Wednesday, 14 March 2012

A story of people and rivers in the Amazon of 5000 years ago
This time I will tell you about a story that began in the Mid-Holocene (5000 years ago) and is set in the Bolivian Amazon. More precisely in the south-eastern part of the Llanos de Moxos seasonally flooded savannah, in what we call the Monumental Mounds Region MMR (Fig. 1).  Here, between 400 and 1400 AD, pre-Columbians built hundreds of monumental earth mounds, known locally as “lomas”. These earth mounds are planned, complex buildings made by one or more pyramids built on top of elevated platforms (Fig. 3-3).  Monumental mounds can be up to 20 meters high and can cover up to 30 hectares. In the MMR there are more than 350 of these pre-Columbian buildings.
Figure 1 The Llanos de Moxos, the MMR (yellow
box) and the paleo-courses of Río Grande (pink lines). 
During the early Holocene (between 11.000 and 5.000 years ago) this portion of Amazonia was relatively dryer than today, inundations were less frequent and rivers transported few sediments. During these stable climatic conditions there was no deposition of fluvial sediments in the savannahs and soils were forming all over the Llanos de Moxos. 

Mid- Holocene: Río Grande built the landscape

But things changed a lot during the Mid-Holocene, between 5 and 4Ky BP. The Rio Grande (pink lines in Fig. 1) entered in a period of frequent avulsions and high sedimentation, probably triggered by a climate change towards wetter conditions. As a result, in the South-eastern LM, it formed a ­fluvial distributary system (FDS) (Fig. 2-1). Suddenly, the landscape was transformed into a large swamp, dominated by something similar to an interior delta. This FDS deposited several levees, crating relief at a local scale (Fig. 2-2), and a sedimentary lobe, creating relief at a regional scale (Fig. 2-3). The former soils were buried and the landscape became a mosaic of patches of savannahs closely interwoven and sometimes enclosed by forested paleo-levees.

Late Holocene: pre-Columbians transformed the landscape

In the MMR, the lobe deposition favoured the development of a complex pre-Columbian society by increasing the region’s agricultural potential. Firstly, it created a convex-up topography, which greatly reduced its susceptibility to ­flooding; secondly, the construction of the elevated ­fluvial levees significantly improved drainage conditions at the local scale. Furthermore, the Río Grande also provided relatively younger sediments derived from its Andean catchment that are rich in nutrients. Thus, the Río Grande removed the two biggest obstacles faced by tropical agriculture in the rest of Amazonia: severe waterlogging and poor soils. But the Río Grande’s job was not perfect: fl­uvial levees enclosed patches of ­floodplain, resulting in ponding and pronounced waterlogging. Thus pre-Columbian people had to transform the landscape through the construction of a drainage system in order to further improve agricultural conditions (Fig. 3-1).

The network of canals had a significant impact on the edaphology of the MMR: it pushed the forest-savannah boundary towards the savannah, eventually increasing the area of well-drained, usable land. The inhabitants of the MMR were very lucky because they also had several lakes placed on the top of the sedimentary lobe. Building canals that transported the water from the lakes to the agricultural fields (Fig. 4), they were able to perform agriculture even during the dry season.
The spatial overlap (Fig. 3-2) between the monumental mounds and the area of deposition of the sedimentary lobe created by the Río Grande during the mid- to late Holocene  suggests that good edaphic conditions favoured the emergence of the monumental mounds culture (Fig. 3-3).

The ­fluvial landscape created by Río Grande was probably an important factor behind the emergence of the monumental mounds culture in the South-eastern LM, as it provided favourable environmental preconditions in terms of soils, nutrients and drainage characteristics. Pre-Columbians additionally modified and improved their environment by building a network of drainage canals.
Figure 4 (a) Drainage canal (A–B) and irrigation canal (C–D) 
built along topographic slopes. (b)The C–D canal in detail;
 the irrigation canal linking Lake San José to the Cotoca mound 
joins pre-existing natural channels. Coring in the middle of 
the canal shows that the bed of the canal is only 50cm below 
the present bed (which is about 50 cm below the modern savannah). 
Therefore, this canal was at most 1 meter deep but went through a slope 
of 3 meters. The only possible function of such a canal was irrigation.

You can find more on this story in my latest paper published on-line in the journal “The Holocene”. It is entitled “Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin” and can be found here. 

Lombardo, U., & Prümers, H. (2010). Pre-Columbian human occupation patterns in the eastern plains of the Llanos de Moxos, Bolivian Amazonia Journal of Archaeological Science, 37 (8), 1875-1885 DOI: 10.1016/j.jas.2010.02.011

Umberto Lombardo, Jan-Hendrik May, & Heinz Veit (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene