Salmon Strategy

Mapping Riparian Land Use within Agricultural Zones

Introduction:
Skagit County's Salmon Policy Resolution (R20070499) directs the county's Geographic Information Systems department to undertake a significant mapping project of riparian areas on agricultural and natural resource lands within the Skagit River watershed. Some 770 miles of watercourse are included. The study area does not include areas outside Skagit County's jurisdiction (cities and towns), diking and drainage districts covered by the Drainage and Fish Initiative and the Tidegate and Fish Initiative agreements, and type 5 streams.

Background
Many counties protect streams using mandatory buffers strips of land bordering the stream where development or farming is not allowed. Because buffers can impose a heavy burden on agriculture, Skagit County does not require buffers on ongoing agricultural lands.

Project Goal
The project's goal was to measure the amount of existing vegetated riparian area and compare it to the amount of potential vegetated riparian area if buffers were required along the watercourses. The project essentially attempted to answer the question, "How much riparian area is already protected on lands with ongoing ag?"

Methodology
The project used high-resolution bird's eye aerial photography that allowed easy identification of vegetation type and accurate identification of even narrow buffers less than 30-feet. For still more precision, project staff used LIDAR imagery that measured vegetation height. With both types of imagery at their disposal, GIS staff examined each watercourse and its surrounding area by hand for accurate ground cover assessment.

Results
Of the 8,031 acres of standard buffer area analyzed, we classified the following land uses: 22% agriculture, 5% developed land or road covered, and 73% forest, wetland, or natural grass. Agricultural activity varied by region ranging from 13% in the Sauk region to 35% in the Nookachamps.

One purpose of this study was to determine how many acres of riparian agricultural land would be eligible for habitat protection. For agricultural use areas within standard buffer areas (1,766 acres), we concluded that there are 42 acres of protected land, 75 acres of public land, and 64 acres of land with no functions or values. Together, these three categories, which would not qualify for habitat acquisition programs, constitute 10% of the agricultural land studied within the standard buffer area. Of the remaining agricultural area, 90% is potentially restorable.

We also compared our land use data with other compiled satellite data sets. These other studies used an automated processing of land cover and have a course resolution of only 30 meters, which detects only large-scale patterns. During this analysis, we found that the high-resolution photography detected many clues and cover types that the 30-meter data set did not detect. In comparison with our study, both course resolution studies over-estimated agricultural land use-NLCD by 123% and RTI by 289%.

Expected Use of Results
Beyond its applications for the Ruckelshaus Center process, Skagit County intends to use the results of this project to evaluate the severity of the riparian protection situation, and to prioritize and focus efforts to enhance riparian areas.

Riparian Mapping Project Summary and Conclusions (Pdf)

White Paper (Pdf)

 

For more information, please contact:

Josh Greenberg, PhD.
Skagit County GIS/Mapping
360-416-1168 · joshg@co.skagit.wa.us

 
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