2017 Nova Science Publishers, Inc. All rights reserved. Flows of rivers throughout the world have been altered by population and economic growth and accompanying water resources development. River flow characteristics are changed by construction of dams and other facilities to control floods, generate hydroelectric energy, and provide reliable water supplies, diversions for agricultural, municipal, and industrial needs, and return flows from surface and groundwater supplies. Impacts on hydrology and water availability associated with climate change due to global warming are also of major concern in hydrologic science and water management. Alterations in flow characteristics differ greatly across the spectrum from low flows to median flows to infrequent extreme flood flows. Gradual permanent increases or decreases in stream flow may be difficult to detect due to the great continuous natural variability that hides long-term trends. This chapter reviews studies of flow alterations found in the literature and then employs a modeling system and databases for the river systems of Texas to investigate characteristics of river flows and long-term impacts thereto resulting from development and other causative factors. Researchers have applied statistical trend analysis methods, watershed precipitation-runoff models, and river-reservoir system management models to quantify flow alterations for river systems throughout the world. Flow alterations in Texas are illustrative of river systems in many other regions of the world. A water availability modeling system developed to manage water resources in Texas provides a unique opportunity to explore alterations to river flows for a broad diverse range of climate and hydrologic conditions, population and economic growth, and water resources development and management practices. Trends of long-term changes in precipitation during 1940-2015 are not evident in Texas. Flows for reaches of major rivers have been impacted significantly, in some cases dramatically, by water development. The flow increases and decreases vary greatly with location.