Water, Silt and Dams: Prehispanic Geological Storage in the Cordillera Negra, North-Central Andes, Peru
bofedales provide optimum pasture conditions for use by herd animals( Maldonado, 2014 / 15). While these occur naturally, there is a long pedigree in the Andean highlands of human-made bofedales( e. g. Flores and Paz, 1986; Flores, Paz and Rozas, 1996; Palacios, 1977, 1981, 1996). To these systems can now be added that of silt dams first identified for the North-Central Andes( Lane, 2006, 2009), and subsequently found in other parts of the Andean highlands( Huamán and Lane, 2014).
A thorough survey of the Upper Nepeña drainage( 1999-2008) – the Loco River and Chaclancayo River – revealed twenty-nine sites with hydraulic features, ranging from major dams to water reservoirs, small check dams and large silt dams( Figure 2). Five of these were silt dams. In turn, these various water and / or silt storage structures combine with fields, terraces and canals to create complex hydraulic systems that can encompass whole valleys. This systemic control of water emphasizes the highlyspecialized relationship that existed between people, water and technology in the past. It was this syncretic relationship that underpinned the development of novel hydraulic constructions, such as silt dams. we concentrate on the former. The scale of these features precludes them being managed at the household( see Aldenderfer, 1998; Carhuallanqui, 1998; Kuznar, 1995), rather it is more in keeping with the large moorlands created and nurtured by the Chichillapi herders of Puno( Palacios, 1977, 1981, 1996). In this community, a complex system of canals was used to irrigate high altitude pastures creating bofedales. In the Cordillera Negra highlands with the construction of silt dams and check dams, large areas of the puna were turned into a rich plant biota for camelids, especially the less robust alpaca( Lama pacos).
The five silt dams identified for the study area combine to cover an area totalling 372,027 m ² of artificial bofedal, and include Oleron Cocharuri [ Cho 2 ], Collpacocha [ Co 1 ], Tsaquicocha [ Pa 5 ], Huancacocha [ Rac 1 ], and, Tsaquicocha [ Uc 3 ]. As calculated by Browman( 1990), these more than 372 ha of deposited silts and moor-like environments would have supplied forage for an extra 1200 animals, calculating at 3.25 animals per hectare of bofedal. While this might seem a paltry sum, one also has to consider the already existing natural areas of bofedal, as well as those created by the silt reservoirs. Jointly, all these areas would have provided an important mosaic of herding options within the suni-puna environment.
Structurally, silt dams are strikingly similar to the normal water dams in that they were constructed of double-faced walls, in-filled with compacted earth, stone and clay, making an almost impermeable barrier behind which sediment and water were stored. 2 Like the water dams, these constructions were gravity structures, in that their solidity and weight is what anchors them in place. The central part of many of the dams was reinforced by step-like walling, given the added soil and water pressure present in this area of the structure.
Unlike water dams, silt dams were not necessarily anchored onto rock, given that water overflow along the sides of the structure was not an issue. They also usually had only one discernible outtake sluice located along the base of the structure, usually at its center. The basic principle governing the silt dam was that of geologic water storage( Fairley, 2003); in this case the accumulated soil basin acted as an aquifer in which water was both stored and purified through the soil. Since the soil also acted as a barrier to water seepage, the sluice should be viewed rather as a‘ sieve’ which siphons excess water out of the dammed basin while the soil retains enough moisture for the growth of a bofedal-type micro-environment.
Figure 2. Detailed map of survey area.
In the study area, two types of hydraulic features were primarily used for the creation and maintenance of bofedales – silt dams and the smaller check dams – here
Also, unlike water dams, the silt dams are located much lower down in altitude, occurring between 3825 and 4425 m( as opposed to 4600-5200 m). The only exception to this is the small silt dam at Tsaquicocha [ Pa 5 ], located at 4625 m. The local geology of loose and eroded earthen hills probably occasioned the swift silting of this basin, which
2
These stone and earth walls were never meant to be completely impermeable. These structures can shift during tremors and earthquakes, usually settling back in with minimal damage to the structure. In contrast, modern rigid concrete dams tend to crack.
Revista de Glaciares y Ecosistemas de Montaña 2( 2017): 41-50
43