John Day Basin BRAT
In this application of the Beaver Restoration Assessment Tool (BRAT), we analyzed all the perennial rivers and streams within the John Day basin (6-digit USGS Hydrologic Unit Code (HUC 6)). The John Day basin was subset into four smaller watersheds for running the BRAT model: Lower John Day, Middle Fork John Day, North Fork John Day, and Upper John Day watersheds (8-digit USGS Hydrologic Unit Code (HUC 8)).
The backbone to BRAT is a capacity model developed to assess the upper limits of riverscapes to support beaver dam-building activities. It outputs an estimated density of dams (i.e. dams per length of stream) and a rough count of an upper limit (i.e. capacity) of how many dams the conditions in and surrounding a reach could support. Both existing and historic capacity were estimated using readily available spatial datasets to evaluate seven lines of evidence: (1) a reliable water source; (2) stream bank vegetation conducive to foraging and dam building; (3) vegetation within 100 m of edge of stream to support expansion of dam complexes and maintain large beaver colonies; (4) likelihood that dams could be built across the channel during low flows; (5) the likelihood that a beaver dam on a river or stream is capable of withstanding typical floods; (6) evidence of suitable stream gradient; and (7) evidence that river is too large to allow dams to be built and to persist. Fuzzy inference systems were used to combine these lines of evidence while accounting for categorical ambiguity and uncertainty in the continuous inputs driving the models. The existing model estimate of capacity was driven with LANDFIRE 30 m resolution vegetation data from 2014, whereas the ‘historic’ estimate represents a pre-European settlement model of vegetation, also from LANDFIRE.
The estimated existing John Day basin (HUC 6) capacity is 120,945 dams or roughly 11 dams/km. By contrast, the same model driven with estimates of historic vegetation types estimated the John Day basin -wide capacity at 169,781 dams or roughly 15 dams/km reflecting a 29% loss compared to historic capacity.
Nearly all of the capacity loss from historic conditions can be explained in terms of riparian vegetation loss, vegetation conversion and degradation associated with high intensity land use including: 1) conversion of valley bottoms to urban and agricultural land uses, 2) overgrazing in riparian and upland areas, 3) conifer encroachment of wet meadow areas. Despite the losses in beaver dam capacity, John Day basin’s waterways are still capable of supporting and sustaining a substantial amount of beaver dam-building activity (120,945 dams).
Identifying these losses in beaver dam capacity incentivizes plans for restoration and conservation opportunities to be considered. To aid groups in their decisions and what possible risks may arise the BRAT model supplies the following management outputs: 1) potential risk areas, 2) unsuitable or limited dam building opportunities, and 3) conservation and restoration opportunities. As such, the BRAT model identifies where streams are relative to human infrastructure and high intensity land use, and conservatively shows how that aligns with where beaver could build dams.
The existing capacity model was verified in the Middle Fork John Day using only one beaver dam, North Fork John Day using only two dams, and the Upper John Day using 9 dams. In these watersheds we found that model performance was good, with electivity indices that effectively segregated out amongst the capacity categories. The existing capacity model was also verified in the Lower John Day watershed using 281 actual dam locations, concentrated in Bridge Creek. These dams helped identify a known limitation of 30 m vegetation data – that it struggles to capture narrow riparian vegetation corridors especially in deeply incised streams such as Bridge Creek (Macfarlane et al., 2017a). In this setting, higher resolution vegetation data or on-the-ground data collection will be required to effectively capture the riparian vegetation present and thus effectively capture beaver dam capacity. The extensive and on-going (as of February 2019) field-based dam collection campaign by North Fork John Day Watershed Council staff and others will further validate the existing capacity model.
The spatially explicit data from this application of BRAT provides stakeholders with the information needed to understand patterns of beaver dam capacity, potential risks to human infrastructure, as well as constraints and opportunities for using beaver in restoration and conservation. Not only does this information help with broad-scale planning efforts, but the resolution is sufficient to support design and on-the-ground implementation of conservation and restoration activities.