Conserving mountain areas
Mountains are steep. Rain runs fast down a steep slope and can carry soil away. A forest slows this in four ways: leaves break the fall of raindrops, leaf litter on the ground soaks up water, roots bind the soil, and the soil acts like a sponge. Together they reduce floods, keep rivers clean and lower the chance of landslides.
Soil that is washed off is gone for a very long time: it takes hundreds of years to form a thin layer of soil. This is why conserving mountain forest is cheaper than repairing a damaged slope.
Erosion is the removal of soil by water or wind. A landslide is when a whole block of soil and rock slips down the slope, often after heavy rain.
Erosion-control works
When a slope is already damaged, people build erosion-control works:
- Check dams: small walls of stone or concrete across a gully. They slow the water, so soil grains settle behind them.
- Planting works: rows of young trees and grass hold the soil and slowly bring the forest back.
- Terraces and slope steps: flat steps cut on the slope so that water walks slowly instead of running.
- Drains and retaining walls: lead the water away safely and hold up weak slopes.
- Forest reserves for slope safety: some forests are marked as protection forests where cutting is limited.
In the 3D, notice that check dams alone cut the soil loss, and trees plus dams cut it most.
Japanese farming and mountain villages
Farming and mountain villages sit at the foot of or on the slopes of mountains, in Japan and in many other countries such as India, Nepal and Switzerland. Villagers farm on terraces, collect firewood and wild foods, and look after the forest. In return the forest gives them water, safe slopes and income.
Many hill villages today have fewer and older people because young people move to cities. When fewer people are left, terraces and forests are not looked after, fields turn to scrub and slopes weaken. Ways to help: community forest work, farm and forest tourism, selling local wood and food, and people from cities helping as volunteers.
Try it: a bottle slope
In the 3D, set trees to 0 and rain to 3, and read the soil units. Then add trees and check dams. At home: fill two trays with soil, put grass on one, tilt both and pour the same amount of water. Compare the muddy water that runs off each.
Key formulas and definitions
- More tree cover = less soil washed away
- Soil loss falls further with check dams
- Roots + leaves + litter = slope protection
- Erosion works: check dams, planting, terraces, drains, retaining walls
Worked examples
1. A slope has 12 tree spots. 6 have trees. In the 3D model with rain 2 and no dams, soil reaching the village = (1 - trees/12) x rain x 100. Find it.
(1 - 6/12) x 2 x 100 = 0.5 x 200 = 100 units.
2. Now add check dams, which reduce the loss to one quarter. What is it?
100 x 0.25 = 25 units.
3. Why can a check dam fill with soil and still be useful?
The soil behind it makes the gully bed flatter and the water flows slower. Plants can then grow on the soil and hold it.
4. A village loses half its young people. Name two slope problems that can follow.
Terraces are not repaired and may fall, and forest is not weeded or thinned, so the slope can slip in heavy rain.
Common mistakes
- Thinking rain alone causes erosion. Bare soil with no roots or leaves is the real reason.
- Thinking check dams stop water. They slow the water so soil settles.
- Thinking erosion-control works are only for the worst slopes. Keeping the forest is the cheapest protection.
- Thinking a village and its forest are separate. People look after both and both give back.