Erosion The transport of earth materials from one place to another. AGENTS Running water (MOST ACTIVE AGENT) Moving air (Wind) Glaciers FORCE driving all agents = GRAVITY

Erosion by water Streams carry sediments four (4) ways: Floatation – Light materials on surface. Suspension – materials between top and bottom, sometimes causing streams to look muddy Solution – Materials that dissolve in water (soluble materials) can’t be seen. Traction – Bed load materials along bottom of stream.

Sediment size & Stream velocity All sediments are moving slower than the stream due to gravity and friction. Size of sediments that can be eroded depend on the velocity of the stream (See pg 6 ESRT). As velocity decreases larger sediments are deposited. As velocity increases lager sediments are eroded. Become familiar with chart on page 6 ESRT and page 261 in text.

DEPOSITION Factors that affect RATE of DEPOSITION: Particle size – The larger particles settle fastest. Particle shape – The roundest ones settle fastest. Particle density – The more dense sediments settle fastest. Stream velocity – As already discussed

Sorting of Sediments GLACIAL DEPOSITS ARE UNSORTED. All mixed up!!! Since some sediments settle faster or slower than others they tend to get separated (sorted) by size, shape & density. SORTING only happens with WIND or WATER GLACIAL DEPOSITS ARE UNSORTED. All mixed up!!!

Two types of sorting Vertical sorting or Graded bedding Associated with still water. Larger sediments on bottom smaller on top. Horizontal Sorting Associated with moving water that is slowing down. Larger sediments near the mouth smaller sediments farter down stream.

Mission of a River Running water (Most active agent of weathering & erosion. The job of a river is to lower the land!!! Steep slopes = high velocity High velocity = lots of erosion Erosion decreases slope Decreased slopes = lower velocity Lower velocity = more deposition usually further down hill.

Erosion vs. Deposition along Meanders! Outside of Meander Faster water (high velocity) Deeper channel due to more erosion *SEE PICTURE Inside of Meanders Slower water (lower velocity) Shallower channel due to more deposition Sand and Gravel bars

Dynamic equilibrium UPLIFT vs. EROSION If the forces of UPLIFT are more active than EROSION that part of the crust will appear to grow higher. If the amount of EROSION is greater than the amount of UPLIFT that part of the crust will be lowered.

Stages of stream development Rivers take hundreds or thousands of years to wear down the land. Rivers go through stages of development as they do their work. Youthful Mature Old age

Youthful Early stage of stream development. Low volume of water (Discharge). Very steep slopes (Gradient). Lots of kinetic energy used to “down cut” a narrow “V” shape (lots of erosion). Straight path (Stream channel) for water. Water attempts to blast through obstacles rather than go around.

Mature Middle stage of stream development. Tributaries and groundwater add to stream volume. Slope (gradient) become less steep Less kinetic energy (erosion & deposition) Meanders Develop Stream channel begins to bend as water goes around obstacles.

Old age Late stage of stream development Stream volume is greatest Kinetic energy is least. Very gentle slopes. Lots of silt and clay transported. Meanders shift side to side across the “wide valley” floor. Oxbow Lakes form as river changes course. Flood often depositing silt on valley floor