Salinity in the Colorado River in the Grand Valley, western Colorado, 1994-95

Seven and one-half square miles, or 4864 acres, of the Grand Valley in Western Colorado consists of high water-using landscapes. Overirrigation of Grand Valley soils flushes 580,000 tons of salt into the Colorado River each year. These salts negatively impact plant and animal health throughout the Colorado River basin. Proper watering of lawns can significantly reduce this problem. Correcting problems with a sprinkler irrigation system can reduce water use by an average of 40%. If water use on all 4864 acres was reduced by 40%, a savings of 11,187 acre feet, or over 3.6 billion gallons, of water would result. The annual historical evapotranspiration (ET) rate for the Grand Valley of Western is ≈61 inches; the ET rate during the irrigation season (April through October) is ≈49 inches. Since a typical sprinkler system is ≈70% efficient, in order to apply 49 inches of water to the soil, ≈70 inches of water is required. Irrigation system problems such as improperly spaced heads, sunken heads and heads not adjusted are typically responsible for 40% more water being applied than necessary. In Western Colorado, this equates to an over-application of 28 inches (2.3 acre feet) of water being applied each year. The 2005 Western Colorado irrigation audit problem covering 18.7 acres of turf. Assuming the problems noted were all corrected, a water savings of 43 acre feet, or 14,013,797 gallons, of water resulted. The 2006 audit program will continue this educational and water-saving effort. A grant from the Department of the Interior-Bureau of Reclamation will help fund the 2006 Irrigation Audit project.

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A constraint on post–6 Ma timing of western Grand Canyon (Arizona, USA) incision removed: Local derivation indicated by ca. 5.4 Ma fluvial deposits below Shivwits Plateau basalts north of Grand Canyon

Geosphere ◽

10.1130/ges02353.1 ◽

2021 ◽

Author(s):

A.T. Steelquist ◽

G.E. Hilley ◽

I. Lucchitta ◽

R.A. Young

Keyword(s):

United States ◽

Detrital Zircon ◽

Colorado River ◽

Landscape Evolution ◽

Colorado Plateau ◽

Grand Canyon ◽

River System ◽

Fluvial Deposits ◽

Western Colorado ◽

Rule Out

The timing of integration of the Colorado River system is central to understanding the landscape evolution of much of the southwestern United States. However, the time at which the Colorado River started incising the westernmost Grand Canyon (Arizona) is still an unsettled question, with conflicting interpretations of both geologic and thermochronologic data from western Grand Canyon. Fluvial gravels on the Shivwits Plateau, north of the canyon, have been reported to contain clasts derived from south of the modern canyon, suggesting the absence of western Grand Canyon at the time of their deposition. In this study, we reassess these deposits using modern geochronologic measurements to determine the age of the deposits and the presence or absence of clasts from south of the Grand Canyon. We could not identify southerly derived clasts, so cannot rule out the existence of a major topographic barrier such as Grand Canyon prior to the age of deposition of the gravels. 40Ar/39Ar analysis of a basalt clast entrained in the upper deposit (in combination with prior data) supports a maximum age of deposition of ca. 5.4 Ma, limiting deposition to post-Miocene, a period from which very few diagnostic and dated fluvial deposits remain in the western Colorado Plateau. Analysis of detrital zircon composition of the sand matrix supports interpretation of the deposit as being locally derived and not part of a major throughgoing river. We suggest that the published constraint of <6 Ma timing of Grand Canyon incision may be removed, given that no clasts that must be sourced from south of Grand Canyon were found in the only known outcrop of gravels under the Shivwits Plateau basalts at Grassy Mountain north of Grand Canyon.

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