CLIMATIC CONTROL ON FLUVIAL LANDSCAPES IN THE LAKE TITICACA BASIN AS EVIDENCED BY REGIONALLY CORRELATIVE FLUVIAL HISTORIES OF THE RIO DESAGUADERO, RIO ILAVE, AND RIO RAMIS (BOLIVIA AND PERU)

Strata and terraces in the Rio Desaguadero (RD), Rio Ilave (RI), and Rio Ramis (RR) 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 and Ramis) 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.

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 Ilave valley (formed between ~4000 and 2500 BP and between ~2200 and 1600 BP) and one inset terrace in the Ramis valley (formed in the last 1457 yrs).