Joseph Ray

Publication Date

12-1975

Advisor(s) - Committee Chair

E.E. Hegan, R.R. Dilamarter, R.H. Foster

Degree Program

Department of Geography and Geology

Degree Type

Master of Science

Abstract

This study deals with the Holocene environmental dynamics experienced by a seasonally wet-dry landscape of tropical Columbia. In light of a high ecological sensitivity, the study area is also analyzed as the resource of man. This is done by intensive study area reconnaissance, mapping, altimeter survey, and radiocarbon dating of fluvial landforms. Also, samples of soil, pasture, and water were collected for later study, and several interviews were completed with local residents.

The study area is a tropical lowland of northcentral Columbia at about 7 ½° north latitude. A maximum local relief about 600 m is maintained by a rather resistant lower Tertiary sandstone hogback. The well-faulted landform lies at an almost vertical attitude because of its boundary with the great Salinas fault, which was probably active during the Tertiary-Quaternary orogeny of the Cordillera Oriental.

Through this landscape flows the Río Lebrija, a major local base level at about 180 m elevation. Draining about 2,700 km2 of the Cordillera’s western flank, this valley contains relict evidence of major ecological fluctuations during Quaternary time. Midway through the Holocene, a marked thinning of plant cover, caused by peak Hypsithermal temperatures and resulting precipitation irregularities, permitted intense surface erosion of hillslopes. This process, maximized by the initiation of short wet periods, delivered considerable quantities of weathered materials to aggrading valleys. The sands and gravels deposited in areas protected from stream migration have formed the alluvial terraces seen today. Charcoal, collected from compact coarse sandy alluvium lying 15 m above the present Río Lebrija, was dated at 6,2535±90 and 5,365±85 B.P., a much younger date than suggested for correlative basin landforms. It appears that there is a need for reassessment of the assumed magnitude and rapidity of the recent, as well as ancient, dynamics undergone by tropical flora, erosion surfaces and fluvial systems. An important environmental concept appears to be a direct correlation between valley aggradation and progressive warmth in continental, tropical wet-dry regions.

Man’s activities are naturally destructive to this sensitive low-land environment. Great earthflows dated at 445±110 B.P. may have resulted from European-type deforestation of unstable slopes. The denudation of these rugged slopes continues today at an expanding rate. The message is reinforced after every heavy rain when the swift Lebrija swirls red-brown with yesterday’s soil.

The initiation of high protein agriculture on the rich pastured bottomlands might lessen the human pressures upon the most sensitive landscapes and would enhance possibilities for supplemental irrigation. Also, the powerful current of the Río Lebrija is a potential source of hydroelectric energy. The harnessing of this power by a proposed waterwheel-barrage facility would deal minimal alteration to the fluvial system, while supplying the community with limited irrigation and domestic energy. If successful, the utilization of land and water resources would be optimized, and the local people would be in a better position to adjust to weather anomalies initiated by an evolving environment. KEY WORDS: Colombia, Cordillera Oriental, tropical wet-dry climate, alluvial terrace, Hypsithermal “environmental flux,” and land use.

Disciplines

Earth Sciences | Geography | Geology | Hydrology | Physical and Environmental Geography | Physical Sciences and Mathematics | Social and Behavioral Sciences

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