The presence of mercury in California's waterways is a serious problem with grave implications for fisheries and humans. It is estimated that 8 million pounds of mercury were lost to the environment during mining operations in the Sierra, much of which remains today pooled in streams, or trapped in sediment, or absorbed in the biota in its methylated form, working its way up the food chain to fish and ultimately to humans. Mercury is a powerful neurotoxin, especially harmful to the brain development of young children and fetuses. The presence of methyl mercury is of particular concern because in this organic form, mercury becomes part of the food chain. Mercury that collects in reservoirs is prone to methylation because of the anoxic conditions present in still water. Mercury is transported downstream and ultimately into the Bay and Delta despite the presence of dams on most major Sierra rivers and streams.

Sierra Streams scientists hypothesize that mercury is transported downstream over dams in the form of either methyl mercury absorbed by algae or fine particles of highly methylation-prone elemental mercury that are trapped in an algae-sediment mixture. The mercury-containing algae or algae-sediment mixture is thought to be mobilized during storm events, during the annual dewatering of reservoirs, and during the routine overflow of dams.

Using Lake Wildwood as a study site, Sierra Streams scientists are testing this hypothesis. Data were collected in monthly reservoir sampling, and in periodic sampling efforts below Lake Wildwood during two storm seasons and one dewatering event. Further sampling will be conducted during the reservoir drawdown planned for fall 2011.

Further study is planned, including sampling in additional dammed streams and further analysis of the algae/sediment mixture to determine the transport mechanism for mercury. With the recent large expansion of flow data-gathering capacity in the lower watershed, Sierra Streams scientists will be able to combine storm sampling and flow data to quantify the mercury being transported downstream. During the summer of 2011 we will collect biological specimens upstream, within and downstream of Lake Wildwood to analyze for mercury content. Specimens will include fish, algae, plants, and macroinvertebrates.

The findings of this study are likely to have widespread application for management of dams throughout the Sierra. We know already that the mercury concentration of algae/sediment in water from the bottom of the Lake Wildwood reservoir is far higher than water from the surface. This suggests that the periodic dewatering, during which water is released from the bottom of the reservoir, allows significant transport of mercury downstream. Changes in management of this process may help to minimize the transport problem.