This report presents methods and data for a Lagrangian sampling investigation into chemical loading and in-stream attenuation of inorganic and organic contaminants in two wastewater treatment-plant effluent-dominated streams: Boulder Creek, Colorado, and Fourmile Creek, Iowa. Water-quality sampling was timed to coincide with low-flow conditions when dilution of the wastewater treatment-plant effluent by stream water was at a minimum. Sample-collection times corresponded to estimated travel times (based on tracer tests) to allow the same "parcel" of water to reach downstream sampling locations. The water-quality data are linked directly to stream discharge using flow- and depth-integrated composite sampling protocols. A range of chemical analyses was made for nutrients, carbon, major elements, trace elements, biological components, acidic and neutral organic wastewater compounds, antibiotic compounds, pharmaceutical compounds, steroid and steroidal-hormone compounds, and pesticide compounds. Physical measurements were made for field conditions, stream discharge, and time-of-travel studies. Two Lagrangian water samplings were conducted in each stream, one in the summer of 2003 and the other in the spring of 2005. Water samples were collected from five sites in Boulder Creek: upstream from the wastewater treatment plant, the treatment-plant effluent, and three downstream sites. Fourmile Creek had seven sampling sites: upstream from the wastewater treatment plant, the treatment-plant effluent, four downstream sites, and a tributary. At each site, stream discharge was measured, and equal width-integrated composite water samples were collected and split for subsequent chemical, physical, and biological analyses. During the summer of 2003 sampling, Boulder Creek downstream from the wastewater treatment plant consisted of 36 percent effluent, and Fourmile Creek downstream from the respective wastewater treatment plant was 81 percent effluent. During the spring of 2005 samplings, Boulder Creek downstream from the wastewater treatment plant was 40 percent effluent, and Fourmile Creek downstream from that wastewater treatment plant was 28 percent effluent. At each site, 300 individual constituents were determined to characterize the water. Most of the inorganic constituents were detected in all of the stream and treatment-plant effluent samples, whereas detection of synthetic organic compounds was more limited and contaminants typically occurred only in wastewater treatment-plant effluents and at downstream sites. Concentrations ranged from nanograms per liter to milligrams per liter.