also potentially broadened the lateral movement of stream water into the surrounding soil.

Questions regarding removal of PL-566 dams as well as dam refurbishment are based on assumptions about the normal function of unrestricted stream flow. Ecological arguments for dam removal are based on effects on fish migration, movement, and landscape connectivity. In addition, concerns about dam integrity are valid where the safety of downstream communities is considered (Doyle et al, 2000). In most cases, the cost of removing dams presumably is less than the cost of refurbishment and rejuvenation of the reservoir. Also, aquatic community connectivity within the landscape is reduced due to dams. However, the function of connectivity is mostly based on northern fish species with specific breeding requirements. Observations from this study indicate that PL-566 dams potentially have important functions in agriculturally dominated landscapes by changing stream flow regimes and promulgating ecological development. These are latent features of mature dams that create habitat both upstream and downstream of the dam, increasing landscape diversity and potential connectivity within terrestrial communities. Increases in riparian corridor development are potentially beneficial ecologically and hydrologically. Large trees near streams decrease bank erosion by enhancing soil stability due to large root structures and decreasing soil saturation by transpiration.

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