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Summary of Colorado State University Research in The Lower Arkansas River Vallev. Colorado . Timothy K. Gates, Civil Engineering Department, Colorado State University Without sound and timely intervention, the Colorado's Lower Arkansas River Valley and similar areas in the western U.S. will eventually succumb to the ill effects of waterlogging and salinization, both on the land and in the larger river ecosystem. Solutions based upon accurate knowledge offield conditions, and in compliance with legal and economic constraints, will be needed to insure sustain ability of the valley's productive agricultural base, to preserve and revitalize its rural communities, and to enhance the overall river environment. Beginning in 1998, Colorado State University has conducted extensive field and modeling studies of the irrigation-stream-aquifer system of the lower Arkansas Valley, More than 120 farmers, and numerous agencies have cooperated to make the research possible. The long-term goal is to redress salinity-related water quality degradation in the Arkansas River Basin of Colorado through widespread adoption of water management practices that will (a) reduce detrimental waterlogging and salinity impacts to agriculture in the Arkansas River watershed, (b) enhance water quality in the Arkansas River by diminishing nonpoint source salt and pollutant (e.g, selenium and iron) loads, and (c) lead to real water conservation in the river by possibly reducing nonbeneficial up flux from high water tables, The prospects for actually achieving this goal are heightened by the extensive database and calibrated modeling tools that already have been developed to date by our research, These models need to be refined and expanded, working toward a comprehensive set of tools that will support wise water management decisions, not only at the field and regional levels, but also at the river-basin scale, An important aspect of the next phase of our research is to address the need for more detailed knowledge of conditions at the field scale, where the "nuts-and-bolts" changes in water management must take place, This requires enhanced understanding of actual crop water demands, under saline conditions, and the efficiency with which these demands are being met by current irrigation practices, Achieving valley- wide reduction in salinity and waterlogging on the land, nonpoint-source loading to the river, and nonbeneficial consumptive use will require strategic implementation of available field-level improvements (increased irrigation efficiency, reduction in canal seepage, etc.) in the best combination over space and time. Such remedial strategies cannot be adopted independently, however. . Actions taken by farmers at the field and regional scales should be informed by guidelines based upon valley-wide objectives and constraints. The entire lower valley system is an interlocking web of scale- dependent components, in which local changes ripple upstream and downstream via irrigation-stream- aquifer interactions and water rights issues. Hence, another prong of our on-going studies seeks to complete the calibration and testing of regional-scale and basin-scale models. These models are being developed to simulate (a) flow and salt transport processes in the irrigation-stream-aquifer system under constraints of Colorado water law and the Arkansas River compact, and (b) the consequent performance measures (increased net economic return from increased crop yield derived from reduced waterlogging and salinity, reduced salt concentration in the river, increased volume of water "liberated" from nonbeneficial consumptive use) associated with current conditions and with alternative improvement strategies across the Valley. .