||In 2000-01, the U.S. Geological Survey sampled the Clackamas River and its major lower-basin tributaries for 86 dissolved pesticides and selected breakdown products during storm runoff conditions. In all, 27 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 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 consistently detected with greater frequency and at higher concentrations in the four lowermost tributaries (Deep, Richardson, Rock, and Sieben Creeks). In these streams, a total of 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-the insecticide diazinon and the herbicides 2,4-D, atrazine, dichlobenil, diuron, imazaquin, metolachlor, simazine, and trifluralin-were detected at relatively low concentrations in five samples of Clackamas River water. Despite these detections, no pesticides were detected in three samples of treated Clackamas River water used for drinking. 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. The greatest number of aquatic-life criterion exceedances occurred in Rock and Sieben Creeks, which drain both agricultural land and urban developments. Concentrations of three pesticides exceeded aquatic-life criteria in each of these streams, and concentrations of the organophosphate insecticides diazinon and chlorpyrifos exceeded their respective criteria by several-fold in both streams. Diazinon was detected in all of the four lowermost tributaries, at concentrations that exceeded the U.S. Environmental Protection Agency aquatic-life criterion (0.1 µg/L) in all five samples collected from Sieben and Rock Creeks. The highest diazinon concentration (0.160 µg/L measured in Sieben Creek in May 2000) exceeded the maximum concentration recommended by the National Academy of Sciences/National Academy of Engineering by a factor of 18. Such aquatic-life criteria consider only one pesticide at a time, and therefore do not evaluate the effects of multiple pesticides detected at once in a sample. To do this, a composite pesticide toxicity index (PTI) was used. The highest PTI values were found in samples collected from Rock and Sieben Creeks, where relatively fewer sensitive invertebrates were observed in other studies conducted in 1997-99. These two streams, with the highest pesticide concentrations and greatest number of pesticide detections, supported fewer high-quality invertebrates (1-10 taxa) compared with other streams sampled (15-31 taxa). During the May 2000 storm, two tributaries -Deep and Rock Creeks-contributed most (65% and 15%, respectively) of the total measured pesticide load from the 6 tributaries sampled. Most (90%) of the total measured pesticide load during the October 2000 storm could be accounted for by inputs from Sieben Creek alone. Note, however, that because of the dynamic nature of precipitation, rainfall runoff, and streamflow during storms, instantaneous pesticide loads constitute best estimates, not precise calculations. Identification of pesticide sources in the Clackamas River Basin is difficult because of the diverse land uses in the basin and the multiple-use nature of many of the pesticides detected. Of the 25 parent compounds detected, 22 have agricultural uses, 24 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. Based on current available crop acreages for Clackamas County and best estimates of chemical application rates in the Pacific Northwest, most (49%) of the pesticide estimated to be applied to agricultural lands in Clackamas County (for those detected) is used for nursery and greenhouse crops. Lesser amounts, ranging from 4 to 15%, are applied to pastureland, Christmas trees, alfalfa and hay fields, hazelnuts, and grass seed fields. Such estimates could be improved with more detailed pesticide use information. Although pesticide use data are available at the county level, such data are too coarse to identify particular areas with subbasins where pesticide contamination is occurring. Also, because only a small fraction of the thousands of pesticide products registered for use in Oregon were tested for during this study, pesticide use information would help ensure that high-use pesticides are included in future studies.