Fluvial history of the Rio Ilave valley, Peru, and its relationship to climate and human history

Abstract

Fluvial strata and landforms in the Rio Ilave valley (Peru) document a history of Holocene aggradation and downcutting that is correlative with regional climatic events and provides an environmental context for human occupation of the river valley. Periods of aggradation correspond to periods of high (or rising) level in Lake Titicaca and elsewhere on the Altiplano, and increased sediment accumulation in the Rio Ilave valley. Downcutting episodes correspond to periods of low level in Lake Titicaca and low or rapidly decreasing sedimentation rates in the Ilave delta. There are five terrace tracts (T1 through T5) present in this southwestern Lake Titicaca tributary. These tracts occur as both paired and unpaired terraces and have average heights from 1.4 to 24.3 m above the valley floor. The major part of the fluvial sequence was deposited during the time period from prior to the Last Glacial Maximum until about 8300 calendar years Before Present (cal BP)^a period of generally high (but variable) precipitation on the Altiplano and high water level in Lake Titicaca. Initial deposition (aggradation) was followed by successive downcutting to the T4 and T3 terrace surfaces. Initial downcutting began immediately after precipitation, runoff, and sediment load decreased while base level dropped. It was followed by a period of episodic equilibrium and minor downcutting that included a prolonged period of soil formation between V8350 and 6780 cal BP. The major pulses of downcutting likely occurred between V6000 and 4500 cal BP and were coincident with periods of decreased precipitation on the Altiplano and decreasing levels of Lake Titicaca. Two final periods of infilling, resulting in deposition of the T2 and T1 terrace sediments at V4000 to 2500 cal BP and V2000 to 1600 cal BP (during periods of rising water level in Lake Titicaca, lacustrine sedimentation in the Rio Desaguadero valley, and increased sedimentation offshore the Ilave delta), were separated by brief equilibrium stages and a brief downcutting event. This fluvial history, when coupled with regional paleoclimatic data, relates to the region's preceramic through Tiwanakuperiod archeological records. Archeological evidence indicates that humans occupied the Ilave valley as early as 10 000 cal BP. The higher terraces (T3, T4 and T5) were occupied for at least 5000 years, but humans did not utilize the lower terraces (T1 and T2) until after V4400^3700 cal BP. Our results confirm that these lower terraces would not have been available for either occupation or agriculture until after V4000 cal BP. ß

Late Archaic–Early Formative period microbotanical evidence for potato at Jiskairumoko in the Titicaca Basin of southern Peru

“…The site itself is situated upon a gravel knoll; as described by Rigsby et al (72) in their documentation of the Ilave Valley's fluvial history, the knoll is a remnant of an ancient river terrace, one of five created as a result of rising and falling lake levels associated with regional paleaoclimatic shifts. The terrace upon which Jiskairumoko sits (T5) was created circa 18,000 cal y BP; a nearby lower terrace (T2) was created more recently, ∼2,300 cal y BC (21,(72)(73)(74). At the time of Jiskairumoko's occupation, T2 would have been found within the river floodplain and subject to frequent inundation and aggradation resulting from then-current high rainfall and lake levels.…”

Four-thousand-year-old gold artifacts from the Lake Titicaca basin, southern Peru

“…Multiple lines of evidence attest to this transition. Geomorphological data from the Ilave river drainage (18) and limnological data from Lake Titicaca (19,20) show that a key river terraceTerrace 2-was created by Ϸ2300 cal yr B.C. as the level of the lake rose from a much lower level in the mid-Holocene.…”

“…(17, ** ). Settlement data from the Ilave basin reflect population growth, settlement aggregation, and the movement of large sites toward the Terrace 2 margins during the Terminal Archaic (17,18).…”

Late Quaternary climate variability and terrestrial carbon cycling in tropical South America

“…Increased precipitation and lower temperatures caused snowlines in the Titicaca catchment to descend to elevations of ~4400-4600 m (Klein et al, 1999), resulting in significant glacial expansion. The presence of glaciers and generally wet conditions also resulted in a long period of sediment aggradation in Titicaca watershed rivers during the LGM and late glacial period, as evidenced by high terraces (~20-40 m, ~10-20 m above modern river level, respectively) in both Ramis and Ilave river valleys thought to be deposited during this time (Farabaugh, 2005;Rigsby et al, 2003).…”

“…The early-mid Holocene was marked by the driest conditions of the last 20 kyr and a ~100 m drop in lake level from ~8-4 ka . Reduced precipitation and lower lake level also contributed to erosion of fluvial deposits and episodic downcutting throughout the early-mid Holocene in the Ramis and Ilave river valleys (Farabaugh, 2005;Rigsby et al, 2003).…”