In this application of the Beaver Restoration Assessment Tool (BRAT), we analyzed 78,835 km perennial rivers and streams in 80 HUC 8 level watersheds within the Sierra Nevada, Cascades, Coast, and Klamath mountains of California.
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 project-wide capacity is 715,548 dams or roughly 6 dams/km. By contrast, the same model driven with estimates of historic vegetation types estimated the project-wide capacity at 1,398,540 dams or roughly 11 dams/km reflecting a 49% 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, California’s waterways are still capable of supporting and sustaining a substantial amount of beaver dam-building activity (715,548 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 nine priority watersheds, using 312 actual dam locations. Results indicate that the model effectively segregates the factors controlling beaver dam occurrence and density 86% of the time. The model verification results also indicate that beavers preferentially dam in reaches with higher modeled dam capacity while avoiding those with lower capacity.
GIS Data Layers
The GIS data layers that make up the maps are available in KMZ for the perennial network of each region, as an example Costal Region data is found here, shapefile here and layer package formats here and enable visualization and querying in GIS programs. We encourage the use of the layer packages because this format provides all the inputs, intermediates and outputs symbolized in a standard format which increases their usability. Viewing the KMZ files in Google Earth or ArcGIS Earth is an effective way to visualize and interrogate these output datasets because of the 3-D capabilities, image rendering speed and the quality of the base imagery. If you need help using the GIS data we have developed a series of tutorial videos and other instructions found here. For non-GIS users we have generated an Esri Story Map of the project that can be viewed here and a map atlas of BRAT outputs which, can be found here.