A stream in Uttarakhand. Source: TanayaPrapti via Wikimedia Commons
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Long before a laboratory test can tell us whether a stream is healthy, its smallest residents already know. Tucked beneath stones, wedged into sediment, and clinging to submerged logs, tiny insects spend their lives quietly recording the condition of the water around them. Few people ever notice them.
Yet in a first of its kind study of the Rawasan stream in Uttarakhand's Pauri Garhwal district published in H2Open Journal , scientists Koshal Kumar and Anoop Dobriyal turned to exactly these overlooked creatures, counting stoneflies, caddisflies, mayflies, mites and bugs at five sites to read the stream's health from the bottom up.
What they found offers a quietly reassuring answer and a reminder of how much a handful of insects can reveal about the water that sustains an entire village downstream.
A wide range of small creatures, including stoneflies, mayflies, caddisflies, dragonflies and their larvae, along with snails, worms and beetles, live at the bottom of and around water bodies such as streams. Scientists call them benthic macroinvertebrates: animals without a backbone, visible to the naked eye, usually found attached to rocks, vegetation, logs and sticks, or burrowed into sand and sediment.
These creatures form an essential part of the food web in and out of water bodies. Some feed on bacteria, algae or plants, while others prey on smaller animals. They help break down wood and leaves into smaller pieces that drift downstream for other species to consume, and they are themselves eaten by fish, amphibians and birds. In this way, they help sustain the health and ecological balance of the water bodies they inhabit.
Scientists consider them exceptionally reliable indicators of water quality. They spend all or most of their lives in water, are relatively easy to collect and identify, often live for more than a year, and have limited mobility. Because they cannot simply swim away from pollution, they absorb and respond to environmental stress, both suddenly and over time, in fairly predictable ways. A stream in good biological condition typically supports a wide variety of macroinvertebrates, including species that cannot tolerate pollution. A stream dominated by only a few pollution-tolerant species, or showing very low diversity overall, is usually a stream under stress.
Among these creatures, the larvae of stoneflies, mayflies and caddisflies are considered especially telling. All three groups are sensitive to pollution and struggle to survive in large numbers where water quality has declined, which is also why fish such as trout favour them as food, particularly when they hatch. A stream rich in all three is generally a healthy one; a stream where only a few kinds persist in large numbers is usually not.
The Rawasan stream is a tributary of the Ganga, flowing through the Shivalik range in the Yamkeshwar block of Pauri Garhwal district, Uttarakhand. It travels a hilly route through Pauri Garhwal and the Lal Dang Forest range before meeting the Ganga at Tanatwali village in Haridwar district. For the people of Tanatwali, the stream is not an abstraction. Its water supports their livelihoods, their daily consumption needs, and their irrigation, making its health a matter of direct, everyday consequence rather than only scientific interest.
To assess that health, researchers selected five sites reflecting different physical characteristics along the stream: Narikatal, Jarpani, Seela, Pupaldanga and Madhan. Aquatic insects were collected across all four seasons, pre monsoon between March and May, monsoon between June and August, post monsoon between September and November, and winter between December and February, over a full year from October 2021 to September 2022.
Alongside the insect counts, researchers measured a wide set of water quality indicators: pH, electrical conductivity, total dissolved solids, dissolved oxygen, biological oxygen demand, free carbon dioxide, total alkalinity, phosphates and nitrates. They also recorded rainfall, air and water temperature, water velocity, turbidity, and the type of streambed and substratum at each site, building a detailed physical picture to sit alongside the biological one.
Location of the stream and sites selected.
Scientists found 87 types of aquatic insects from 8 groups in the Rawasan stream. Most belonged to three sensitive groups that only live in clean water:
Caddisflies (25%)
Stoneflies (25%)
Mayflies (22%)
A few belonged to pollution-tolerant groups:
Mites (3%)
Aquatic bugs (2.5%)
The top three orders of aquatic invertebrate insects namely caddisflies, stoneflies and mayflies were found in high abundance and dominance, which indicated the cleanliness, and purity of the water across all five sites under study. This is because all the three insects are very sensitive to pollution and cannot live in large numbers in places that are polluted.
Common stonefly.
Water quality varied across the study sites, reflecting the local environmental conditions and minor anthropogenic influences in the streams, but remained within the permissible limits set by the Bureau of Indian Standards and the World Health Organisation. There were some differences in water velocity, which was highest at Site 5 (Madhan) and lowest at Site 1 (Narikatal). High velocity can help in enhancing oxygenation and reduce sediment deposition. There were some differences in conductivity by sites, with it being lowest at Site 4 (Pupaldanga) and highest at Site 1 (Narikatal), indicating differences in dissolved ions. The water temperature varied, with the lowest values at Site 1 (Narikatal) and the highest at Site 5 (Madhan), which affected the metabolic activity of aquatic organisms.
Total hardness, dissolved oxygen (DO) and total dissolved solids (TDS) indicated moderate mineral content and good water quality. Biological oxygen demand (BOD) and free CO₂ values were slightly elevated at Site 4 (Pupaldanga), suggesting minor organic enrichment. Overall the stream had healthy, supportive conditions for aquatic life, with Site 4 (Pupaldanga) showing some localised impacts due to urbanisation and land use changes. The phosphate levels were high at Site 3 (Seela) due to organic input and runoff from surrounding areas and nitrate values were high at Site 2 (Jarpani) linked to agricultural or domestic runoff in the catchment area of the stream indicating moderate enrichment without severe eutrophication.
Mayfly found in Kanjirapally, India.
Taken together, the findings paint a reassuring picture. The Rawasan stream supports a rich and varied community of benthic macroinvertebrates, dominated by pollution sensitive groups whose presence, backed by supportive water quality data, confirms a stream that remains clean, resilient and largely undisturbed. In an unexplored Himalayan ecosystem, this study shows how nature's own small residents can offer a reliable, low cost way of tracking freshwater health, without complex equipment or laboratory infrastructure alone.
That reassurance, though, comes with a responsibility to keep watching. The early signs of organic enrichment at Pupaldanga, Seela and Jarpani are a reminder that even a healthy stream can shift under the pressure of agricultural and domestic runoff, and that continuous monitoring will be essential to catch such changes early, protect biodiversity, and guide conservation efforts in this ecologically fragile region.
Caddisfly larva in the water.
The stakes extend beyond the insects themselves. Changes in the aquatic insect community can directly affect the diversity and health of fish populations further up the food chain, a link that matters well beyond ecology alone. Across many developing countries, rapid population growth has driven strong demand for protein-rich diets, with fish often the most accessible source amid widespread protein deficiency. Conservation efforts, the study suggests, cannot focus on fish alone. They must also protect the small, easily overlooked creatures such as benthic insects and macroinvertebrates that form the base of the food chain and on which the fish, and ultimately the communities who depend on them, both rely.