In 2018, the Skagit River System Cooperative (SRSC) formed the Skagit Culvert Working Group together with project partners Skagit County, the Skagit Fisheries Enhancement Group, and the Upper Skagit Tribe, to complete the initial phase of a project to update fish passage barrier data for the Skagit River Basin (Mickelson et al. 2020). The work was intended to facilitate identification and development of barrier correction projects in the watershed, and involved reconciling and updating existing barrier datasets, collecting new habitat and barrier assessment data, and building a database to house it all. Although a statewide barrier inventory database is maintained by the Washington Department of Fish and Wildlife (WDFW), maintaining a local database allowed our working group to more easily create queries to help identify and compare potential projects.
This effort led to the selection and successful development of several barrier correction projects throughout the Skagit Basin while also updating data for a large proportion of barrier crossings. However, it was also clear that significant data gaps remained, so a Phase 2 project was launched (Crowley et al. 2024). A major focus of the new work was to address difficulties in comparing the potential habitat benefits of candidate barrier correction projects. Field-based surveys are the best way to assess the quantity and quality of habitat upstream of barrier crossings, but these can be labor-intensive and time-consuming to complete. In addition, field surveys commonly require traversing multiple properties owned by public and private landowners. While Washington State asserts ownership of all navigable waterways in the state (WAC 332-30-106), thus allowing crews to legally access mapped streams for habitat assessments, surveys commonly require crossing private property at some point, and permission for access is often challenging or impossible to secure. As a result of these complications, a great number of crossings in the watershed have no information about upstream habitat conditions, making comparisons of the potential habitat benefits of candidate barrier correction projects difficult.
These difficulties are not new, and a previous project completed in the Skagit Watershed (Smith and Waldo 2003) estimated habitat upstream of barrier crossings using GIS-based tools. That effort combined hydrography and topography GIS layers with land use, precipitation, fish distribution, and barrier location layers to estimate habitat length and area, weighted by land use categories. The habitat estimates were presented as one means of comparing candidate sites for the first phase of our project (Mickelson et al. 2020). However, the 2003 tool produced habitat estimates that are not directly comparable to field survey data; it estimated habitat area only to the next upstream culvert crossing, while field-based surveys characterize all accessible upstream habitat (WDFW 2019). The Smith and Waldo tool also (by necessity) relied upon hydrographic and topographic data that are less accurate than those available currently, making it challenging to accurately measure reach length and slope attributes.
The recent development of an improved hydrography layer (Hyatt et al. 2022) based on high-resolution lidar topography has made it possible to develop a new GIS tool for estimating habitat upstream of barrier crossings, which we describe here. The tool utilizes a GIS-based data analysis process referred to as a geometric network, which links our hydrography to a stream crossing point layer, allowing estimates of upstream habitat, sorted by geomorphic class. The network also can generate counts of upstream and downstream barrier crossings as well as estimate the amount of habitat between a given barrier and the next upstream barrier (in addition to total upstream habitat including tributaries). All of these are primary considerations when comparing potential barrier correction projects.
The tool, which we have called the Skagit Barrier Correction Habitat tool (SBCH), was developed using ArcPy scripts (ESRI ArcGIS 10.7, ArcPy 2.7) allowing updates to be incorporated by re-running the scripts after new culvert data are collected or as hydrography layers are refined. We stress that the tool is not aimed at mapping fish distributions but rather is intended to be a uniformly available resource for initial comparison of candidate sites based on estimated productive habitat for a variety of anadromous species, in the context of the presence of other barriers on the system. It is meant to be supplemented by other data such as field-based habitat surveys, fish surveys, and knowledge about individual sites before making decisions about project development.