Origins and Development of Fresh Water Lake, Dominica, West Indies, and Exploratory Study of Traces of Catastrophic Events in its Sediments

We describe the origins and development of Fresh Water Lake on the island of Dominica, West Indies, and interpret indications in its sediment of past disturbances of the catchment by tropical cyclones, earthquake-induced slope failures, and human impacts. The descriptions are based on both the paleorecord and historical record. The lake occupies a small and steep-sided basin at the foot of a volcano and is surrounded by montane rainforest. It was enlarged by construction of a series of increasingly larger dams between 1961 and 1991. Basal contents of a 976-cm sediment core indicate that the lake originated ~730 AD, when a pyroclastic flow dammed a forested stream valley. In the lake’s first ~230 years, 436 cm of gravels, sands, and silts accumulated, indicating unstable catchment slopes. The next 340 cm, deposited in the ~935 years to 1896 AD, with high organic content and abundant plant debris from the terrestrial catchment, include evidence of frequent non-anthropogenic disturbances. In the most recent century, 200 cm of relatively minerogenic sediment accumulated, indicating additional destabilization of the catchment, largely due to human impacts. The sediment-accumulation rate peaks at 1990 AD when the final and largest dam was constructed. An exceptionally high 210Pb inventory in the upper part of the core results from a combination of excessive rainfall (~700 cm yr-1), high catchment-erosion, and in-lake focusing. The upper 540 cm of the core contains a series of major negative excursions in the loss-on-ignition profile, each indicating deposition of mixed minerogenic sediment associated with disturbance. We developed informal guidelines for distinguishing between the sedimentary traces of tropical cyclones and terrestrial and subaqueous slope failures at Fresh Water Lake, and applied them to the excursions. However, attributions of the excursions to these different causes contain uncertainties due to lack of real-time data on fluxes of materials to deep-water sites-of-deposition that result from these types of catastrophic events in similar lake/catchment systems.