Water, Silt and Dams: Prehispanic Geological Storage in the Cordillera Negra, North-Central Andes, Peru Agua, Limo y Represas: Almacenaje Geológico Prehispánico en la Cordillera Negra, Andes Norcentrales, Perú Kevin Lane1 ([email protected]) 1CONICET - Instituto de Arqueología, Facultad de Filosofía y Letras, Universidad de Buenos Aires, Argentina https://doi.org/10.36580/rgem.i2.41-50 …Y después mandó los señores reyes ingas guardar la costumbre y ley de que no meneasen todas las dichas acequias, agua de regar las dichas sementeras, hasta los pastos de ganado regaban en los altos y quebradas… (Guaman Poma de Ayala, 1993 [1615],780) Abstract a las dificultades en mantener un flujo regular de agua, especialmente durante la estación seca, esto constituye una Over the past few decades there has been an increasing respuesta lógica a una necesidad urgente. Sin embargo, la interest in building micro-dams across the whole of the falta de agua no es un fenómeno moderno, es un problema Cordillera Negra, North-Central Andes, Peru. Given the de larga data que se abordó de forma similar en el pasado. difficulties in maintaining a regular flow of water, especially Por lo tanto, el conocimiento del paisaje y las habilidades during the dry season, it is a logical response to a pressing indígenas llevaron a la selección de sitios para hacer need. Yet, lack of water is not a modern phenomenon, it is a represas. long-standing problem that was tackled in a similar fashion in the past. Then, indigenous skill and landscape knowhow Los ingenieros modernos han seguido su ejemplo, informed the selection of sites for damming. de modo que las microrepresas usualmente ocupan los mismos espacios que las que fueron realizadas en Modern engineers have followed their lead, such that contextos prehispánicos. Además de la obvia destrucción present-day micro-dams often occupy the same space as del patrimonio cultural – generalmente sin un estudio the prehispanic ones. Aside from the obvious destruction of arqueológico apropiado – existe otro problema con esta cultural heritage – usually without a proper archaeological política: no todas las antiguas represas reconstruidas en survey – there is one other problem with this policy: not la actualidad fueron originalmente hechas para retener all the old dams which are now being reconstructed were solamente agua. A partir de un profundo conocimiento del built for retaining just water. With an intimate knowledge ambiente andino, las poblaciones humanas adaptaron y of their Andean environment, humans adapted and altered alteraron la tierra que los rodeaba. Con este objetivo, además the land around them. To this end, alongside water dams, de las represas para contener el agua, se construyeron otras other structures were built, such as silt dams. estructuras como las represas para retener limo. Although outwardly like water dams, silt dams evinced Si bien son exteriormente similares a las represas a very different purpose. These structures acted as large hidráulicas, las estructuras para contener limo tuvieron check dams which accumulated run-off silt and other un propósito muy distinto. Estas represas permitieron sediments, fostering a bofedal-type microclimate, while controlar y acumular flujos de limo y otros sedimentos, likewise trapping and purifying water within the silt. lo cual fomentó un microclima de tipo bofedal, al mismo Nowadays, these unique structures are under threat from tiempo que retenían y purificaban el agua contenida en el engineers with little understanding of the true purpose of limo. Actualmente, estas estructuras únicas se encuentran these hydraulic monuments. This article describes these silt amenazadas por ingenieros con escasa comprensión del dams, using a case-study from the Upper Nepeña Valley, verdadero propósito de estos monumentos hidráulicos. Este their form and function. artículo describe estas represas para controlar y acumular limo, usando un caso de estudio del valle alto de Nepeña, Keywords: Ancash, silt dams, geological storage, principalmente su forma y función. prehispanic, bofedales Palabras clave: Ancash, represas de limo, almacenaje Resumen geológico, prehispánico, bofedales En las últimas décadas, hubo un importante incremento en la construcción de microrepresas a lo largo de la Cordillera Negra, Andes Norcentrales del Perú. Debido Revista de Glaciares y Ecosistemas de Montaña 2 (2017): 41-50 41 K. Lane Introduction This study presents and describes a series of structures known as represas de limo or silt dams. Silt dams were Over the past few decades, the complexity and diversity structures primarily built to contain erosion (a sort of of Andean hydraulic engineering have been studied and massive check dam), while concomitantly providing a highlighted as never before (Denevan, 2001; Herrera, stable platform for the creation of artificial bofedales 2011; Lane, 2014; Ortloff, 2009). This is especially true (irrigated moorland). Although initially identified for the of the highlands, which until recently were singularly Cordillera Negra, Ancash, new research has uncovered under-researched. Nevertheless, there