La Revista Digital 1 Versión Final Revista No. 2 - Prueva | Page 44

K. Lane
Introduction
Over the past few decades, the complexity and diversity
of Andean hydraulic engineering have been studied and
highlighted as never before (Denevan, 2001; Herrera,
2011; Lane, 2014; Ortloff, 2009). This is especially true
of the highlands, which until recently were singularly
under-researched. Nevertheless, there is still much new
material to be investigated and assimilated, as studies into
the amunas systems of the Central Andes demonstrate
(Alencastre, 2012; Apaza, Hurtado and Alencastre, 2006).
This study presents and describes a series of structures
known as represas de limo or silt dams. Silt dams were
structures primarily built to contain erosion (a sort of
massive check dam), while concomitantly providing a
stable platform for the creation of artificial bofedales
(irrigated moorland). Although initially identified for the
Cordillera Negra, Ancash, new research has uncovered
evidence of similar constructions in the Upper Ica Basin
(Huamán and Lane, 2014), suggesting that these type-
sites were more common across the Andes than originally
envisaged (Figure 1).
Figure 1. Map of study area – Pamparomás District, Huaylas Province, Ancash.
Although outwardly similar to prehispanic water dams,
these silt dams were very different in their use and location.
Nevertheless, the general similarities in structure and
construction have led to misidentification of these features
as water dams, in turn leading to the destruction of some
of them (Lane, 2013). In this article, we describe these silt
dams – their state of preservation, form and function – in
the Cordillera Negra, Department of Ancash.
Silt Dams
Silt dams are large stone and fill structures that
essentially serve three basic functions:
1. Accumulate silts and sediments that erode from the
mountain slopes;
2. Retain and store water within the trapped silts and
sediments;
3. Provide a rich soil matrix platform for the creation
of an artificial bofedal.
These three functions are obviously interlinked. In
addressing the first function, given the annual high-energy
water discharge associated with the austral summer rains
and therefore the propensity for landslides and avalanches
(huaicos), it makes logical sense to construct barriers to
halt and control the flow of water and sediment. In this
remit, silt dams act like large check dams which cut across
the path of main water flow (Denevan, 2001). While the
features described by Denevan (2001: 171-185) were
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grouped and stepped, built across very narrow ravines – and
therefore more analogous to the silt reservoirs described
for the Ancash region (Lane, 2009) – these silt dams are
large constructions (usually over 80 m in length) spanning
across a whole valley or side-valley floor.
Indeed, this function was highlighted by Freisem
(1998), who identified them as secondary erosion dams,
which ameliorate the effects of huaicos and thereby protect
the lower-lying cultivation fields and settlements. Yet this
is only part of the story. Silt dams, like the silt reservoirs,
were more than just devices to prevent erosion flooding.
The sediment generated by hill-wash and general erosion
provided an important platform for pasture, and retained
mineral salts that were in turn consumed by animals.
Likewise, the function of trapping silts and sediments
(Point 2 above) does not preclude the fact that these
structures also store and contain water. In this case,
silt dams act not so much as dams but rather sieves that
filter hydraulic overflow while maintaining the trapped
sediments behind the wall structure saturated with water.
In so doing, these features act to store water and help in
replenishing underground aquifers (sensu Fairley, 2003).
Finally, (Point 3 above) the dual accumulation of sediments
and water creates the conditions for bofedales (moorland
ecotone).
Bofedales are important highland ecotones that help
create and sustain particular niche habitats. In this case,
Revista de Glaciares y Ecosistemas de Montaña 2 (2017): 41-50