The response of riparian vegetation to PL-566 flood control structures.

In this study, the response of riparian trees to increased stream water availability due to hydrologic alteration by PL-566 dams was investigated through three successive studies. To determine site water balance within a specific riparian site downstream from a PL-566 dam, a three-year study was established along a second-order stream. The results showed stream water contribution to transpiration, via a highly dynamic hyporheic flow (further enhanced by riparian trees), exceeded both rainfall and groundwater flow. Leaf area development was closely coupled with stream and groundwater fluctuations, indicating sensitivity of site water balance for small dam systems. A second study used oxygen-18 stable isotopes to characterize source water contributions within this same riparian zone and an additional site. The results showed a linear trend between isotopic enrichment and tree distance-to-stream, with the stream contributing as much as 80% of source water to tree uptake. The study demonstrated that riparian trees provide a historical record of the close connectivity between stream water availability and riparian transpiration. A third study examined the large-scale and long-term effects of PL-566 dams on riparian systems. A land-use change detection analysis using remotely-sensed Landsat Multispectral Scanner and Thematic Mapper images was performed on a headwater basin containing several PL-566 dams along its intermittent tributaries. Comparisons were made of vegetation shifts in the riparian zones of impounded and non-impounded reaches over a ~30 year period (1973-2001). The results showed a compositional shift toward increased riparian species along the impounded reaches, from 6% of land-cover in 1973 to 39% by 2001. Hydrologic modeling of the watershed showed the dams act individually to increase water residence in the downstream reach, some by almost 2000% above flow, if the same stream were non-dammed. Collectively these dams increase basin-wide groundwater recharge (3.2 mm/yr), deep aquifer recharge (0.22mm/yr), and revaporation to the soil layers (5.7 mm/yr). The overall outcome from these studies is that upstream impoundments have increased riparian vegetation productivity by influencing movement of stream water to storage in the groundwater system. Just as importantly, the trees have responded in a positive feedback mechanism to further increase hyporheic exchange of stream water.