Sand mining and river bottom-dredging: impacts on river ecology and the environment

East Zone outlook

Nachiket Kelkar1

1Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore, India

India Rivers Forum discussion series: “Is sand mining killing our rivers?” 21st November 2020

Map courtesy: Tarun Nair

Subarnarekh, Brahmani-Baitarani, and allied basins

Teesta

Meghna basin

Map courtesy: Dharmadhikary & Sandbhor (Manthan Strategic Report on Inland Waterways, 2018).

Nearly 6000 km of waterways planned across India

Inland Waterways in India

Map source: Nayak, R. & Srinivas, V. (FERAL), Figure from Krishnaswamy et al. (2019) Climate change and aquatic biodiversity. MoEF-CC publication

River Interlinking: India

Adapted from Jacobs & Moroka, 2014

In-stream sand mining (wet pit mining, sediment traps): worst direct impacts

Floodplain mining (dry/wet pit mining, bar scalping): direct and indirect impacts

Terrace mining (bar scalping, dry/wet pit mining): indirect impacts

Flow direction

Modes of sand mining

Padmalal & Maya, 2014

Terrace

Floodplain

In-stream

In-stream

older floodplain

Fresh deposits

Sand bars

Floodplain wetlands

meander scrolls

side-channels

Terrace

Floodplain

In-stream

Floodplain wetlands

In-stream

Fish larval dispersal

older floodplain

Fresh deposits

Sand bars

meander scrolls

River bottom-dredging

Four Rs: Resuspension, Release, Residual, Risk

Photos: Subhasis Dey

Dredging can impair the pollution buffering capacity of sediment deposits to adsorb heavy metals and biological contaminants (Grimes 1975, Eggleton & Thomas 2004, Höss et al. 2010, Singh et al. 2018)

Mayukh Dey

Effects on vegetation

• Detrimental effects on aquatic plants

• Increase in riparian tree cover after floodplain-terrace mining (Koehnken et al. 2020)

• Changes in riparian cover affect habitat of sand-dwelling species

Impacts on invertebrates and plankton

• Zooplankton species richness and abundance declined by 15% to 50% at high levels of sand mining and wave-induced bank erosion in the Ganga and Hooghly Rivers (Prabhakar et al. 2019, CIFRI studies)

• Similar impacts likely due to mining or dredging disturbances on insects, freshwater mussels, prawns, shrimps (Meng et al. 2018, Brim Box & Mossa 2008)

• Cascading effects on river trophic webs and ecosystem services (esp. fisheries)

Impacts on fish and fisheries

• Changes in turbidity, dissolved oxygen, fine sediment flux, and disturbance to gravel badly affect adult fish, eggs, larvae (Bellamy et al. 1992, Arun et al. 2006, Kemp et al. 2011, Padmalal & Maya, 2014, Koehnken et al. 2020)

• Floodplain mining impacts on lateral movements and physiology of young fish, deformities and injuries (Wolter et al. 2004)

Example. Gandak River: capture of aquarium fish species (loaches, gouramis) and aggregate removal by small-scale fishers

Impacts on turtles and crocodiles

Gharial Narrow-headed Softshell Turtle

Impacts on breeding success as floodplain nesting sites are directly disturbed by sand mining (Lang et al. 2019)

Changes in thermal regimes and water turbidity

Tarun Nair Wikipedia commons

Impacts on bird nesting sites and habitats

• Threatened species of sand-nesting terns and Indian skimmer (e.g. Mahanadi river; Kar & Debata 2019)

• Sediment mining may threaten habitats of the Black-necked Crane and White-bellied Heron in NE India (e.g. Menzies et al. 2020)

Indian Skimmer Black-bellied Tern Black-necked Crane

Smooth-coated otters use sand bars and islands for resting, play, and breeding

Jagdish Krishnaswamy

Ganges River dolphin(India’s National Aquatic Animal)

16Sushant Dey

Dredging impacts on river dolphins

• Avoidance of dredged channels by dolphins

• Combined stress from sediment disturbance and noise, which can interfere with feeding activity (Todd et al. 2015)

- Subdued and elusive surface activity and longer diving patterns (e.g. at Bhagalpur, Dey et al. 2019)

• In-river habitat loss

• Detailed studies urgently needed (Todd et al. 2015, Fisher et al. 2018)

17

Sand mining and Ganges river dolphins

• Studies by Wildlife Institute of India noted a reduction in dolphin habitat use and fishing activity, after increase in sand mining (2016-2019), in Kulsi River

• Similar impacts on river dolphins in the Barak River (Choudhury et al. 2019)

Mayukh Dey

Taxa Invertebrates Fishes Reptiles Birds Mammals

Mode of sand mining

In-stream mining

Plankton, crustaceans, insects ✔

All fish ✔

Turtles ✔Crocodiles ✔

Waterfowl ✔

River dolphins ✔

Floodplain mining

Shrimps ✔ Younger fish ✔

Turtles ✔Crocodiles ✔

Sand-nesting birds ✔

Otters ✔

Terrace mining

Insects ✔ ✗? ✗? Sand-nesting birds ✔

Otters ✔

Variable impacts of sand mining on aquatic species

River biodiversity: current status

San

d m

inin

g/d

red

gin

g im

pac

t

Low

High

High

Mahanadi

GandakBrahmaputra

Son

TeestaDamodar

Hooghly

Sundarbans

Kosi

Other rivers in NE India

Mitigation and restoration

• Dam re-operations for ecological flows: adequate release of downstream flows might reduce need for dredging

• Bubble curtains and technical options to reduce dredging impacts

• Sand mining site restoration and spatial zoning

• Small-scale mining?

21

(Bridges et al. 2008, Gavriletea 2017, Torres et al. 2017, Koehnken & Rintoul 2018, Fisher et al. 2018, Koehnken et al. 2020)

Loss of connectivitySediment removal and de-siltation upstream of dams and barrages: (e.g. Farakka barrage)

Thakur et al. (2011), Natural Hazards

Sonkar & Gaurav (2020)River Research and Applications

Artisanal sand mining: is small really beautiful?

• Demands for sand mining rights: traditional sand harvesting

• “SLOSS” arguments with sand mining

Mayukh Dey