Innovative approach to restoring urban water bodies in rapidly developing catchments

Bengaluru, the “Silicon Valley of India”, faces environmental pressures, with degradation of its urban lakes among the most visible. Historically, three interconnected catchments with cascades of manmade reservoirs supported irrigation, domestic use and groundwater recharge. Rapid urbanisation and a shift from agriculture to the service sector led the city to import water from the Cauvery River, about 100 km away, reducing reliance on local lakes. Wastewater infrastructure did not keep pace with expanding water supply, and many lakes became sinks for untreated or partially treated sewage, resulting in fish kills, algal blooms and odour problems.
In response, public agencies, corporate social responsibility initiatives and citizen groups undertook lake restoration projects, but most interventions focused on civil works such as stone pitching, desilting, deepening of basins and planting of exotic species, with limited ecological rationale. Restoration success has typically been assessed against national “Class B” bathing water standards. Because most lakes fail to meet these norms, restoration is often portrayed as unsuccessful.
This study presents a context specific framework to benchmark lake health and guide restoration in rapidly developing catchments. We evaluated 32 lakes during critical season (Feb to May 2025), randomly selected from about 180 across Bengaluru, representing diverse intervention types, such as sewage treatment plants, sedimentation ponds and constructed wetlands, and different levels of community stewardship. Lake condition was assessed along three dimensions: water quality and hydrology (Secchi depth, total phosphorus, dissolved oxygen), biodiversity (plant and bird diversity) and community engagement.
To develop an operational benchmark, we focused on three indicators with strong ecological relevance: water clarity (Secchi depth), nutrient status (total phosphorus) and plant species richness. These were weighted to reflect their relative importance (0.5, 0.3 and 0.2 respectively). Indicators were normalised across lakes; beneficial values were scored positively and detrimental values inversely. Composite scores were calculated for each lake, and a benchmark threshold of 0.6 was proposed.
Fewer than 20 percent of lakes exceeded this benchmark. Higher scoring lakes had Secchi depth greater than 0.4 m, total phosphorus less than 0.8 mg/L, well maintained shoreline vegetation and received treated wastewater through sedimentation zones or in lake constructed wetlands, together with strong community stewardship. Lower scoring lakes were characterised by total phosphorus greater than 1.2 mg/L, Secchi depth less than 0.25 m and degraded shorelines, reflecting inflows of untreated or partially treated sewage and weak local engagement.
The framework helps establish restoration targets that are achievable in rapidly urbanising catchments. Rather than focusing solely on bathing water standards, agencies can use the benchmark to design interventions (type and scale) that help lakes achieve threshold 0.6 and above.  Additionally also help identify catchment and in lake factors associated with higher water clarity and better ecological condition. Well performing lakes can serve as reference systems, and the benchmark can act as a performance indicator. These lakes can also be developed as living laboratories where schools and communities monitor simple indicators such as Secchi depth, total phosphorus and plant diversity, helping to sustain lake health and the effectiveness of interventions.