Abstract The Kathmandu Valley has recently seen a rapid population growth and expanding urbanization, with a vast increase in urban trash and untreated sewage entering the Bagmati River. We sampled and compiled historic data on the aquatic macroinvertebrates from the past three decades from five sites along 30 km of the river, and quantified land-use categories in the catchments from satellite imagery. Our aims were (1) to study current longitudinal patterns in macroinvertebrate and water quality, (2) to demonstrate long-term patterns in macroinvertebrate quality (i.e. the Nepalese biotic index NEPbios), and (3) to relate changes in macroinvertebrate quality to land-use properties of the respective catchments. Time series regressions of the NEPbios index scores revealed that at the site upstream of the city of Kathmandu, NEPbios scores were consistently high through time. All three middle sites in the city showed decreasing NEPbios scores over time, with progressively lower mean levels. The site immediately downstream of Kathmandu consistently had the lowest scores. Throughout the time period, NEPbios scores followed a strong and positive linear relationship with the proportion of natural vegetation, and a negative logarithmic relationship with the proportion of urbanized area in the catchments. Already at 5% urbanization, the stream macroinvertebrate quality was greatly reduced, and at 10–20% urbanization, the ecological status of the river deteriorated to near its lowest condition. These findings show how sensitive river quality is to even low levels of urbanization in the catchment, with implications for urban and environmental planners.
Thapa et al. (Thu,) studied this question.