As the annual draw of the Rio Grande river approaches nine times its prescribed capacity, new efforts are being employed to obtain the best available data to improve evaporation loss estimates in the hope of improving management efficiency of an increasingly scarce resource. Current estimates rely on simple approaches developed over 30 years ago. Long-term eddy covariance measurements over a large reservoir in southern New Mexico are being conducted in response to a need to accurately quantify evaporative water losses from shallow open bodies of water in semi-arid regions. A 25 m tower was erected on the north end and near the middle of the Elephant Butte Reservoir where an eddy covariance (EC) system was mounted 3 m above the water surface. Net radiation, wind speed and direction, air temperature and humidity were also mounted on the tower. In addition a 15 meter water temperature profile was deployed to monitor water temperature changes so that monthly radiation storage may be computed. Periodically short, intensive field campaigns are conducted to study and evaluate turbulence structures over the open water surface. During these times, the data acquisition system is modified to collect 10 Hz turbulence data . The first intensive field campaign took place in September of 2001. Power and co- spectra of variance turbulence components were computed and are presented. Expected typical turbulence structures are inferred and often found. Daily evaporation losses are computed. However, a unique and unexpected event took place during a short afternoon period. Eddy covariance measurements came under the influence of a strong isolated thunder cell outflow. Turbulence characteristics and scalars can be seen to respond immediately and suggest intense turbulent exchange occurs between the outflow from the cell and the under lying water surface. In a matter of minutes, latent heat fluxes were observed to increase from typical values of 150 W m-2 to values in excess of 950 W m-2. During the course of the monsoon season in the SW U.S. these isolated or groups of isolated cells can be a daily or weekly feature across the surface of a reservoir. Implications of these periodic events on water losses from shallow open water reservoirs will be discussed.