Why does rain wash soil off bare slopes?

How it works

Raindrops fall from high up and hit harder than you might think. On bare soil they splash and break up the little crumbs. In heavy rain, the ground can't soak up water fast enough, so it runs down the slope, faster and faster, carrying loose soil with it and carving little gullies.

A slope covered in plants is different. Leaves and grass catch raindrops so the water drips down gently. A thick layer of fallen leaves, called leaf litter, covers the soil like a blanket and soaks up water like a sponge. Underground, roots weave a net that grips the soil.

Roots also leave tiny tunnels that make soil loose and spongy, so rain soaks into the ground instead of racing across the top. That way, more soil stays on the hill, less mud washes into rivers, and the water stays clearer.

Where to see it in New Zealand

On the North Island's East Coast, around Tairāwhiti / Gisborne, many hillsides lost their original forest and now slip easily in heavy rain, turning rivers brown with mud. Many farmers plant poplar and willow trees on steep slopes so their roots help hold the soil together.

Try it together

Fill two plastic containers with soil. Leave one bare and cover the other with grass turf or a layer of leaves. Tilt both, with a cup under each to catch the runoff. Sprinkle the same amount of water on both with a watering can. Which cup collects muddier water?