During 1936 to 1965 water resources research emphasized quantitative, model-driven approaches to planning and management. The period saw a shift toward probabilistic storage design, using long records of annual discharge to determine reservoir storage that guarantees a specified draft, and the integration of hydraulic geometry, drainage evolution, and geochemical interpretation to understand river form and watershed dynamics. These methods created a cohesive framework linking storage planning with process-based hydrology and water-quality reasoning, supporting more resilient resource decision making.