This report summarizes historical water-quality data for six National Park units that compose the Rocky Mountain Network. The park units in Colorado are Florissant Fossil Beds National Monument, Great Sand Dunes National Park and Preserve, and Rocky Mountain National Park; and in Montana, they are Glacier National Park, Grant-Kohrs Ranch National Historic Site, and Little Bighorn Battlefield National Monument. This study was conducted in cooperation with the Inventory and Monitoring Program of the National Park Service to aid in the design of an effective and efficient water-quality monitoring plan for each park. Data were retrieved from a number of sources for the period of record through 2004 and compiled into a relational database. Descriptions of the environmental setting of each park and an overview of the park's water resources are presented. Statistical summaries of water-quality constituents are presented and compared to aquatic-life and drinking-water standards. Spatial, seasonal, and temporal patterns in constituent concentrations also are described and suggestions for future water-quality monitoring are provided.