RC02a - Bocatoma Cañete

Site Code: RC02a. GPS: S13°03575, W76°205815. Elevation: 294m asl.

Water Quality Data

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

RC02a July 2019 November 2019 November 2021
Season: Austral Winter Austral Summer Austral Summer
Date: 9.07.2019 17.11.2019 8.11.21
Time: 16.00pm 17.15pm 11.30am
Width (m) 51.0 30.0 68.0
Depth (Av.) (m) 0.43 0.21 0.43
CSA (Av.) (m²) 21.78 6.24 29.24
Velocity (Av.) (m/s) 0.83 0.87 0.57
Discharge (Av.) (m³/s) 16.21 5.45 16.67
Riverbed sediment Pebbles, cobbles & boulders Pebbles, cobbles & boulders Mainly cobbles & boulders
Temp. (Atmospheric) (°C) 23 22.5 21
Temp. (Water) (°C) 20.51 23.85 21.81
Atmospheric pressure (mbar) 978 960  
Dissolved oxygen (mg/l) 6.82 8.82 3.39
Dissolved oxygen (%) 78.0 108.4 38.9
pH 5.91 8.63 8.39
Conductivity (µS/cm) 408 405 507
Total Dissolved Solids (ppm) 204 202 253
ORP (mV) 348.8 176.3 127.6
Turbidity Clear: slight green colouration Clear: slightly milky Clear:
Alkalinity (mg/l CaCO₃) 115 (1.6N) 122 (1.6N) 132 (1.6N)
Alkalinity Field Tester (ppm/ CaCO₃) - - -

Data Analysis

It is the case that the amount of dissolved oxygen at 100% saturation at sea level and at 20°C would be 9.03 mg/l or slightly less as the site is at 300m asl. At 6.53-6.97 mg/l in July the DO reading is low with water temperatures at around 20°C, but within the tolerances of most higher order invertebrates. In November, 8.65-8.99 mg/l represents a level of super-saturation with water temperatures at around 23°C. What might reduce DO levels in July when waters are colder required further investigation. High concentrations of nutrients (particularly phosphorus and nitrogen) fuel algae blooms, which initially boosts dissolved oxygen levels but then rapidly reduce DO. When the algae die, bacterial decomposition spikes, using up most or all of the dissolved oxygen available. This creates an anoxic, or oxygen-depleted, environment where few organisms survive. Such nutrient levels can occur naturally but are more often caused by pollution from fertilizer runoff or poorly treated waste waters which could apply to this site below Herbay Alto. At these recorded levels, denitrification takes place as bacteria use nitrates to decompose organic matter. These conditions are referred to as ‘dead zones’ and are often found near larger human populations in estuaries and coastal zones.

The conductivity reading at this site is 405-408 µS/cm across all seasons for these sample times. The level of conductivity in the Rio Canete at RC02 is an indicator of some contamination from waste waters and agricultural runoff. This is, however, lower than that found at RC00a or b, where sewage and agricultural waste would be expected to raise conductivity. Generally, agricultural runoff and sewage will increase conductivity due to the additional chloride, phosphate, and nitrate ions. Nitrate levels recorded at RC00b were also lower. Throughout the catchment nitrate levels ranged from ‘under range’ to 6.79 mg/l NO3 perhaps underlining the limited extent of intensive agriculture in the catchment.

The pH in November of 8.6 falls within the tolerances of most aquatic vertebrates and invertebrates while the July readings require confirmation.

Site description

Site RC02 is located in the tropical zone just below Puente Socsi and the long linear settlement of Lunahuana. It is flanked by intensively farmed lands on riverine terraces irrigated by waters extracted from the river at several points higher up. The town of Lunahuana is directly upriver and largely located atop a high riverine terrace on the south side of the channel. The valley floor at this point is a narrowing flat flood plain bounded north and south by arid desertic hills with little or no vegetation.

The channel flow is highly variable from the dry to wet season and from year to year, especially in El Nino years when the valley may be subject to catastrophic flooding. However, the river holds water year-round, unlike many rivers entering the coastal plain along the west flank. More generally water levels are a function of the levels of abstraction for agriculture and urban demands. The channel itself is very similar to that at RC01, braided and made up largely of gravel and smaller boulders. Discharge in July was 16.2 m³/s and would have been high following the end of wet season in April. During November the discharge is approaching its lowest. A discharge of 5.45 m³/s was recorded for one of the two channels at Puente Socsi. During the wet season discharge is considerably higher at this point, possibly rising to over 25 m³/s.

The banks are lined with extensive river cane scattered acacia and stands of stunted willow. This area is designated as a ‘bosque de proteccion’ affording some degree of protection to this relictual area of natural vegetation. This area is an important reserve for populations of Slender-billed Finch, a near threatened species with a limited distribution from northern Chile to the Rio Canete, in Peru. This area also holds populations of mountain Parakeet and endemic Black-necked Woodpecker.

There is little or no natural vegetation remaining in the lower valley and much of what was open desert on the coastal plain has been irrigated and is now highly productive agricultural lands. The water quality in the river at lower elevations is severely impacted by agricultural run-off and sewage disposal. The wider environment and fauna in the valley are largely determined by agriculture with many species expanding their range and abundance to occupy newly created agricultural lands. Desert fauna and flora has decreased accordingly. The impact of water quality on the health of the environment and populations of wildlife occupying these lands is the subject of some aspects of our study here. The valley is intensively cultivated at this elevation. The production of avocado dominates along with viticulture, ajo (garlic), aji (peppers) together with limited areas of apple, pear, and citrus orchards. There are smaller areas of sugar cane, a crop which dominates land use further north.

RC02a View upstream Site RC02a - Study site location: below Puente Socsi & above Socsi irrigation outflow. (John Forrest)

Site RC02 - the study site. (John Forrest) Site RC02a - the study site. (John Forrest)

RC11 View upstream Site RC02a - View upstream. (John Forrest)

RC08 View downstream Site RC02a - View downstream. (John Forrest)