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Last modified
1/26/2010 4:16:55 PM
Creation date
8/6/2007 10:04:55 AM
Metadata
Fields
Template:
Water Supply Protection
File Number
8271.300
Description
Colorado River Basin Salinity Control Program - CRBSCP - General Information-Publications
State
CO
Basin
Colorado Mainstem
Water Division
5
Date
7/6/1998
Author
Nancy J Bauch - Noeman E Spahr
Title
Salinity Trends in Surface Waters of the Upper Colorado River Basin - Colorado - Nancy J Bauch and Norman E Spahr - 07-06-98
Water Supply Pro - Doc Type
Report/Study
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<br />001933 <br /> <br />BAUCH & SPAHR: SALINITY TRENDS IN SURFACE WATERS OF THE COLORADO RIVER BASIN <br /> <br />653 <br /> <br />results. Monthly trends in dissolved-solids loads mostly <br />were downward and highly significant, with P values <br />generally :s0.001, whereas significance levels and P val- <br />ues for annual trends were more variable (Tables 4 and <br />5). In any statistical test, including trends tests, sample <br />size affects P values. For a given trend magnitude and <br />data variability, P values tend to decrease as sample <br />size increases and time period lengthens, and it becomes <br />more likely that a trend will be indicated when a trend <br />is actually present. Thus, more trends are reported for <br />monthly loads than for annual loads. Trend results also <br />depend on the periods chosen for analysis. As presented <br />in Tables 3 to 5, trend results can differ depending on <br />the period of record selected for analysis. Helsel (1993) <br />states that "trend is not a static phenomenon." <br />There were fewer upward or downward trends for <br />1980 to 1993, compared to other analysis periods. Most <br />of the results for dissolved-solids concentrations and <br />loads for 1980 to 1993 indicated no significant trends. <br />During 1980 to 1993, streamflow in the Colorado and <br />Gunnison River Basins was highly variable. Streamflow <br />was above normal in 1983 to 1986 and below normal in <br />1988 to 1992. The results of the streamflow adjustment <br />of loads and concentrations for 1980 to 1993 may not <br />have fully reflected the extreme variability in stream- <br />flow during that time, masking some trends in monthly <br />loads as a result. <br />In the Gunnison River Basin, results for the Uncom- <br />pahgre River at Delta site indicated significant upward <br />trends in periodic dissolved-solids concentrations and <br />monthly and annual dissolved-solids loads for 1986 to <br />1993. Because this site is located downstream from the <br />Lower Gunnison Basin Unit salinity-control projects, <br />downward trends in dissolved-solids concentrations and <br />loads would be expected rather than upward trends. <br />With the Lower Gunnison Basin Unit salinity-control <br />work beginning in 1988 and 1990, the analysis period <br />of 1986 to 1993 probably is not appropriate for study <br />of trends at the Uncompahgre River at Delta site. The <br />first 2 yr of study, 1986 and 1987, were prior to any <br />salinity-control work in the basin, and 1988 to 1993 may <br />be too short a period to show changes in salinity relative <br />to salinity-control work. Another factor affecting the <br />trend results may be Ridgway Reservoir (Fig. 1). Lo- <br />cated on the Uncompahgre River upstream from Delta <br />and the Lower Gunnison Basin Unit salinity-control <br />projects, this reservoir came on-line in 1988. Whereas <br />most of the dissolved-solids load enters the Uncompah- <br />gre River system downstream from the reservoir, the <br />reservoir may have some effect on streamflow in the <br />river. This could affect the results ofthe flow-adjustment <br />procedure, which then could affect the trend test results. <br /> <br />Land-Use Changes <br /> <br />Another factor that could be affecting salinity levels <br />in the Upper Colorado River Basin is the change in <br />land use from agriculture to urban. When subsurface <br />irrigation return flow picks up dissolved solids as it per- <br />colates through salinity-bearing formations like the <br />Mancos Shale, the dissolved solids are carried along <br />in the groundwater-flow system until discharged into <br />surface streams. In most counties in the basin, there <br /> <br />have been changes in land use from agriculture to urban. <br />In Pitkin County, where there are outcroppings of the <br />Mancos Shale and equivalent formations, the popula- <br />tion increased 20% from 1 July 1986 to 1 July 1993. <br />Similarly, the population of Garfield County increased <br />21.6% for the same time period (Bureau of Census, <br />1986, 1993). Depending on how the new commercial or <br />residential urban lands that overlie saline materials are <br />developed and irrigated, less subsurface percolation of <br />water may occur and less saline groundwater may enter <br />into streams in the basin, or there may be more percola- <br />tion and greater saline groundwater, draining into sur- <br />face-water streams. The change from agricultural land <br />use to urban land use could account for part of the <br />downward trends in dissolved-solids concentrations and <br />loads in the basin. <br /> <br />Conflicting Results <br /> <br />Two sites in the study showed conflicting trend results <br />for concentrations and loads: the East River at its conflu- <br />ence with Taylor River and the Colorado River near <br />Dotsero. For the East River site, the trend in flow- <br />adjusted periodic dissolved-solids concentrations for <br />1986 to 1991 indicated increasing concentrations, <br />whereas the monthly load trend indicated decreasing <br />loads. Trends were based on 6 yr of data, and the short <br />period of analysis may have affected trend results. The <br />trend in monthly dissolved-solids loads for the Colorado <br />River near Dotsero site for 1986 to 1993 indicated in- <br />creasing loads, whereas the trend in annual loads for <br />the same analysis period indicated decreasing loads. Ad- <br />ditional study is needed to determine why this anomaly <br />is observed. <br /> <br />Study Needs <br /> <br />Further information about groundwater contribution <br />of major ions, dissolved solids, and land-use changes is <br />needed to more completely understand salinity trends <br />in the Colorado River Basin. Analysis of flow and salt <br />discharge in the springs around Glenwood Springs could <br />determine if the springs are affecting trends at the Colo- <br />rado River near Cameo site. Information about dis- <br />solved solids in the tributaries between Glenwood <br />Springs and Cameo also could help explain the down- <br />ward trends in dissolved solids reported at the Cameo <br />site by Butler (1996). For some sites, longer periods of <br />records of dissolved-solids data could provide additional <br />data that could result in better flow-adjustment relations <br />or better load estimates. Additional data also could help <br />verify the upward trends in dissolved-solids concentra- <br />tions and loads for the Uncompahgre River at the Delta <br />site for 1986 to 1993 and could allow a change in the <br />analysis period to better- match the timing of the Lower <br />Gunnison Basin Unit salinity-control work. The effects <br />that the agricultural to urban land-use change have had <br />on dissolved solids in the Colorado and Gunnison River <br />Basins need to be determined. Such studies will increase <br />the knowledge about dissolved~solids concentrations, <br />loads, and trends in the Upper Colorado River Basin <br />in Colorado. <br />
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