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Watershed Management
> Dam Position Statement
> Mosquito Abatement
Searsville Lake: Position of the Jasper Ridge Advisory Committee – October 2007
Searsville Dam Searsville Lake is a major feature of the Jasper Ridge Biological Preserve. It plays a role in all aspects of the Preserve's mission, including research, conservation, and education. As Stanford and other stakeholders evaluate options for managing the reservoir, it is important for all parties to understand the relationship of the reservoir to the Preserve's strategic goals.

In that spirit, the Jasper Ridge Advisory Committee has assessed options for managing Searsville Lake into the future.

The goal of this assessment is to inform the decision-making process and help Stanford develop a unified and well-supported plan of action. The recommendation of the Jasper Ridge Advisory Committee is not based upon or intended to be a comprehensive analysis. It is based solely on the impacts of possible management options on the Preserve's mission. It does not account for numerous other factors that need to be addressed in the final decision, including costs, offsite environmental impacts, and consequences for upstream and downstream stakeholders.

The judgment of the Jasper Ridge Advisory Committee is that the continued existence of a reservoir provides important values for the Preserve. The available evidence indicates that careful dredging to maintain open water can sustain nearly all of these values, and that the impacts of the dredging can be managed in a way that does not create unacceptable levels of damage or risks to the Preserve's goals, operations, environment, and values. Further, none of the alternative options we have considered presents a comparable balance of environmental benefits, costs, and risks, based on the Preserve's mission.

Searsville Lake provides a number of important benefits to the Preserve. Ecologically, it supports a range of habitats, including the reservoir itself, the associated wetlands, and all of the habitats with species that use the reservoir and wetlands for feeding or breeding. Searsville Lake has a diverse aquatic community. It is a key habitat for migratory and breeding waterfowl and provides important foraging resources for bats. In addition, the reservoir helps maintain shallow water tables that support wetland habitat on the Corte Madera and Sausal Creek alluvial floodplains. The reservoir and associated wetlands are home to a number of species of conservation interest. Aquatic and wetland sites are rare on the San Francisco Peninsula, on university campuses, and in urban areas. As a consequence, Searsville Lake is a unique educational and research resource. It provides opportunities for students to have direct experience with a range of globally and locally important habitats, environmental issues, and engineering topics. It is also an important resource for research. Recent projects have pursued questions in biogeochemistry, hydrology, atmospheric chemistry, remote sensing, animal behavior, and sedimentology.

The Jasper Ridge Advisory Committee assessed the consequences for the Preserve of five general management options. Each of these could have a number of variants. We focus on impacts that are characteristic of each class of management options. These are:
  1. Allow the reservoir to fill with sediments and transition to meadow habitat.
  2. Remove the dam and restore Corte Madera Creek to steelhead trout habitat.
  3. Alter the dam and dredge the reservoir to maintain open water in a smaller reservoir at lower water surface elevations.
  4. Alter the dam to provide downstream flood mitigation.
  5. Leave the dam but remove sediments to maintain open water.

From the perspective of the Jasper Ridge mission, each of these options has costs, risks from unknowns, and benefits, and each creates opportunities and challenges.

Based on the information currently available, our assessment of each of the options is:
  1. Allow the reservoir to fill: The loss of the wide range of habitats and species supported by the reservoir is the major risk and disadvantage resulting from this option. Other risks include the potential for increased abundance of invasive species in the new meadow and for increased prevalence of sudden oak death in upstream areas that become moister. This option would also require increased channel maintenance to mitigate risks of upstream flooding. The succession to meadow would provide research opportunities, but so would having a reservoir in the future. The lack of disruption from future construction or deconstruction activities is a significant advantage of this option.

  2. Remove the dam: This massive undertaking would result in significant disruption to the Jasper Ridge mission. The impacts could be immense over a shorter period or, if the project is phased over time, of lower intensity over a longer period. In either case, the cumulative impacts would be large. A restored stream provides the best opportunity for steelhead habitat, but successful restoration is not certain. The loss of habitat for a wide range of other species is certain and a major disadvantage. Managing a century's worth of sediment accumulation so the impacts are benign presents enormous challenges. Removing Searsville Dam would be a highly experimental project. The large size of Searsville Dam, the alluvial fan character of the creek, and the urbanized downstream floodplain combine to make removal complex and challenging. Dam removal would substantially reduce or eliminate the jurisdictional wetlands at the south end of the reservoir, resulting in the loss of rich bird habitat and an important foraging ground for bats. Of the management options considered here, dam removal is the least reversible.

  3. Lower the reservoir water surface via modifications to the dam: This option combines most of the negative impacts of dam removal and of dredging, without providing additional benefits to the mission of Jasper Ridge. It does, however, potentially provide benefits to the Preserve by reducing impacts of upstream flood-risk mitigation. Lowering the reservoir water surface would likely impose logistical impacts and ecological risks similar to those of dam removal, and it would also likely require dredging. Lowering the dam and the water level would decrease the size of the reservoir and wetlands and decrease the benefits they currently provide. This option would create risks of increased problems from invasive species on the newly exposed shoreline and would not provide any potential for restored steelhead habitat.

