Streamflow characteristics change over time as a result of water resources development and management projects, water use, watershed land use changes, and climate changes. The main objective of this thesis is to assess the significance of the impacts of human activities such as construction of reservoirs, water supply diversions, increased water use and return flows on streamflows by the recently completed Texas WAM (Water Availability Modeling) system. The major river basins in the state of Texas were selected as suitable study basins. The particular objective is accomplished by the assessment of WAM monthly and annual naturalized and regulated flows, based on using the WRAP (Water Rights Analysis Package) model, which represents the river/reservoir management model. WAM flow frequency analysis was performed for the simulated flows. The flow ratio indices developed showed the divergence of the actual flows from their natural behavior for the entire monthly flow frequency flow spectrum ranging from minimum flows to high flows. This study describes the combined effects of reservoir construction, increased water use, water resources development projects and land use changes on the river flow regime.