Climatic control on fluvial landscapes in the Lake Titicaca Basin as evidenced by regionally correlative fluvial histories of the Rio Desaguadero, Rio Ilave, Rio Ramis, and Rio Huancane (Bolivia and Peru)
by
Catherine A. Rigsby
East Carolina University, U.S.
Coauthors: Renee L. Farabaugh (Idaho State University, U.S.), Paul A. Baker (Duke University, U.S.), Mark S. Aldendefer (University of California, Santa Barbara, U.S.)

Strata and terraces in the Rio Desaguadero (RD), Rio Ilave (RI), Rio Ramis (RR), and Rio Huancane (RH) valleys suggest that climate variability is the primary control on fluvial landscapes in the Lake Titicaca (LT) region. Specifically, base-level (controlled by precipitation) has overwhelmed smaller-scale variables, resulting in regionally correlative sedimentation and terrace formation in these otherwise dissimilar tributary (Ilave, Ramis, and Huancance) and distributary (Desaguadero) valleys.

Freshwater lacustrine clays in the RR valley were deposited before 40,170 BP, occur 150m above LT, and are correlative with paleolake "Minchin." Desiccation of this large paleolake resulted in rapid, but episodic, fluvial downcutting of the RR valley between 40,170 and 37,700 BP. Subsequent fluvial aggradation occurred between 35,140 and 10,421 BP. The aggradation was episodic (paleosols formed during brief equilibrium periods) and coincident with periods of high water levels in LT. In the RI valley, fluvial aggradation started before the LGM (cold and wet in the LT basin) and lasted until at least 8250 BP. In the RD valley, correlative strata are dominantly lacustrine, but a short period of fluvial aggradation is documented between 16,000 and 14,000 BP. Similar, but as yet undated, lacustrine strata are present in the RH valley to the north.

During the region's most arid period (7900 to 4500 BP), lake level dropped 85m, tributary rivers were severely downcut, and the RD was braided. RI valley downcutting probably began by 8000 BP and fill-cut terraces in the RI and RR valleys suggest that downcutting continued in both valleys between 6000 and 4500 BP. A single fill-cut terrace in the southern RD valley is underlain by braided fluvial sediments dated from ~7000 to 3200 BP.

At ~4500 BP, increasing precipitation caused LT to rise. The lake overflowed into the RD valley at ~3500 BP. In the northern RD valley, episodic aggradation (lacustrine deposition from ~4525 to 3900 BP and from ~2200 to 2000 BP) and downcutting (fill-cut terraces at ~4000, 3600, and after 2000 BP) were intitiated by minor changes in LT outflow. The tributary valleys also responded to these more recent changes in base level. Increasing base-level, for example, is recorded by the aggradation of two inset terraces in the RI valley (formed between ~4000 and 2500 BP and between ~2200 and 1600 BP) and one inset terrace in the RR (formed in the last 1457 yrs) and RH valleys.

The fluvial history of these river valleys relates to the region’s pre-ceramic through Tiwanaku archaeological records. Archaeological evidence indicates that humans occupied these valleys as early as 10,000 BP, a time when the valleys were relatively wet and LT was high, but erratic. In the RI valley the highest terraces were occupied for at least 5000 years and occupied the lower terraces from after ~ 4400 to 3700 BP. In the RR valley, cochas (currently used for agriculture and grazing animals) are located preferentially on the highest terrace. This terrace is held up by impermeable paleolake strata, suggesting that, although the cochas have been certainly manipulated by humans, their location and orientation may be primarily stratigraphically controlled.

Date received: November 15, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caod-16.