Abstract

Available secondary suspended sediment data from the 1980s was analyzed together with new data collected during 2001–2003 at instrumented sites upstream and downstream of open-cast mining activities (∼4.2 km²) in an enclosure within the Kudremukh National Park in south India. More than 50% of the suspended sediment load in both the Bhadra River and Bhadra Reservoir comes from mining-affected lands that occupy <1% of the total catchment area (1968 km²). For baseflow conditions during the post-monsoon period of 2001, 0.02 Mg km⁻² day⁻¹ of suspended sediment was discharged at the upstream site compared with 0.74 Mg km⁻² day⁻¹ downstream of the mine. During the 2002 and 2003 monsoons, these rates increased to about 1.99 and 7.89 Mg km⁻² day⁻¹ for upstream and downstream sites, respectively. The specific sediment yield above the mine is 239 Mg km⁻² year⁻¹ versus 947 Mg km⁻² year⁻¹ downstream. Sediment concentration downstream was significantly higher than upstream for all conditions. The current annual suspended sediment load below the mine ranges from 100,000 to over 150,000 Mg, depending on the size and frequency of large rain events. During the 2002 and 2003 monsoon seasons, 20–30% of the total suspended sediment load during the sampling period of 67 and 123 days, respectively, was transported in one single day and over one-third of the total recorded suspended load is discharged in less than 4 days in each monsoon. Daily rainfall and maximum hourly rainfall intensity were reasonable predictors of daily sediment loads downstream of the mine (R² = 0.71 and 0.575, respectively; p < 0.001). However, the upstream response was not predicted well by these variables, suggesting an absence of rapid overland pathways and/or reduced availability of easily transportable sediment on the less-disturbed lands above the mining area. Large episodic sediment events downstream of the mine were associated with daily rainfall >150 mm and hourly intensities exceeding 20 mm. One hundred and nine such episodic events occurred between 1990 and 2001 alone. A conservative estimate of the total suspended sediment load in the Bhadra River after mining commenced in the 1980s is 1.37 × 10⁶ Mg, although the actual value could be considerably higher, up to 10⁷ Mg. Comparison of historic data and another study in 1994, with recent measurements confirm that mining and associated activities in Kudremukh National Park are the greatest sources of sediment entering the Bhadra River; and the Bhadra river carries considerably more sediment now than before mining started damaging riverine ecosystems and disrupting downstream water resources.