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2.11 inches for July and August (data obtained from the Western Regional Climate Center in Reno, Nevada, at http://www.wrcc.dri.edu/ accessed November 2001). The rain may have caused an unusual amount of flushing of soluble salts from the soil that could increase selenium and salt loading the Montrose Arroyo area during the post-project period. If that happened, then some of the load reduc- tions resulting from the lateral project could be offset by increased loads caused by the excess rainfall. Comparison of the individual samples collected during this period to samples from the summer of 2000 does not indicate any unusual changes in loads during July- September 1999. A potential effect on selenium and salt loads in the Montrose area is conversion of agricultural land to residential and commercial development. However, the effect of land-use changes on selenium and salt loads has not been documented for the study area. Concerns have been expressed by members of the Task Force and at public meetings that new development, especially on previously nonirrigated land in shale areas, might contribute new selenium and salt loading to the Uncompahgre and Gunnison Rivers. New loading sources could partly offset some of the load reductions that result from remediation projects such as the Montrose Arroyo demonstration project. In November 2000, the USGS began collecting monthly samples at sites MA3 and MA2 for the NIWQP to monitor long-term changes in selenium and salt loads in Montrose Arroyo. SUMMARY Selenium and salinity are water-quality issues in the Upper Colorado River Basin. The east side of the Uncompahgre Valley in west-central Colorado contains soils and bedrock with high salt and selenium concentrations. Irrigation in the area has increased salt and selenium loads to the Gunnison River. The National Irrigation Water Quality Program (NIWQP) and the Gunnison Basin Selenium Task Force were examining remediation methods to reduce selenium loading in the lower Gunnison River Basin because some water bodies in the basin have selenium concen- trations that exceed the State water-quality standard (5 µg/L). Lining or placing irrigation canals and laterals in pipe to eliminate canal and lateral leakage has been used to reduce salt loads in the Colorado River Basin. A demonstration project was initiated in 1998 to replace 8.5 miles of open-ditch laterals (five different laterals) with pipe and determine the effects on selenium and salt loads in the Montrose Arroyo Basin. Montrose Arroyo has high selenium and salinity concentrations and is significantly affected by irrigation drainage. Biweekly samples were collected from June 1998 through October 2000 by the U.S. Geological Survey (USGS) to monitor selenium and salt loads at five sites in the Montrose Arroyo Basin. Data were separated into pre-project and post-project data sets for each site to determine the effects of the lateral project on selenium and salt loads. Water-quality data were analyzed to estimate surface-water and ground- water loads. Changes in ground-water loads were used to estimate the effects of the lateral demonstration project. The placing of five laterals in pipe caused a significant reduction in selenium loads in the post- project period at all monitoring sites except at the control site, where selenium loads did not change. At the outflow monitoring site on Montrose Arroyo (MA2), the selenium load decreased about 194 lb/yr, or 28 percent of the pre-project load, and more than 90 percent of that decrease was in the ground-water load. The selenium load decreased more than 50 percent in a tributary drainage ditch (site D2MA). Salt load also decreased in the post-project period at all monitoring sites except the control site. The percent decreases in salt loads were smaller than the percent decreases in selenium loads. The decrease in salt load at the outflow site on Montrose Arroyo (MA2) was about 1,800 tons/yr (ground-water portion of the total load). If a small increase in salt load at the control site is accounted for, the estimated salt-load decrease in Montrose Arroyo was about 1,980 tons/yr. All of the potential effects of the lateral demon- stration project on selenium and salt loads might not have been measured at the monitoring sites. Part of the leakage from some of the laterals prior to their being placed in pipe did not necessarily discharge into Montrose Arroyo upstream from Niagara Street. The lateral project had a greater affect on selenium loads than on salt loads. The greater decrease in selenium loads relative to salt loads may have been partially the result of decreases in selenium concentrations in the ground water in some areas of the demonstration-study area in addition to the decreases in loads caused by decreases in ground-water discharge. SUMMARY 13