The Water Department Historical Files, an artificial collection of public documents that were gathered in the process of writing an agency history, contains two types of records. The first type is comprised of the records that Mary McWilliams compiled from several sources within the department. These include correspondence, reports, leases, ordinances, specifications, articles and speeches, contracts, financial records, and other records relating to the history of the Water Department and the City's water system. The second type consists of McWilliams' research notes and a typescript of her book manuscript.

The departmental records that McWilliams pulled together are a valuable compilation of materials that cannot be found elsewhere. Examples are Superintendent Luther B. Youngs' correspondence (1906-1923); various reports on the water system and water-related issues by Benezette Williams, Reginald H. Thomson, and Virgil Bogue; forest management correspondence and reports by the department's forester, Allen E. Thompson; health and sanitation records related to the watershed; and documents providing information on logging and railroads in the watershed. The records also provide a glimpse at the organizational structure of the department, municipal utility finances, and other detailed minutiae including equipment inventories and civil service issues.

A small body of material was added to the collection after McWilliams finished the book. Presumably this was done with the idea that the collection would serve as a living body of records that would serve a reference function for the department.

McWilliams' research notes include drafts of topical sections, partial transcriptions of interviews, transcribed copies of records that she was not allowed to compile into this collection, and various other notes. Examples include 13 pages she transcribed from the 1916 Joint Report of the City Engineer and Superintendent of Water Works, and a 1934 report by R.H. Thomson on the possibility of incorporating the Tolt River watershed into the City's water system. In using these notes, researchers should be aware the dates given to the folders are the dates McWilliams compiled the notes. The dates of issues and events covered by the notes are often found in the folder titles.