Hydrology has had the good fortune of attracting the intellectual interest of Harold A. Thomas, Jr. Ever appreciative of the work of others, he claims to be no more than a part-time hydrologist. But only under the narrowest definition of hydrology does the claim have merit. The momentum provided by the Harvard Water Resources Program spurred hydrology to reach beyond its traditional interests in physics, chemistry, meteorology, and geology and to embrace the economic, social, and political sciences. Thomas was one of the principals of that program, instrumental in helping to lay the quantitative foundation that today supports the nation's water planning and management activities.