A freshwater ecologist explains why the mountain is less a postcard and more a slow-leaking, bug-monitored water machine.

Table Mountain is a wetland. Not in the reedy, duck-filled sense people usually imagine, but in a much more surprising way. It's an ancient sponge, a seep, a slow-release valve. It's a piece of prehistoric plumbing that catches rain, holds it, filters it and feeds Cape Town both above and below ground, long after the clouds have moved on.

Water doesn't just rush off the mountain in streams. Much of it sinks into cracks and layers of sandstone, trickling downslope underground, recharging aquifers in the city far below.

If you want to understand how well this system is functioning, you don't start with pipes or charts or satellite images. You start with bugs.

Dr Ruth Fisher is a freshwater ecologist with Table Mountain National Park, which means she spends her days monitoring rivers that most people barely register. She reads water the way others read faces.

Her informants are small, soft-bodied and uncharismatic by Instagram standards: stoneflies, black fly larvae, ghost frog tadpoles. Creatures that tell her stories without saying a word. "These systems are sensitive," she says, carefully. "And the organisms tell us what's happening."

Freshwater ecologists call them macroinvertebrates, but think of them instead as the mountain's internal...