Ackerson Meadow and South Ackerson Meadow make up the largest mid-elevation meadow complex in Yosemite National Park. These scenic meadows are important habitat for the State endangered great grey owl and little willow flycatcher, as well as a suite of additional at-risk wildlife species. Currently, a large erosion gully network, up to 14 feet deep and 100 feet wide, is actively draining 90 acres of former wetlands in the meadow complex and threatening an additional 100 acres of wet meadow habitat. The gully network is a result of over a century of landscape manipulation including domestic water diversion, farming, ranching, and timber harvest. Yosemite National Park and the Stanislaus National Forest jointly propose to implement actions to reduce erosion and restore wetland functionality at Ackerson and South Ackerson Meadows. Additional goals of the project include:
- Protect existing intact wetlands from advancing gullies and headcuts, and re-establish hydrologic processes and conditions characterized by sheet flow and shallow dispersed swales.
- Restore the former extent of wetlands in Ackerson and South Ackerson Meadows by re-establishing sustained high water tables (water table within 12 inches of the soil surface for 21 days per year).
- Minimize and mitigate impacts related to restoration actions.
- Restore natural habitat for at-risk wildlife species.
- Enable tribal participation in ecological restoration, tending, and gathering of traditional use plant materials.
- Provide continued grazing on US Forest Service-managed lands while protecting recovering wetlands, riparian areas, and archeological resources.
- Remove invasive plant species that threaten native species.
- Preserve wilderness character. In designated wilderness, minimize impacts to wilderness character by limiting restoration activities and tools to the minimum required to restore water tables and prevent further degradation.
Several restoration methods to restore the meadow were formulated with these goals in mind. In summer 2020, the park requested input on issues that the planning team should address in the upcoming planning process, additional alternatives to meet the purpose of the plan, information the park should consider in the upcoming analysis, and other feedback. Comments were accepted through August 25, 2020.
The project team analyzed the public comments from the civic engagement period and refined alternatives for the Ackerson Meadow Restoration Environmental Assessment (EA). The planning team developed the following three action alternatives to meet the purpose and need of the project.
- Alternative 1 (Preferred alternative). Completely fill the erosion gullies to the level of existing meadow terraces to restore original topography, hydrology, and vegetation. Fill material will be generated from a combination of nearby upland hillslope soil excavations and locally generated wood chips and biochar. Approximately 151,000 cubic yards of fill will be needed. This alternative will maximize the acres of protected and restored existing and former wetlands.
- Alternative 2. Encourage sediment deposition and re-direct erosive flow energy within the erosion gullies by installing more than 350 hand-built structures from natural materials. These hand-built structures include beaver dam analogs (BDA) and function to create a stair-step sequence of ponds about 4 feet deep over the length of active channels. This treatment will require hand tools and manual labor, no fill or heavy equipment will be needed. Annual long-term maintenance of structures will be required. This alternative would not fully restore the gullies to natural meadow topography, rather it would enhance the wetland and floodplains within the gully network and eventually form an inset floodplain. This alternative would protect and restore the least acres of existing wetlands and former wetlands. The park has also considered the introduction of beavers to the meadow for restoration purposes but considers that action infeasible at this time.
- Alternative 3. Apply individual prescriptions of the fill or hand-built alternatives to specific reaches of the gullies based on depth of incision to restore meadow hydrology and reconnect with the floodplain. This hybrid alternative would use soil from the same sources as the other fill alternatives in the deeper portion of the gullies and use BDAs in the areas where the gullies are less than 3-5 feet deep. This would require less fill than the full fill option and more fill than the intermittent fill option, and it would require annual long-term maintenance of the BDA structures. This alternative would protect and restore a moderate number of existing and former wetlands.
Next an impact analysis of the three alternatives was conducted. Analyzing impacts means considering how the condition of the resources would change, either negatively or positively, as a result of the implementation of the alternatives under consideration. The environmental assessment presented the results of the impact analysis. The Ackerson Meadow Restoration Environmental Assessment underwent public and agency review from June 2, 2021 to July 8, 2021. A Finding of No Significant Impacts (FONSI) and the Determination of Non-Impairment were signed by NPS on September 21, 2021.