The Geology of Running Water

C15

Where does water go when it rains?

• Accumulates

1. Velocity - Slope

2. Discharge - How far down a stream you are?

3. Deposits

Stream Order

• Numbers on stream indicate stream order

• Dashed Lines are drainage divides (not all divides and basins are shown)

What is stream discharge?

• The volume of water that passes through an imaginary cross section across a stream.

  • Expressed at some volume per time

• Generally, increases downstream

• controls amount of material a river can carry

What is stream velocity?

• The rate of flow (not amount of water)

• Discharge and velocity not equivalent

What is competence?

The degree of resistance of a rock to deformation, erosion, or flow, essentially its relative mechanical strength. (Maximum clast size a stream can carry.)

What is the Thalweg?

• The deepest part of the channel (an usually the highest current velocity)

• Produces turbulence

Why is flow in channels usually turbulent?

• Streams occur, break, abrade, and dissolve material → possible ways to do it

• More discharge = More materials (Size depends on Velocity)

What are the three types of sediment transport (definitions)?

• The material moved by streams is the sediment load

1. Suspended Load - consists of tiny, solid grains (generally silt or clay) that swirl along with the water without settling to the streambed.

2. Bed Load - consists of large clasts (sand, pebbles, or cobbles)

   1. Uses Traction move by rolling or slipping along the streambed

   2. Saltation a process during which grains bounce along in the direction of flow.

3. Dissolved Load - consists of the ions that a stream carries in solution.

Why do cross section change with discharge?

A stream’s cross section changes with discharge because the channel’s width and depth adjust in response to the volume of water flowing, driven by hydraulic balance, erosion, and sediment transport processes

What is the long profile?

• long profile of a river is a side view showing how its gradient decreases from the steep source in the upper course to the gentle slope at the mouth.

  • Lake Travis

  • Cannot erode pass base level

What is the longitudinal profile?

• Toward the mouth of a stream, the gradient flattens, discharge increases, sediment grain sizes are smaller, and channels develop broad meander belts.

• (C) flooding

• Base Level - the lowest point to which a stream can erode

What is base level?

• Base level is the lowest point to which a stream can erode.

• Base-level changes cause stream readjustments. Raising base level results in an increase in deposition. Lowering base level accelerates erosion.

Why does the base level happen?

Steep makes water erosive … Smooth → Longitudinal

What is the relationship between base levels and waterfalls?

Waterfalls often form where a river encounters a local base level — a temporary “floor” that limits downward erosion — creating a sudden drop in elevation in the river’s profile

What are meandering streams?

Channels from intricately looping meanders along the lower gradient portion of the longitudinal profile

How do meandering streams happen?

• erosion on the cut bank

• cutting of the meander neck

What are the different features of cut bank and point bar?

Cut Bank - outside bank of a curve or meander, along a stream, where erosion takes place.

Point Bar - wedge-shaped deposit of sediment on the inside bank of a meander.

How is incision driven by changes in base level?

What are alluvium’s in stream deposits?

Leaves - New Orlean hurricane natural vs man made

What are Deltas?

• When rivers enter open water such as an ocean, they deposit sediment as they lose energy

• Greek historians originally termed these deltas because the Nile river delta is roughly shaped like the Greek letter delta. Deltas can have many shapes

What is a river dominate delta?

• Mississippi

• River sediment exceed removal delta prograde out rapidly subject to

• deliver so much water, lots of sediment

What is a wave dominated delta?

• High wave energy, Chanel prograde slowly sand is well sorted, parallel beach ridges of redistributed Seds

What is a tide dominated delta?

High tidal range distributary channels are widened by tides

How do you classify deltas (marine)?

1. sediment delivery (rivers)

2. sediment redistribution (waves)

3. sediment removal/erosion (tides)

What is Delta evolution?

What is subsidence at deltas?