Critical creek health assessment in the Duwamish River urban watershed

Background/Question/Methods

Efforts to restore the Duwamish River, a federal superfund site in Seattle, WA, depend upon both remediation of legacy contaminants and assessment of the health of the surrounding urban watershed.  Our research focused on the ecology of urban creeks that make up the watershed to expand on work dedicated to the river itself.  To detect patterns in water quality parameters, we compared multiple creeks and locations within those creeks.  We measured aquatic environmental variables at headwater, midstream, and outfall locations on six creeks: Codiga, Hamm, Moses, Marra, Longfellow, and Puget.  Variables measured include: ammonia, nitrate, nitrite, total nitrogen, phosphate, total phosphorus, salinity, temperature, chemical oxygen demand (COD), copper, turbidity, and dissolved oxygen (DO).  We hypothesized that (A) outfall locations would have lower water quality than headwaters at all creeks due to accumulation of nutrients and contaminants; and (B) creeks surrounded by more urban development would have lower water quality due to the potential for greater runoff volume. 

Results/Conclusions

Examining creeks from the headwater to the outfall, nitrate decreased while ammonia increased, which may indicate how runoff into creek headwaters undergoes reactions as part of the nitrogen nutrient spiral.  COD also increased towards creek outfalls, suggesting potential enhanced bacterial activity with accumulation of nutrients in runoff.  Significant differences among creeks were found for ammonia, nitrate, COD, DO, and total nitrogen, but patterns did not directly correlate with levels of surrounding development or percent impervious surface as determined by satellite image analysis.  Cluster analysis of creeks based on all environmental variables revealed the creeks outside the designated superfund site (Codiga and Moses) to differ from the rest, while the two creeks that traverse industrial and residential development adjacent to the river were the most similar (Hamm and Marra).  Further statistical analyses revealed significant interactions between creeks and locations, suggesting that while some water quality patterns were consistent from headwaters to outfalls, other variables may be sensitive to more local watershed conditions.  Overall, our research provides an assessment of the ecological status of the creeks that make up the urban watershed for the Duwamish River, generating valuable data for restoration efforts critical to the future health of the river.