Resiliency is the ability to recover from disruptive change and withstand ongoing pressures. The ability to recover from change is directly proportional to the size and extent of the disturbance and the degree to which an ecosystem can tolerate disturbance. Ecosystems have developed around “natural disturbance regimes” where change is a constant and critical component of a dynamic ecosystem function (water levels rise and fall, temperatures change with seasonality). As a result of continuous change, the dynamics of systems select for resiliency. In other words, an ecosystem’s resiliency is its ability to tolerate disturbance without collapsing into a different state that is controlled by a different set of processes.
TSF is interested in addressing the disturbance regimes critical to ecosystem resiliency in the Sierra Nevada including the flow regime, sediment regime, fire regime and climate regime. The natural variability of these disturbance regimes is fundamental to restoring ecosystem resiliency but this variability has been fundamentally altered by the impacts of the California Gold Rush.
Water Flow Regime
In the 1800s, California’s streams and rivers were engineered to deliver water to the mines and today are operated to deliver water and power during periods of peak demands. As a result, the flow release schedule developed by reservoir operators dominates the current flow regime and may lack critical pulse flows for species and environmental flows. The system’s ability to meet the increased demands for water and power, as well as environmental flows, may depend on the functioning of the “green infrastructure” of the headwaters and the restoration and protection of key areas, such as meadows, that hold water higher, longer and release it slowly over extended periods of time.
Sediment Regime
The hydraulic mine sites were denuded of soil, resulting in a landscape littered with scars which delivered unprecedented amounts of sediment to the streams and rivers. For example, more than three times as much material excavated during the construction of the Panama Canal was washed down the Yuba and Bear River watershed from 1860-1884, leaving deeply incised channels cut to bedrock and terraces disconnected from the floodplain. These scars continue to erode and release sediment contaminated with mercury, choking streams with turbid conditions and filling reservoirs. The system’s ability to handle increased sediment loads from denuded mine-scarred landscapes and from overgrazed meadows is dependent on both remediation efforts that abate erosion and on sediment removal efforts coupled with reservoir maintenance that are sophisticated enough to address mercury contamination.
Fire Regime
In the fire prone landscape of the Sierra Nevada, fire is a natural part of the ecosystem function and resiliency of the region. Fire plays a role in seed germination and forest stand diversity. The region-wide removal of timber for the mining operations resulted in today’s single-age stands, which are choked with underbrush and full of invasive species. Fire suppression has had devastating effects on this disturbed landscape, in many ways precluding it from recovering from the impacts of the Gold Rush. The result is an ongoing threat of severe mega fires, the likes of which the region has not seen before, including in areas that are now populated. The system’s ability to handle fire, and to recover from severe fire, is directly proportional to continued coordination of efforts on topographically specific forest thinning, invasive species removal, and fire treatments.
Climate Regime
The greatest pressure the Sierra Nevada ecosystems face today is climate change. Longer droughts and warmer temperatures mean that precipitation will be dominated, not by snow, but by rain fall, resulting in a ”flashy” system
characterized by increased power for erosion and less water late in the season due to decreased snow pack. Under these conditions, erosional forces will dwarf the absorption capacity of the headwaters resulting in mine-scarred
landscapes, such as hydraulic mines contaminated with mercury, delivering more sediment and further choking streams, rivers, and reservoirs. Sedimentation will also put increased pressure historic debris control dam infrastructure, originally designed to hold back hydraulic mine debris and tailings, that now litters the landscape with antiquated, overburdened dams.
