Yakama Nation Reservation BRAT

Yakama Nation BRAT

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Executive Summary

This webpage presents an application of the Beaver Restoration Assessment Tool 2.0.0 (BRAT;, a decision support and planning tool for beaver management (Macfarlane etal. 2017). In this application we analyzed all the perennial rivers and streams within the Yakama Nation Reservation, which includes portions of the Lower Yakima, and Klickitat watersheds (8-digit USGS Hydrologic Unit Code (HUC 8)). As part of Contract Modification 1, we expanded our analysis to include Rock Creek watershed (10-digit USGS Hydrologic Unit Code (HUC10)), which is presented in the body of this report. As part of Contract Modification 2 we again expanded our analysis and included the White Salmon, Little White Salmon and Wind River watersheds (HUC 10’s) which is presented in the Appendix A because this modification was awarded after this report was drafted.

The backbone to BRAT is a capacity model developed to assess the upper limits of riverscapes ability to support beaver dam-building activities. Both existing and historic capacity were estimated using readily available spatial datasets to evaluate five key lines of evidence: 1) a perennial water source, 2) availability of dam building materials, 3) ability to build a dam at baseflow, 4) likelihood of dams to withstand a typical flood, and 5) likelihood that stream gradient would limit or completely eliminate dam building by beaver. Fuzzy inference systems were used to combine these lines of evidence while accounting for uncertainty.

The estimated existing Yakama Nation Reservation-wide capacity is 30,688 dams or roughly 10 dams/km. By contrast, the same model driven with estimates of historic vegetation types estimated the Yakama Nation Reservation-wide capacity is 40,760 dams or roughly 13 dams/km reflecting a 25% loss compared to historic capacity. Nearly all of this capacity loss can be explained in terms of vegetation loss and degradation associated with land use including: i)urbanization in the Yakama valley, ii) conversion of valley bottoms to agricultural land uses, iii) overgrazing in upland areas, iv) conifer encroachment of wet meadow areas. Despite the losses, the uniform ownership of the reservation and good condition of many streams in the reservation means Yakama Nation’s waterways are still capable of supporting and sustaining a substantial amount of beaver dam-building activity. By contrast, the estimated existing beaver dam capacity outside the Yakama Nation Reservation but within the Lower Yakima, Klickitat and Rock Creek watershed is 14,507 dams or roughly 5 dams/km and historic estimates is 28,080 dams or roughly 10 dams/km reflecting a 48% loss compared to historic capacity.

The capacity model estimated existing capacity for Lower Yakima, Klickitat, and Rock Creek watersheds combined at 45,195 beaver dams (8 dams/km) and the historic capacity at 68,840 dams (12 dams/km), reflecting a 35% loss compared to historic capacity. At the individual watershed level beaver dam capacity was found to be as follows. Lower Yakima, 18,753 beaver dams (7 dams/km) and the historic capacity at 35,593 dams (14 dams/km). Klickitat 24,945 beaver dams (8 dams/km)and the historic capacity at 31,328 dams (10 dams/km) and Rock Creek 1,497beaver dams (11 dams/km) and the historic capacity at 1,920 dams (14 dams/km).

We verified the performance of the existing capacity model using 122 existing dams throughout the full extent representing 0.4% of the 5996 kilometers of perennial streams. We found that model performance was spatially coherent and logical, with electivity indices that effectively segregated out amongst the capacity categories. That is, only one beaver dam was found where the model predicted no dams could be supported, beaver exhibited avoidance of reaches predicted as supporting rare or occasional densities, and beaver exhibited preference for areas predicted as having pervasive dam densities. Of the total 93 stream segments with validation dam counts 15 exceeded the capacity estimates indicating that the model effectively segregates the factors controlling beaver dam occurrence and density 84% of the time.

The decision support and planning tool side of BRAT uses simple geospatial analysis and rule systems to account for human conflict potential with beaver dam building and the recovery potential of riparian habitat to segregate the stream network into various conservation and restoration zones. BRAT categorized 52% of the reservation as ‘Low-hanging Fruit’ streams signifying habitats that are either currently inhabited by beaver or are in relatively good condition for beaver re-colonization and/or reintroduction.

The model would benefit from additional actual dam count data based on field reconnaissance data especially in more remote portions of the reservation. These data could be used to further validate the model and could also be used to identify source and sink zones throughout the reservation. Accurate identification of source and sink zones will help Yakama Nation biologists continue to manage beaver populations, especially nuisance beaver.

The spatially explicit outputs from this application of BRAT provides Yakama Nation biologists with the information needed to effectively identify where nuisance beaver can be relocated, where ‘Living with Beaver’ strategies may be needed and where beaver can be used for watershed restoration efforts to have the greatest potential to yield increases in biodiversity and ecosystem services. Not only does this information help with broad-scale planning efforts, but the resolution is sufficient to support detailed design and on the ground implementation of translocation and restoration activities.

Scope and Extent of the Project

Yakama BRAT Deliverable Data Products Include: