How Erosion Changes Landscapes

Understanding How Erosion Changes Landscapes

Erosion begins only after material is available to be picked up. Cracks in rock widen when water freezes, plant roots grow, or temperatures change. These processes make loose fragments.

Flowing water then sorts those fragments by size. Fast water can roll large pebbles along the bed, while slower water may carry only sand, silt, or clay. Grains scraping along a stream bed act like sandpaper.

This wears away the channel, especially on the outside of river bends where water moves fastest. The inside of a bend often gains a bar of sand because the current loses energy there.

A river does not carry the same amount of sediment everywhere. Its carrying ability changes after heavy rain, snowmelt, drought, or the building of a dam. Floodwater can spread across a floodplain and leave layers of fine mud when it slows down.

This can make floodplain soils fertile, but it can bury roads, fields, and homes. Dams trap sediment that would normally move downstream.

As a result, a beach or delta beyond a dam may receive less new material. Riverbanks can then wear away more quickly because the river has less sediment available to deposit.

On coasts, waves move sand along the shore in a process called longshore drift. Waves often arrive at an angle, pushing sand up the beach diagonally. The returning water pulls sand straight down the slope.

Repeated wave action shifts sediment along the coastline. A groyne or seawall can protect one location, yet it may reduce sand supply farther along the beach. Cliffs can fail when waves cut a notch at their base.

The unsupported rock above becomes unstable and falls. People living near coasts need to notice changing beach width, exposed roots, cracks near cliff edges, and storm forecasts.

Ice changes land in a different way because it moves slowly but has enormous weight. A glacier can pull blocks from fractured bedrock. It can grind the surface beneath it with rocks frozen into its base.

This produces deep valleys with broad floors and steep sides. When the ice melts, it leaves piles and ridges of unsorted material. These deposits can redirect streams and create lakes.

Wind works best where dry, bare ground leaves particles exposed. It can remove fine dust from fields, then deposit it far away. Plants reduce this loss because roots hold soil while leaves slow the wind near the ground.

Erosion rates are often too small to notice during a school day, yet single events can cause rapid change. A storm can cut gullies into a bare construction site within hours. Farmers use cover crops, contour plowing, and strips of grass to slow runoff.

Engineers use drainage systems to keep water from soaking and weakening slopes. When studying landscapes, look for clues about energy and direction. Rounded pebbles suggest repeated movement in water.

Sharp fragments may have traveled only a short distance. Layers that become finer upward often show water slowing down. Linking the sediment to the moving agent helps explain how a landform developed.