Section 3 Freshwater - Honors Science

What can flowing water do?

Weather rock, erode and transport sediment, deposit material, and reshape landscapes

The way water moves is influence by:

• The shape of the landscape

How are water and landscape related?

They influence one another and are a Dynamic System

Stream

Flowing surface water contained within a channel (describes flowing water of many sizes)

Why are channels not permanent?

Because velocity and discharge change the location of erosion

River

A larger stream

Headwaters

Where the river system begins, it is the upper portion of the drainage network

Mouth

Where the drainage network eventually reaches and flows into another river, a lake, or the ocean

What is important to know about rivers that are used a bunch?

They barely contribute to discharge at the mouth (ex. the ocean)

Tributary

A smaller stream that joins the main channel along the way

Gradient

The change in elevation of a stream over a given horizontal distance (the steepness of slope of a stream or river) (it influences how water moves)

What is the Gradient equation?

Rise/Run or Elevation/Distance

Steep Gradient

• Elevation lost rapidly

• Gravity accelerates water downhill

• Higher water velocities are favored

Gentle Gradient

• Less gravitational energy available

• Flow more slowly

What is velocity influenced by?

• Channel shape

• Roughness

• Depth

• Volume of flowing water

• Channel widens

• Obstruction is met (a bend in the river)

Velocity

Describes both the speed and direction of moving water (commonly expressed in m/s) (The distance the water in a stream travels in a given time)

How does the velocity of water throughout the stream change?

Friction slows water in contact with stream bed and banks

What does the amount of water in a stream depend on?

The size of the channel

Cross sectional area

Area of stream channel occupied by water when viewed as if the channel had been sliced from one bank to the other

Simplified Rectangular Channel Equation

Cross-Sectional area = width x average depth

How do Hydrologists determine the velocity of a channel (Refer to Figure 3.2)

They determine the velocity of smaller subsections and divide the channel to get a more accurate estimate

Discharge

The amount or volume of water passing a particular location in a stream in a given amount of time

Discharge Equation

Q = A x V

Q = Discharge, A =Cross-sectional Area (Determined using depth x width), V = Average Velocity

When is D (Cubic meters per second (m³/s) used?)

If A = square meters (m²) and V = meters per second (m/s)

When will discharge increase downstream

As gradient decreases because as a river goes downstream, tributaries add to the main channel and Watershed gets larger. Channel becomes wider and deeper.

When can discharge decrease

If water is diverted for agricultural use

What are the three main processes in how streams shape landscapes?

1. Weathering

2. Erosion

3. Deposition

Weathering

Breaks rock into smaller pieces or alters its composition (Disintegration or Decomposition at or near Earth’s surface)

Erosion

Removes and transports material from one location to another (the process by which material is removed and carried away)

Deposition

Occurs when transported material settles in a new location (water loses energy and drops sediment it is carrying)

Physical Weathering (Disintegration)

Breaks rock into smaller pieces without changing its chemical composition (Ex. Abrasion, freezing and thawing of water in cracks, physical breakup of rock by plant roots)

Chemical Weathering (Decomposition)

Changes chemical composition of minerals within rock

How do PW and CW frequently operate together?

PW exposes new mineral surfaces for CW

What does water being a Polar Molecule allow it to do?

Dissolve substances and participate in chemical reactions

Sediment load

Formed by material that is transported by the river

What is the order of sediment sizes?

Clay → Silt → Sand → Gravel → Cobble → Boulder

(Larger particles generally settle before smaller particles as energy decreases)

What is important to know about fine clay?

Cohesive particles can require high velocities initially to erode

What influences how easily flowing water transports sediment?

The sediment size

Competence

Largest particle size a stream can transport

Velocity increases → Competence increases

Capacity

Total amount of sediment a stream can transport (large volume of rapid water = greater capacity)

What can happen when velocity or discharge decreases?

Deposition because the stream can no longer carry the sediment load

Meander

Bend or curve in a river channel

Outbank

The outside of the meander which usually experiences greater erosion

Point Bar

Where there is lower velocity, encouraging deposition at the inside of the meander

Outside VS Inside of Meander

Outside of bend → higher velocity → erosion → cutbanks

Inside of bend → lower velocity → deposition → point bar

(Channel migrates across floodplain gradually because of this)

What happens as the channel migrates across the floodplain

Meander bends can become more pronounced as meanders migrate (oxbow lakes begin to form)

Oxbow lake

When adjacent portions of the channel become separated as erosion narrows the strip of land, causing water to cut across for a shorter route during high flow, separating bend from the channel

Steps of an Oxbow Lake

1. Meander develops

2. Meander enlarges

3. River cuts through the neck

4. Deposition isolates an oxbow lake

Floodplain

Relatively flat land adjacent to a river that is periodically covered by water when the river exceeds the capacity of the channel

What shapes a flood plain overtime?

Velocity decreases and sediment is deposited during a flood as water spreads across the floodplain

• The plain becomes truly shaped through repeated flooding, erosion, deposition, and channel migration

What do floods do?

• Damage human structures

• Redistribute sediment tributaries

• Contribute to the development of river ecosystems

Near Headwaters (General Trends)

• Streams are commonly narrow and relatively steep

• Channels can contain coarse sediment, gravel, cobbles, and boulders

Farther Downstream (General Trends)

• Tributaries add water and discharge increases

• Channel becomes wider and deeper, gradient decreases

• Sediment becomes smaller, rounded, weathered, and sorted

Near the Mouth (General Trends)

• Large rivers have high discharge and low gradients

• Fine sediment may be transported long distances

Delta

Forms when sediment accumulates at the mouth of the river faster than it is removed, causing the deposited material to build outward

For deltas, how does sediment accumulate?

When velocity decreases in a sediment-carrying river entering an ocean or lake making small channels called distributaries

Watershed

Where water flowing through a river comes from

• Tributaries collect water from the different portions of a watershed to feed the main river

What becomes altered when there is a change in precipitation, runoff, infiltration, groundwater input, and land use?

• Stream discharge

• Velocity

• Erosion

• Sediment Transport

• Deposition

River systems are shaped by:

• Interactions among gradient

• Velocity

• Cross-Sectional area

• Discharge

River System

A river and all of it’s tributaries

Divide

The high land that separates one drainage basin from another

Load

The eroded rock and soil materials that are transported downstream by a river

Bed load

Sand, pebbles, and boulders that are too heavy to be carried in suspension, which are moved along the streambed through rolling, sliding, or saltation (bouncing)

Suspension

The state of a substance when its particles are mixed with but undissolved in a fluid or solid

Suspended Load

The portion of sediment, like silt and clay, that is carried within the water and floats along without settling to the bottom

Solution

The state in which mineral matter dissolved from bedrock is carried in a river

Dissolved Load

Consists of minerals and other soluble materials that are carried in the water after being dissolved from rocks and soil

What are the stages of a stream

• Youthful

• Mature

• Old age

Youthful

• Steep gradient

• V-shaped valley

Mature

• Less steep gradient

• Stream begins to meander

Old Age

• Shallow gradient

• Wide flood plain

• Stream meanders

V-shaped valley

A valley formed by flowing water, or river valley, is usually V-shaped

Natural Levee

Raised banks formed from sediment build up along the edges of the river during floods, creating barriers that help prevent future flooding

Base level

The lowest point a stream can erode its channel

Gravel

Made from Boulders, Cobbles, and Pebbles