Watershed characteristics, study streams, sample sites, mills, and mill effluents are provided for 4 streams included in a long-term study to assess potential effects of pulp and paper mill effluents on US receiving waters. The study streams are Codorus Creek (Pennsylvania, USA), Leaf River (Mississippi, USA) and McKenzie and Willamette rivers (Oregon, USA) and were chosen to represent a blend of mill process types, effluent concentrations, and coldwater/warmwater stream systems. The described effluent quality, water quality, and habitat data sets encompass the initial 7 to 8 y of a study anticipated to continue >10 y and provide a backdrop to a series of articles describing periphyton, macroinvertebrate, and fish community properties in these same streams. The mean in-stream waste concentration (IWC) for these 4 effluent discharges was 32.4%, 2.0%, 0.5%, and 0.2% v/v for Codorus Creek and Leaf, McKenzie, and Willamette rivers, respectively, as compared with a median of 0.4% for US mills. Effluent quality measurements included Selenastrum capricornutum, Ceriodaphnia dubia, and Pimephales promelas chronic bioassays as sanctioned by the US Environmental Protection Agency for estimating effluent effects on receiving-water aquatic communities. Based on mean bioassay inhibition concentration for a 25% effect and on mean IWC, a margin of safety against adverse biological effects of 2, 25, 137, and 150 times was indicated for Codorus Creek and Leaf, McKenzie, and Willamette rivers, respectively. Habitat and water quality assessment was carried out over a gradient of sample sites above and below the effluent discharge to determine nonmill-related conditions that might interfere with interpretation of effluent effects. Noneffluent related localized differences in conditions for some parameters, including current velocity (McKenzie River), and surface incident photosynthetically active radiation (Codorus Creek and Willamette River) occurred at the sample stations immediately upstream or downstream of the effluent discharge. In addition, broader watershed differences were evident on Codorus Creek, where a relatively rich riparian corridor and stream structure occurred upstream in contrast to areas of canopy and stream-structure loss in the downstream urban area. The mill effluent discharges contributed to increases in receiving-water color and conductivity, although upstream tributaries contributed additional conductivity to Codorus Creek and color to the Leaf River. The McKenzie River provided the only example of a nutrient increase immediately downstream of a mill discharge. This increase in total nitrogen (0.11 vs 0.16 mg/L) could not, however, be differentiated with respect to whether it was of mill effluent or tributary stream origin. Tributary streams were potentially important total nitrogen contributors on Codorus Creek and the Willamette River. As an integrated study, the effluent quality and physical/chemical watershed descriptions provided here represent 1 component of the broader study addressing potential point-source effluent effects within the context of the larger watershed and a multiyear timescale. The absence of effluent-related in-stream chemical/physical responses, other than increases in conductivity and color, and a considerable bioassay-based margin of safety, provides for a working hypothesis that there will be no effluent-related biological population/community responses from these 4 mill discharges. This hypothesis, as it relates to periphyton, macroinvertebrate, and fish communities, will be addressed in other articles in this series.