Case Study: Assessment of Historical Changes to Riparian Vegetation

Client: WRIA 31 Planning Unit Location: Rock Creek basin, WA Contact for water resource projects: Tim Flynn

Project Background & Challenges

Rock Creek, a salmonid-bearing stream, was added to the state’s 2004 303d list for impaired water quality because of high water temperature. The available evidence suggests that the elevated temperatures are due to natural conditions, at least in part. As part of the watershed planning process for WRIA 31, Aspect Consulting assessed water quality, documenting historical changes to the creek channel and surrounding vegetation. The objective was to assess whether the elevated water temperatures result from land use changes that reduced vegetation — and thus shade — in the valley. The study also provides insight about how much creek channel stability and shade might be reasonably attainable.

Solution

To evaluate historical changes to vegetation, we compiled and rectified four sets of aerial photographs (1938, 1969, 1996, 2002) that covered the mapped extent of salmonids in the mainstem of Rock Creek. We then used GIS methods to map vegetated areas based on spectral differences for each of the four periods and evaluated them statistically by year and stream reach. The results indicate that vegetation across the Rock Creek valley has increased and become more stable since 1938 — most likely because of fire-suppression over the past century. However, the analysis also showed that some reaches of the lower stream channel are highly dynamic, migrating across the valley floor regularly. These reaches are rocky, braided, and have little vegetative cover; consequently, confining them into a single channel — a suggested remedy for lowering stream temperature — would be infeasible. Likewise, planting vegetation to increase riparian shade may also be infeasible. Our work can help guide efforts under the State’s Total Maximum Daily Load (TMDL) program to reduce the temperature of Rock Creek. We also developed database software with a custom interface, which the Eastern Klickitat Conservation District (EKCD) can use to manage, evaluate, and report water quality data for Rock Creek.