ABSTRACTThe ground‐water system in many of the irrigated areas of the Colorado River Basin is derived almost entirely from deep percolation of irrigation water and seepage from irrigation conveyance and tail‐water collection systems. Salt pickup rates from irrigated soils in the basin vary in the different areas. Among the high salt pickup areas is the Grand Valley in western Colorado, estimated at 8 tons/acre/year. Water entering the ground‐water supply from irrigation practices in the valley amounts to about 145,000 acre‐feet/year and contributes about 690,000 tons/year to the salt load of the Colorado River. Samples of base‐flow water from the weathered Mancos Shale aquifer in the valley vary in salinity from about 1,500 to about 9,000 mg/l with a mean of 4,100 mg/l; while water samples from alluvial aquifers range from 305 to 124,000 mg/l with a mean of about 11,500 mg/l. Base‐flow returning to the river in the drains and washes has concentrations that average about 4,200 mg/l.Water losses and quantities of irrigation return flows can be reduced by improving farm irrigation efficiencies and by partial or complete lining of canals, laterals, and ditches. Increasing on‐farm irrigation efficiency through system improvements and irrigation scheduling is the most cost‐effective measure. However, achievement of higher efficiencies will require changes in water laws to encourage conservation and revised water pricing policies that d