is still much new evidence of similar constructions in the Upper Ica Basin material to be investigated and assimilated, as studies into (Huamán and Lane, 2014), suggesting that these type- the amunas systems of the Central Andes demonstrate sites were more common across the Andes than originally (Alencastre, 2012; Apaza, Hurtado and Alencastre, 2006). envisaged (Figure 1). Figure 1. Map of study area – Pamparomás District, Huaylas Province, Ancash. Although outwardly similar to prehispanic water dams, grouped and stepped, built across very narrow ravines – and these silt dams were very different in their use and location. therefore more analogous to the silt reservoirs described Nevertheless, the general similarities in structure and for the Ancash region (Lane, 2009) – these silt dams are construction have led to misidentification of these features large constructions (usually over 80 m in length) spanning as water dams, in turn leading to the destruction of some across a whole valley or side-valley floor. of them (Lane, 2013). In this article, we describe these silt dams – their state of preservation, form and function – in Indeed, this function was highlighted by Freisem the Cordillera Negra, Department of Ancash. (1998), who identified them as secondary erosion dams, which ameliorate the effects of huaicos and thereby protect Silt Dams the lower-lying cultivation fields and settlements. Yet this is only part of the story. Silt dams, like the silt reservoirs, Silt dams are large stone and fill structures that were more than just devices to prevent erosion flooding. essentially serve three basic functions: The sediment generated by hill-wash and general erosion provided an important platform for pasture, and retained 1. Accumulate silts and sediments that erode from the mineral salts that were in turn consumed by animals. mountain slopes; 2. Retain and store water within the trapped silts and Likewise, the function of trapping silts and sediments sediments; (Point 2 above) does not preclude the fact that these 3. Provide a rich soil matrix platform for the creation structures also store and contain water. In this case, of an artificialbofedal . silt dams act not so much as dams but rather sieves that filter hydraulic overflow while maintaining the trapped These three functions are obviously interlinked. In sediments behind the wall structure saturated with water. addressing the first function, given the annual high-energy In so doing, these features act to store water and help in water discharge associated with the austral summer rains replenishing underground aquifers (sensu Fairley, 2003). and therefore the propensity for landslides and avalanches Finally, (Point 3 above) the dual accumulation of sediments (huaicos), it makes logical sense to construct barriers to and water creates the conditions for bofedales (moorland halt and control the flow of water and sediment. In this ecotone). remit, silt dams act like large check dams which cut across the path of main water flow (Denevan, 2001). While the Bofedales are important highland ecotones that help features described by Denevan (2001: 171-185) were create and sustain particular niche habitats. In this case, 42 Revista de Glaciares y Ecosistemas de Montaña 2 (2017): 41-50 Water, Silt and Dams: Prehispanic Geological Storage in the Cordillera Negra, North-Central Andes, Peru bofedales provide optimum pasture conditions for use by we concentrate on the former. The scale of these features herd animals (Maldonado, 2014/15). While these occur precludes them being managed at the household (see naturally, there is a long pedigree in the Andean highlands Aldenderfer, 1998; Carhuallanqui, 1998; Kuznar, 1995), of human-made bofedales (e.g. Flores and Paz, 1986; rather it is more in keeping with the large moorlands Flores, Paz and Rozas, 1996; Palacios, 1977, 1981, 1996). created and nurtured by the Chichillapi herders of Puno To these systems can now be added that of silt dams first (Palacios, 1977, 1981, 1996). In this community, a complex identified for the North-Central Andes (Lane, 2006, 2009), system of canals was used to irrigate high altitude pastures and subsequently found in other parts of the Andean creating bofedales. In the Cordillera Negra highlands with highlands (Huamán and Lane, 2014). the construction of silt dams and check dams, large areas of the puna were turned into a rich plant biota for camelids, A thorough survey of the Upper Nepeña drainage especially the less robust alpaca (Lama pacos). (1999-2008) – the Loco River and Chaclancayo River – revealed twenty-nine sites with hydraulic features, ranging The five silt dams identified for the study area combine from major dams to water reservoirs, small check dams to cover an area totalling 372,027 m² of artificial bofedal, and large silt dams (Figure 2). Five of these were silt dams. and include Oleron Cocharuri [Cho 2], Collpacocha [Co 1], In turn, these various water and/or silt storage structures Tsaquicocha [Pa 5], Huancacocha [Rac 1], and,Tsaquicocha combine with fields, terraces and canals to create complex [Uc 3].