RC06 - Below Calachota

Site Code: RC06i.
July 2019 - GPS: S12°40057, W75°57335. Elevation: 1606m asl. Site Code: RC06 November 2019 - GPS: S12°63846, W75°975771. Elevation: 1695m asl.

Water Quality Data

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

RC06 July 2019 November 2019 November 2021
Season: Austral Winter Austral Summer Austral Summer
Date: 12.07.2019 19.11.2019 12.11.21
Time: 13.10pm 10.30am 10.45am
Width (m)   35.0 36.0
Depth (Av.) (m)   0.13 0.87
CSA (Av.) (m²)   4.52 31.32
Velocity (Av.) (m/s)   0.24 1.82
Discharge (Av.) (m³/s) 15.00 1.06 57.18
Riverbed sediment Cobbles & boulders Cobbles & boulders Mainly cobbles
Temp. (Atmospheric) (°C) 27 23 25
Temp. (Water) (°C) 15.71 16.88 16.52
Atmospheric pressure (mbar) 820 825  
Dissolved oxygen (mg/l) 6.62 5.58 7.81
Dissolved oxygen (%) 80.57 74.90 96.7
pH 6.52 8.79 8.35
Conductivity (µS/cm) 454 424 556
Total Dissolved Solids (ppm) 227 212 278
ORP (mV) 342.3 156.0 50.0
Turbidity Clear: very slightly milky Clear: slight green colouration Clear:
Alkalinity (mg/l CaCO₃) 104 (1.6N) - 142 (1.6N)
Alkalinity Field Tester (ppm/ CaCO₃) - - 123

Data Analysis

Discharge at this site varies significantly seasonally. It is significantly reduced downstream by the El Platanal HEP scheme. The river is also impacted by gravel workings in the channel just downstream from the site.

At just over 1650m asl the DO readings at RC06 are as expected. Again, the increased elevation and diurnal fluctuations in temperature in narrow, sheltered sections of the valley produces fluctuations in maximum absolute oxygen capacity and, in consequence, percentage saturation. This then fluctuates with that rhythm accordingly. At DO levels from 5.25-6.8 mg/l and 75-83% oxygen saturation aquatic vertebrates survive and invertebrates thrive.

The conductivity readings at this site (423-454 µS/cm) are as expected at this altitude.

pH readings of 8.76 November 2019 are as expected. At these levels the alkalinity of the river is suitable for aquatic life and not approaching the tolerances of most aquatic vertebrates and invertebrates. The July data requires confirmation.

Site Description

Site RC06 is located just below the settlement of Calachota and above the mid-catchment reservoir at Capillucas at 1660m asl in the arid subtropics. The valley here is broad with a highly braided channel. There are significant areas of cultivation, requiring irrigation waters. These are abstracted in the traditional fashion with many channels carrying water along valley side contours to feed fields by superficial watering. The valley is deeply incised with steep arid slopes rising from the flood plain. Valley sides at this elevation remain very dry, mainly supporting the xeric vegetative communities found lower down in the valley system. At this point in the sub-catchment farming is mainly subsistence, with some limited production being offered to local markets down the valley. There are areas devoted to avocados, citrus and some plantains.

As elsewhere in the drainage basin discharge is highly variable with a marked seasonality. Discharge measurements from January to March being considerably greater. The channel itself is braided with large area of ephemeral gravel islands found in the channel. In addition to the abstraction of water the extraction of gravels and sands is undertaken just below the village.

Natural vegetation is limited to scattered willows and river cane along the banks. The fauna and flora in the valley remain depauperate. Scrub Blackbird, Black-necked Woodpecker and Bare-faced and Croaking Ground Doves are common.

Site RC06i (July 2019) is located further downriver where the road to San Lorenzo de Putinza crosses the river. This site has been replaced by RC06 for future data and sample collection. The narrow flood-plain and lands below and around the village are farmed with citrus fruit and avocado orchards as well as maize, onions, garlic, and pepper production. The river here is lined with acacia and some willow.

RC06 View upstream Site RC06 - Study site location: below the settlemnt of Calchota and just above the riverbed gravel workings. (John Forrest)

Site RC06 - the study site. (John Forrest) Site RC06 - the study site. (John Forrest)

RC06 View upstream Site RC06 - View upstream. (John Forrest)

RC06 View downstream Site RC06 - View downstream. (John Forrest)