WSP04503

• File Number: 8142.600.20
• Description: Trinidad Project - Environmental Studies
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 3/1/1974
• Author: Corps of Engineers
• Title: Draft Environmental Statement - Trinidad Lake Project - Purgatoire River - Colorado
• Water Supply Pro - Doc Type: Report/Study

f. Geolo~y. The principal rocks of the upper
Purgatoire River basin range in age from pre-Cambrian through
Tertiary, with minor amounts of Quaternary gravels and alluvium.
Except for the small representation of pre-Cambrian plutonics and
metamorphics in the extreme headwaters of the basin, the only igneous

or metamorphic material present occurs as dikes and flows of Tertiary

age. The Pierre formation of Cretaceous age occurs along the area
of transposition from the foothills to the plains. Above the Pierre

are the coal-bearing Trinidad and Vermejo formations of Cretaceous

age whi~h form the abrupt break between the foothills and the plains.

The other Cretaceous formations form relatively flat mesas with

shallow soil mantles. The Cretaceous and Tertiary formations comprise

the foothills, which have steep slopes and narrow valleys.

g. Water Quality. Generally, the quality of surface

waters above Trinidad is good. .water flowing into Trinidad is

influenced or degraded primarily by natural geochemical processes,

the agricultural industry, past and, perhaps, current mining activities,

and the influence of small, localized areas of human habitation or

utilization. Water quality of the Purgatoire River is monitored by

the Colorado Department of Health, the results of which are presented

in Appendix C. Also included in the appendix are results of water

quality analysis conducted by the corps of Engineers. To insure the

quality of the resource, the State of Colorado has instituted atandards

which form the foundation for the classification of State waters and

subsequent enforcement. The following data reflects the application

for which the water can be, or i8 being used.

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