WSP06806

• File Number: 8276.450
• Description: McElmo Creek Unit - Colorado River Salinity Control Program
• State: CO
• Basin: Colorado Mainstem
• Water Division: 7
• Date: 5/1/1982
• Title: Possible Improvements for Onfarm Irrigation Systems to Reduce Salinity
• Water Supply Pro - Doc Type: Report/Study

MAY 1982

rolling plain with gently sloping hills, dissecting streams and occasional

steep-walled canyons. The elevation at Cortez is about 6,030 feet above

mean sea level.

Geology
Shale of the Mancos Formation underlies much of the Montezuma Valley.

To the north and west, the valley is bordered by outcrop areas of sandstone

of the Dakota Formation and sandstone and shale of other older sedimentary

formations as well as small areas of younger volcanic rocks as shown in

Figure 111-2. The Mancos Formation consists mainly of gray silty clay shale

of marine origin. It is the principal source of salt in the Me Elmo Creek

area. The salts are concentrated in certain areas within the shale and the

younger shale-derived alluvial deposits, usually at depths of a few feet

below the ground surface in the lower more arid portions of the valley.

Here, natural infiltration of precipitation and runoff water was not

sufficient to leach the salts froD the soil and bedrock profile. However,

the application of irrigation water has increased the amount of water moving

through the soil profile and has greatly increased salt loading in Mc~lmo

Creek.

Another smaller source of salt in the area is concentrations of salt

associated with the coal bed lying in the upper part of the Dakota

Formation. This coal bed is overlain by several feet of resistant sandstone

and only infrequently can be seen in outcrops or road cuts. Where this

sequence of coal and sandstone lies near the surface in irrigated areas, the

slow infiltration of irrigation water through fractures in the sandstone and

the coal bed results in the gradual leaching and the delivery of salts to

the stream system.

111-2

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