The Philadelphia Water Department (PWD) is currently completing Source Water Protection Assessments (SWA) for eight drinking water intakes on the Delaware River. One of the intakes, PWD's Baxter drinking water intake, is located on the Delaware River downstream of Trenton, in a portion of the river that is tidally influenced. Because of its locations, the Baxter intakes present several unique challenges, and the resulting Source Water Assessment is one of the most complex of any assessment in the country. The contributing watershed covers over 3,000 square miles, and includes numerous tributaries. Watershed land use ranges from heavily urban (Philadelphia, Trenton), to suburban areas north of Philadelphia, to rural and undeveloped areas near the New York/Pennsylvania border. This case study shows, however, that it is not only possible to do a thorough and effective SWA, but that the size of the watershed does not necessarily mean that the assessment needs to oversimplify. In fact, the Delaware River SWA would not have provided any useful results without addressing the inherent complexities. The primary technical difficulties that were overcome included: developing a database and collecting enough data throughout the watershed to identify potential sites; programming vital links to a geographic information system to calculate the time of travel from each potential source to each intake; assessing pollutants carried by storm water by developing a storm water model using the US Environmental Protection Agency's Storm Water Management Model (SWMM); effectively dealing with the issue of intakes influenced by tidal oscillations within the estuary using a tidal zone water quality model; using a decision software package EVAMIX to successively screen point sources, and to simultaneously evaluate point and nonpoint sources in a single evaluation; and, including stakeholder participation in the process of evaluation to solidify cooperation of upstream communities. The complex approach needed to integrate all the diverse elements of the study into a coherent assessment was developed to meet the challenge of producing over 50 SWAP reports for intakes on both the Delaware River and Schuylkill River. It represents a template for developing a consistent, comprehensive assessment for large river systems, and its use for the Baxter Intake SWA is an excellent case study to demonstrate a successful application. Includes 3 references, table, figures.