The River Sample

Every Tuesday for a year, Devika had hiked the same half-mile stretch of river to log water temperature, oxygen levels, and the number of mayflies she could count in ten minutes. Her original hypothesis was simple: warmer water in July would mean fewer mayflies, since the insects preferred cooler currents.

The early data agreed with her. But by August, something odd appeared — a discrepancy between what her thermometer showed and what the mayfly counts implied. The water was warm, exactly as predicted, yet the insects kept appearing in unusually large numbers.

She brought her notebook to her advisor, half-expecting to be told she'd measured something wrong. Instead, he suggested they check a nearby weather station's records. If shade cover along that bend had increased — new growth on the bank, perhaps — it could corroborate an alternate explanation: the insects weren't responding to water temperature at all, but to sunlight hitting the surface.

They spent a rigorous two weeks re-walking the same stretch, this time logging canopy cover at each site, refusing to skip a single Tuesday even in the rain. The pattern held. Shade, not heat, explained the numbers.

Devika's advisor warned her not to extrapolate too far from one river. "This bend isn't every river," he said. "Test it somewhere else before you write the paper." She nodded, already planning her next hike, further downstream, into a stretch of river she had never surveyed at all.