  4. Alter the dam and change its operation to provide downstream flood mitigation benefits: This option is unacceptable to the Preserve because it seriously degrades the value of the reservoir and surrounding habitats for the Preserve's mission of conservation, education, and research. Altering the dam and dredging would entail disruptions and logistical risks from construction activities. Shifting operating goals to flood mitigation would create substantial challenges involving operations, management, and control. The manipulation of water level needed to accommodate downstream flood-damage mitigation would disrupt ecosystems on the reservoir margins, in the open water, in the sediments on the bottom of the reservoir, and in the nearby wetlands. Implementation of the altered management would be difficult and possibly dangerous, most likely limiting or eliminating a wide range of educational and research uses.

  5. Dredge to maintain open water and wetlands habitat: The primary benefit of this option from the Preserve's perspective is the maintenance of open water habitats and jurisdictional wetlands, along with their environmental services. This benefit sustains many dimensions of the Preserve's mission, including research in several disciplines, conservation of many native species, opportunities for conservation-oriented research, and a wide range of educational opportunities. Dredging will have impacts, and the challenge of this option is finding a way to minimize the adverse impacts. This challenge is heightened by the need for repeated dredging. With the best available technologies, however, the impacts of dredging and the risk of unforeseen ecological changes appear to be manageable. In addition, dredging in the near term leaves open the option of pursuing a different alternative in the future, should management needs or circumstances change. Even so, dredging is a difficult task. It will require careful planning, technical expertise, and ongoing collaboration between the project's engineers and the Preserve's scientists and staff.
Planning for and implementation of a plan to dredge Searsville Lake should involve input from the Jasper Ridge Advisory Committee, researchers, and staff, from initial feasibility studies, continuing through every stage. Issues that will demand careful evaluation include:
  • Strategies to minimize the spread of invasive species,
  • Strategies to minimize adverse impacts of heavy equipment,
  • Careful monitoring of sensitive habitats and important species,
  • Careful attention to the impacts of sediment transport, storage, and dewatering,
  • Sensitivity to impacts on program activities, and
  • A continuing commitment to capitalize on the research opportunities associated with the project.
The Jasper Ridge Advisory Committee is committed to working with other Stanford units and other stakeholders to develop a plan and a process consistent with the Preserve's mission, as well as with the needs of the University.


Mosquito Abatement

Helicopter Applies Bacterial Agent to Control Mosquitoes Mosquito Report for Jasper Ridge Biological Preserve - December 2006
The San Mateo County Mosquito Abatement District works with Stanford University and the staff of the Jasper Ridge Biological Preserve (JRBP) to control mosquitoes arising from the cattail marshes surrounding Searsville Lake. This report is a summary of the work done at this site and its results to date.

Click here to download the report (PDF format)



Mosquito Abatement at JRBP
The San Mateo County Mosquito Abatement District (SMCMAD) has treated the Preserve's wetlands south and west of Searsville Lake with a bacterial agent that attacks mosquito larvae, Bacillus thuringiensis subspecies israelensis, for at least a couple of decades. However, due to the density of vegetation, especially in the tules and cattails, the District is unable to access some areas they consider to be potential mosquito threats to nearby residents.

With the increased concern about West Nile virus, SMCMAD used a helicopter to spray about 25 acres of wetlands in the Preserve with another bacterial agent, Bacillus sphaericus in September of 2003, and again in July, August, and September of 2004. Mosquito larvae ingest the bacteria, which produce a toxin that disrupts the gut in the larvae by binding receptor cells present in insects that are not found in mammals. The product is typically marketed under the trademark name of VectoLex and is generally effective for up to 4 weeks.

To date, most of the information indicates that this larvicide and Bacillus thuringiensis subspecies israelensis, have little or no significant environmental impacts. The District intends a more aggressive monitoring and management regime in 2004 than has been the case historically at Jasper Ridge Biological Preserve that includes more frequent spraying and increasing the area to include all of Searsville Lake. The Preserve not only bears the costs associated with the aerial spraying, but also makes an effort to keep nearby residents informed about upcoming spraying activity.

San Mateo County is using "sentinel" chicken flocks as an early detection mechanism for West Nile virus and Jasper Ridge Biological Preserve's flock of 10 chickens is part of the early warning network. Technicians from the San Mateo County Mosquito Abatement District take blood samples from the chickens every two weeks to test for the presence of West Nile virus and other mosquito-borne diseases.

SMCMAD posts monthly updates on West Nile virus and the sentinel chicken monitoring system in their entomology reports located at http://www.smcmad.org/news1.htm.


For further information on West Nile virus:
  California Dept. of Health Services
  NBII Wildlife Disease Info Node
  Center for Disease Control






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