Effects of sedimentation on water quality in the Metolong Reservoir, Maseru, Lesotho
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Date
2025
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Publisher
National University of Lesotho
Abstract
Reservoir sedimentation is a significant environmental challenge affecting the
sustainability of water resources, particularly in Lesotho, where soil erosion and land
degradation are prevalent due to intensive land–use activities and fragile ecosystems.
Metolong Reservoir is a critical water supply for approximately two–thirds of Lesotho's
population, challenged with rapid sedimentation which lowers the water quality and
threatens the long–term viability of the reservoir, yet no known studies have been done
to correlate sedimentation and water quality in the area. This study investigated how
sedimentation affects water quality in the Metolong Reservoir by quantifying sediment
accumulation in the Metolong Reservoir from 2020 to 2022 and analyzing the impact
of sedimentation on key water quality parameters. The study employed historical
bathymetric and water quality data (2020–2022), complemented by GIS–based spatial
analysis and R statistical modeling to assess the spatial and temporal relationships
between sediment deposition and changes in water quality parameters, following the
causal–comparative research design. Water quality parameters analyzed were
aluminum, Electrical Conductivity (EC), iron, manganese, nitrates, nitrites, phosphates,
sulphates, TDS and turbidity. Bathymetric analysis revealed a total sediment
accumulation of approximately 1,705,583 m3 (2.68% of reservoir capacity) between
2020 and 2022, concentrated primarily near the reservoir’s middle and towards the
dam, resulting in an annual storage loss of 1.34%.Linear regression analysis revealedturbidity as the most significant at (p<0.05).The study also identified turbidity and
nitrates as key water quality parameters significantly influenced by sedimentation, with
turbidity showing the strongest correlation (r = 0.60) and nitrates showing a moderate
correlation (r = 0.2) with sediment volume suggesting possible links with upstream land
use and nutrient runoff. There was an increase in nutrients and heavy metals
concentration from 2020–2022, indicating a need for intervention, though most water
quality parameters were still within WHO and South African water quality standards.
The findings confirm that sedimentation negatively affects water quality, underscoring
the need for integrated catchment management strategies, including sediment control,
land–use planning, and systematic water quality monitoring to safeguard reservoir
operations and public health.