K. Lane
of the structure is 5.4 m. In marked difference to other silt dams, Collpacocha has three sluices, located at different heights. The first sluice, located in the north section of the dam, is set at a middle height in the dam wall just 95 cm below the top level of the dam. The sluice has a height of 60 cm and a width of 72 cm. The sluice decants onto a pond that then channels the water onto the main stream. The present outflow from the sluice is low, probably a reflection of the high degree of water flow through the river breach at the center of the structure. This outflow is orange in color, demonstrating a high level of iron content in the trapped soil behind the structure.
Figure 7. Plan of Co 1 – Collpacocha. Direction of water flow is from east to west( right to left).
The second sluice is also located along the northern half of the structure, although closer to the center of it. A large 60 cm by 60 cm opening still shows a modest orange water flow that joins the same channel as the first sluice. Just beyond this sluice is the main water outflow. This outflow is actually an erosion break in the dam wall. As this high-energy outflow cascades downwards, it is slowly undermining the integrity of the base of the structure. The last sluice is located at the extreme south of the structure cut into the rock outcrop to which the dam is anchored. This sluice is level with the basin and probably represented the main water outflow when the dam was in working order. The drop-pond for this outtake is cut into the natural stone to a level below the earthen channel that subsequently takes the water to the central stream. This feature would have regulated the speed of the water outflow away from the dam, causing a substantial drop in water flow speed. This drop-pond feature is a prime component of modern dams, and serves to show the engineering knowledge of these prehispanic builders.
Sedimentation has banked all across the dam wall, while – as mentioned above – the river has cut through the center of the structure. The bofedal itself is rich in hard ichu grass and other types of forage plants, but the drying of the sides of the dam due to the present unregulated water flow means that the moor-like area is now limited to a narrow central strip along the center of the basin. Manual augering( Dutch and gauge auger) of the dam was undertaken to understand the functioning of the structure. Sixteen auger holes were drilled into the silt basin covering an area 560 m in length and 177 m in width. Although the basin continues towards the modern village of Breque, the main area of the bofedal was that covered by our augering survey.
Preliminary results from the boreholes show that the lowest layer is mainly composed of a dark blue-grey clay layer, probably the original water or silt basin that existed before the construction of the dam. From 4.5 m downward, the deposits were very water-logged, confirming the existence of a permanent aquifer at this depth. The residual nature of the organic remains up to a depth of 2.5 m below the surface suggests that the aquifer was previously substantially larger. Between 2.5 and 4.5 m there were layered sections of low-energy deposited organic loam interspersed with medium to high energy sand deposits. These layers probably correspond to separate stable bofedal periods, interspersed with high energy episodes, perhaps huaicos. Above these layers, between 0 and 2.5 m, there was significant geoturbation of the soil with many high-energy sand and rock deposits, most likely as a consequence of the river frequently changing course. The lack of association between layers in different boreholes implies a degree of soil matrix truncation by the river. These upper layers probably date to the abandonment and general disuse of the site and area from the 16 th or 17 th century onwards.
The augering of the dam at Collpacocha [ Co 1 ] proves that the main function of the site was that of generating and sustaining a bofedal-type environment, while the prevalence of grey organic silts and clays in the lower layers of the first boreholes shows that a water pond existed for a period of time right against the dam wall itself. The bofedal at Collpacocha [ Co 1 ] would have allowed the periodic concentration of animals, which might well have been a major consideration in the placement of the Inca administrative site of Intiaurán in close proximity.
Tsaquicocha [ Pa 5 ] Silt Dam
Located at 4625 m and adjacent to Tsaquicocha Lake, it lies directly to the south of the Carhuacocha Dam [ Pa- 6 ], representing the southern source of Collapampa stream. The center of the wall has collapsed and the water drains through it. The dam is 13.7 m long, with a varying width of
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Revista de Glaciares y Ecosistemas de Montaña 2( 2017): 41-50