RT11B - Upper Vilca

Site Code: RT11B. GPS: S12°093520, W75°811620. Elevation: 4080m asl.

Water Quality Data

The following river and water quality data was collected from this site:

T11b November 2021
Season: Austral Summer
Date: 21.11.21
Time: 12.45pm
Width (m) 3.0
Depth (Av.) (m) 0.04
CSA (Av.) (m²) 0.12
Velocity (Av.) (m/s) 0.67
Discharge (Av.) (m³/s) 0.080
Riverbed sediment Mainly pebbles
Temp. (Atmospheric) (°C) 10
Temp. (Water) (°C) 7.70
Atmospheric pressure (mbar)  
Dissolved oxygen (mg/l) 5.56
Dissolved oxygen (%) 75.1
pH 8.42
Conductivity (µS/cm) 1010
Total Dissolved Solids (ppm) 505
ORP (mV) 113.0
Turbidity Clear:
Alkalinity (mg/l CaCO₃) 172 (1.6N)
Alkalinity Field Tester (ppm/ CaCO₃) -

Data Analysis

Discharge at this point on the Rio Vilca was very low suggesting that the rains had not yet started in the headwaters of this sub-catchment. At nearly 4100m asl the DO readings at T11b fall within the range to be expected for a November data set. The conductivity reading at this site (1010 µS/cm) is higher than is expected for a site located above all major settlements within a sub-catchment but probably reflects local geological conditions and weathering processes in the river channel, but this is yet to be determined. At this level it is beginning to present some challenges for some species of aquatic life. The pH reading of 8.42, is as expected. At these levels the alkalinity of the river is highly suitable for aquatic life and indicative of clean water.

Site description

Site T11b lies in the Rio Vilca valley at 4080m asl. The valley at this point is narrow with a v-shaped profile and a narrow flood-plain. The river has incised down in to a rock ledge and flows out from a small narrow cave approximately 10 metres deep. Bed load in the river channel is comprised almost entirely of smaller stones, larger material being held back in the channel above the rock ledge. River flow in the Rio Vilca is year-round and makes a small input to the Rio Cañete. There is no agricultural cultivation taking place, the banks and hillsides are lined with open pasture where cattle and alpacas graze. However, there is evidence of more intensive land use having taken place in the past; old corrals being located nearby. Andean goose, Andean and Puna Ibis are found feeding in the wetter soils and marshland.
There are no permanent settlements higher up the valley.

Site T11B - the study site. (John Forrest) Site T11B - the study site (Nov.2021). (John Forrest)

Site T11B - the study site. (John Forrest) Site T11B - the view upstream (Nov.2021). (John Forrest)

Site T11B - the study site. (John Forrest) Site T11B - the view downstream (Nov.2021). (John Forrest)