4.2 Erosion, Deposition & Rivers

Erosion and the Agents That Move Sediment

Erosion is the transport of weathered rock material from one place to another. Deposition (sedimentation) is the dropping or settling of that material when the moving agent slows down and loses energy. The Regents groups four agents of erosion:

• Running water (streams and rivers): the dominant agent worldwide; it shapes most of New York's stream valleys.
• Wind: important in deserts and on beaches; moves only small, light particles such as sand and silt.
• Glaciers (moving ice): scour and carry everything from clay to house-sized boulders.
• Gravity (mass movement): landslides, slumps, and rockfalls move material directly downslope.

For every agent the rule is the same: erosion needs energy. When the agent has high energy (fast, steep, abundant), it can move large amounts of large sediment. When it loses energy, deposition occurs.

Gravity acts on all of them as the ultimate driver: streams flow downhill, glaciers creep downslope, and mass movements drop material straight down. Mass movement ranges from sudden rockfalls and landslides to slow slump and creep, the gradual downhill flow of soil that tilts fence posts and telephone poles over years. Recognizing gravity as an erosional agent in its own right is a common Regents point.

Stream Velocity vs. Particle Size (ESRT Relationship)

The Earth Science Reference Tables (ESRT) include the chart Relationship of Transported Particle Size to Water Velocity. You must be able to read it.

The core rule: the faster the water moves, the larger the particle it can carry. Slow water can move only clay and silt; fast water can move sand, then pebbles, then cobbles, then boulders. The chart lets you read off the largest particle a given velocity can transport.

What speeds a stream up?

• A steeper gradient (slope).
• A greater discharge (volume of water, as after heavy rain or snowmelt).
• The outside of a meander bend and the center/surface of a channel, where friction is lowest.

Where a stream slows down, near its mouth, on the inside of a bend, or where it floods over its banks, it deposits sediment. A common cluster question gives a velocity and asks the largest particle the stream can move, or gives a particle size and asks the minimum velocity required.

Deposition and Sorting

When a transporting agent slows, particles settle out in a predictable order. Settling depends on three properties:

• Size: larger particles settle faster and first.
• Shape: rounder, more spherical particles settle faster than flat or angular ones of the same size.
• Density: denser particles settle faster than less-dense particles of the same size.

Because of this, water and wind produce sorted sediment (particles separated by size). Sorting appears in two patterns:

• Horizontal sorting: along a stream, the largest particles drop first near the source and the smallest are carried farthest, so particle size decreases downstream.
• Vertical sorting (graded bedding): in still water, a single batch of mixed sediment settles with the largest particles on the bottom and the finest on top.

This table is the fastest way to answer an origin question: sorted and rounded = water or wind; unsorted and angular = glacier.

River Features

A stream carves and builds recognizable landforms as it ages:

• V-shaped valley: a young, steep stream cuts downward, producing a narrow V-shaped valley.
• Meanders: on gentle gradients the channel loops; erosion attacks the fast outside (cut) bank while deposition builds a point bar on the slow inside.
• Floodplain: the flat valley floor covered with fine sediment when the stream overflows.
• Oxbow lake: a cutoff meander loop left behind when the stream takes a straighter path.
• Delta: a fan of deposited sediment where a stream enters a standing body of water (a lake or ocean) and abruptly loses velocity.

These features all reflect the velocity rule: erosion where water is fast, deposition where it is slow.

Glacial Features

Most of New York State was shaped by continental glaciers during the Ice Age, so glacial landforms are a major Regents topic. Erosional features form where ice scrapes bedrock; depositional features form where ice melts and drops its load.

• U-shaped valley: a glacier widens and deepens a former V-shaped stream valley into a broad U.
• Striations (scratches) and grooves: parallel scratches gouged into bedrock by rock fragments frozen in the ice; they record the direction the ice flowed.
• Moraine: a ridge of unsorted sediment (till) bulldozed and dumped at the edge of a glacier; the terminal moraine marks the farthest advance (Long Island contains terminal moraines).
• Drumlin: a streamlined, elongated hill of till shaped under the ice; its long axis parallels ice flow, with the steep, blunt end facing the direction the ice came from. Western New York has classic drumlin fields.
• Erratic: a boulder transported and dropped far from its source bedrock; its mismatched composition proves long-distance ice transport.
• Kettle lake: a depression left when a buried ice block melted, common across New York.

The quick origin test for any glacial deposit is the unsorted, angular signature that no flowing water or wind could produce.