La Revista Digital 1 Versión Final Revista No. 2 - Prueva | Page 46

K. Lane
made the creation of a silt dam at this location desirable.
In the other instances, they are located sufficiently low
enough in a valley to best trap hill-wash and silt from the
surrounding area. Currently for the area, elevations above
4200 m are rarely visited by modern goat and sheep herders.
Therefore, these abandoned silt dams are used by the few
cattle and donkeys that roam semi-wild in the sierra.
More commonly, the silt dams were located downriver
from one or more larger bodies of water and dams. In these
circumstances Tsaquicocha [Uc 3] is set below the large
water dam of Agococha/Negrahuacanan [Uc 2]. The silt
dam of Oleron Cocharuri [Cho 2] is likewise sited below
the water dams of Yanacocha [Cho 1] and Orconcocha
[Cho 6], while the silt dam of Huancacocha [Rac 1] is
located below Sacracocha [Rac 2]. The large silt dam of
Collpacocha [Co 1 at 3950 m] accumulates the sediment
discharge from the source of the Chaclancayo River located
upslope in the puna.
Silt dams are the result of a long process of construction
and years of careful management. In the main, the initial
silt basin was probably small and would have grown slowly
through silt accretion during the annual rains. This system
of accretion is sometimes known as varve-formation
(Leet, Judson and Kauffman, 1982). The overflow and
eroded conditions seen today on many of these silt dams,
such as Collpacocha [Co 1] and Huancacocha [Rac 1],
are not indicative of how these dams would have looked
and functioned in the past. It is probable that, in the past,
parts of these structures were periodically de-silted. This
occurred with the Indian gabarband silt-traps (Possehl,
1975). It is possible that the excess silt thus recovered was
relocated to nearby terraces and cultivation fields. Below
we describe the five silt dams present in the study area.
Oleron Cocharuri [Cho 2] Silt Dam
Cho 2 is located in the Chorrillos Valley, which branches
out to the north from the main Chaclancayo Valley. The site
itself is composed of three sectors (A-C) between 4185 m
and 4220 m and lies altitudinally 450 m below the water
dam of Yanacocha [Cho 1] on a type of natural platform
(Figure 3).
Sector A is the main silt dam itself and describes a
C-shaped walled structure about 76 m long (Figure 4). The
wall is composed of two parallel stone walls, infilled with
packed earth and stone, with a width that varies between
2.15 m and 4.30 m. The height varies between 0.50 m and
0.57 m at the extremities, rising to a maximum of 2.50
m along the central part. Part of the height discrepancy
is due to the V-shape of valley itself, which makes the
central section higher while maintaining an almost level
dam crest. The dam uses large, naturally occurring rock
outcrops as integral parts of the structure. The dam is in
a good state of preservation, with only the center having
partially collapsed. The Chorrillos stream flows through
this eroded center. The two ends of the dam are anchored
onto the valley flanks, a clear indication that this is a silt
44
Figure 3. Photograph of Cho 2 – Oleron Cocharuri,
Chorillos River. Observe bofedal behind dam wall.
Photo: K. Lane, 2 002.
dam. A single sluice located at the base of the center of
the structure drains excess water. The level area behind the
dam wall recreates a bofedal-type environment of 53,125
m 2 .
Sector B is located approximately 350 m upstream
to the north of Sector A. It comprises a seasonal natural
pool (ojo de agua) circular in shape, and approximately
4 m in diameter. It is located on a bedrock outcrop that
stands about 8 m in height. It does not seem to have been
artificially modified, and it feeds into Cho 1. Finally,
Sector C (Patoparanán) is located downstream from Sector
A comprising the edge of the natural platform on which
Cho 1 is situated. It comprises two small, low, C-shaped
stepped walls. The structures are no more that 70-80 cm
high and both have a sluice located in the central part near
the base. Behind these walled structures, a small bofedal
has formed. These two structures represent silt reservoirs
or check dams, which have been described elsewhere
(Lane, 2009).
Collpacocha [Co 1] Silt Dam
The lack of a management system can be most aptly
appreciated at the site of Collpacocha [Co 1] silt dam
(Figure 5). Here, the large 28.5 ha silt basin is slowly
drying out. Although the primary function of the site is
under threat, the bofedal created by the partial working of
Revista de Glaciares y Ecosistemas de Montaña 2 (2017): 41-50