In 2000-01, the U. S. Geological Survey sampled the Clackamas River and its major lower-basin tributaries during storm runoff conditions for 86 dissolved pesticides and selected breakdown products. Twenty-seven compounds, including 18 herbicides, 7 insecticides, and 2 pesticide breakdown products, were detected in 18 stream samples. The most commonly detected pesticides, in decreasing frequency, included atrazine, simazine, diazinon, metolachlor, and diuron, which variously occurred in 46-92% of samples collected from the tributaries. Of these, atrazine, simazine, and metolachlor, plus six other compounds, also were detected in the main-stem Clackamas River.

Pesticides were detected more frequently and at higher concentrations in the four lowermost tributaries (Deep, Richardson, Rock, and Sieben Creeks). In these streams, 12 to 18 pesticides were detected per stream in samples collected during spring and fall. Pesticides always occurred with at least one other pesticide, and about half of the samples, including one sample from the Clackamas River in October 2000, contained six or more pesticides. Nine pesticides, including the insecticide diazinon and the herbicides 2,4-D, atrazine, dichlobenil, diuron, imazaquin, metolachlor, simazine, and trifluralin, were detected in five water samples of Clackamas River water. No pesticides were detected in three samples of treated Clackamas River water used for drinking-water supply. Concentrations of six compounds--carbaryl, chlorpyrifos, diazinon, dieldrin, malathion, and the breakdown product of DDT (p,p'-DDE)--exceeded established or recommended criteria for the protection of aquatic life in some of the tributaries, sometimes for multiple pesticides in one sample.

Identifying the sources of pesticides detected in the Clackamas River Basin is difficult because of the diverse land use in the basin and the multiple-use nature of many of the pesticides detected. Of the 25 parent compounds detected, 22 have agricultural uses, 23 have urban uses, 16 are applied to golf courses, 11 are applied along roads and other right-of-ways, and 5 have or had forestry applications. Because only a small fraction of the thousands of pesticide products registered for use in Oregon were tested for in this study, future monitoring could benefit from knowledge of what pesticides are applied so that potential problems can be identified and managed.

Summary
Introduction
Study Design, Methods, and Data Quality Control
Results and Discussion
    Stream Conditions During Sampling
    Pesticide Detections
    Potential Pesticide Source Areas
    Pesticide Concentrations Relative to
       Water-Quality Standards and Suggested
       Guidelines to Protect Human Health
       and Aquatic Life
    Potential Effects of Pesticideson Aquatic Life
    Potential Uses of Pesticides in the Clackamas
       River Basin
    Pesticide Management and Future Studies
Acknowledgements
References
Appendix