Abstract

Elevated levels of dissolved solids in water (salinity) can result in numerous and costly issues for agricultural, industrial, and municipal water users. The Colorado River Basin Salinity Control Act of 1974 (Public Law 93–320) authorized planning and construction of salinity-control projects in the Colorado River Basin. One of the first projects was the Lower Gunnison Unit, a project to mitigate salinity in the Lower Gunnison and Uncompahgre River Basins.

In cooperation with the Bureau of Reclamation (USBR), the U.S. Geological Survey conducted a study to quantify changes in salinity in the Gunnison River Basin. Trends in salinity concentration and load during the period water years (WY) 1989 through 2004 (1989–2004) were determined for 15 selected streamflow-gaging stations in the Gunnison River Basin. Additionally, trends in salinity concentration and load during the period WY1989 through 2007 (1989–2007) were determined for 5 of the 15 sites for which sufficient data were available. Trend results also were used to identify regions in the Lower Gunnison River Basin (downstream from the Gunnison Tunnel) where the largest changes in salinity loads occur. Additional sources of salinity, including residential development (urbanization), changes in land cover, and natural sources, were estimated within the context of the trend results. The trend results and salinity loads estimated from trends testing also were compared to USBR and Natural Resources Conservation Service (NRCS) estimates of off-farm and on-farm salinity reduction from salinity-control projects in the basin. Finally, salinity from six additional sites in basins that are not affected by irrigated agriculture or urbanization was monitored from WY 2008 to 2010 to quantify what portion of salinity may be from nonagricultural or natural sources.

In the Upper Gunnison area, which refers to Gunnison River Basin above the site located on the Gunnison River below the Gunnison Tunnel, estimated mean annual salinity load was 110,000 tons during WY 1989–2004. Analysis of both study periods (WY 1989–2004 and WY 1989–2007) showed an initial decrease in salinity load with a minimum in 1997. The net change over either study period was only significant during WY 1989–2007. Salinity load significantly decreased at the Gunnison River near Delta by 179,000 tons during WY 1989–2004. Just downstream, the Uncompahgre River enters the Gunnison River where there also was a highly significant decrease in salinity load of 55,500 tons. The site that is located at the mouth of the study area is the Gunnison River near Grand Junction where the decrease was the largest. Salinity loads decreased by 247,000 tons during WY 1989–2004 at this site though the decrease attenuated by 2007 and the net change was a decrease of 207,000 tons.

The trend results presented in this study indicate that the effect of urbanization on salinity loads is difficult to discern from the effects of irrigated agriculture and that natural sources contribute a fraction of the total salinity load for the entire basin. Based on the calculated yields and geology, 23–63 percent of the estimated annual salinity load was from natural sources at the Gunnison River near Grand Junction during WY 1989–2007. The largest changes in salinity load occurred at the Gunnison River near Grand Junction as well as the two sites located in Delta: the Gunnison River at Delta and the Uncompahgre River at Delta. Those three sites, especially the two sites at Delta, were the most affected by irrigated agriculture, which was observed in the estimated mean annual loads. Irrigated acreage, especially acreage underlain by Mancos Shale, is the target of salinity-control projects intended to decrease salinity loads.

The NRCS and the USBR have done the majority of salinity control work in the Lower Gunnison area of the Gunnison River Basin, and the focus has been in the Uncompahgre River Basin and in portions of the Lower Gunnison River Basin (downstream from the Gunnison Tunnel). According to the estimates from the USBR and NRCS, salinity-control projects may be responsible for a reduction of 117,300 tons of salinity as of 2004 and 142,000 tons as of 2007 at the Gunnison River near Grand Junction, Colo. (streamflow-gaging station 09152500). USBR and NRCS estimates account for all but 130,000 tons in 2004 and 65,000 tons in 2007 of salinity load reduction. The additional reduction could be a reduction in natural salt loading to the streams because of land-cover changes during the study period. It is possible also that the USBR and NRCS have underestimated changes in salinity loads as a result of the implementation of salinity-control projects.

First posted August 29, 2012