Ch. 16 - Running Water

Class: GEOL-101

Notes:

16.1 Earth as a System: The Hydrologic Cycle

List the hydrosphere’s major reservoirs and describe the different paths that water takes through the hydrologic cycle.

Unique Earth

Presentation content:

• Earth is unique in the solar system
  • Right size and distance from the Sun to have liquid water
  • Mantle convection brings water to Earth’s surface through volcanism
• The hydrologic cycle describes the movement of water through Earth’s four spheres
  • Geosphere, hydrosphere, atmosphere, biosphere

Movement Through the Hydrologic Cycle

Presentation content:

• Movement Through the Hydrologic Cycle
  • Evaporation/Transpiration
  • Precipitation
    • Soaks into the ground (infiltration),
    • Runs over the surface (runoff),
    • Evaporates, or
    • Is stored as part of a snowfield or glacier
• The Hydrologic Cycle is balanced:
  • water is constantly moving from one reservoir to another, but the total amount on earth remains the same

16.2 River Systems

Describe the nature of drainage basins and river systems. Sketch and briefly explain four basic drainage patterns.

Runoff vs. infiltration

Presentation content:

• The difference between runoff and infiltration depends on
  • Intensity and duration of rainfall
  • The amount of water already in the soil
  • The type of soil
  • Slope of the land
  • Nature of the vegetative cover
• When the surface is impermeable or saturated, runoff is dominant
  • Runoff is high in urban areas due to buildings, roads and parking lots

Runoff

Presentation content:

• Runoff Will Start as Sheet Flow
  • Sheet flow develops into tiny channels called rills
  • Rills meet to form gullies
• Gullies join to form brooks, creeks, or streams
  • A stream is any water the flows in a channel, regardless of size
  • A river carries a substantial amount of water and has many tributaries

Drainage Basins

Presentation content:

• A stream drains an area of land called a drainage basin or watershed
• The imaginary line separating one basin from another is called a divide
  • Sometimes visible as a high ridge in a mountainous region
  • Sometimes hard to determine in subdued topography

• A continental divide splits a continent into different drainage basins
• If you observed streams over several years, you would see many lengthen by headward erosion

River Systems

Presentation content:

• Rivers drain much of the land area
  • Exceptions: extremely arid or polar regions
• The variety of rivers that exist reflect different environments
• Climate differences and human intervention influence the character of a river
• River systems can be divided into three zones
  • Sediment production (erosion dominates)
  • Sediment transportation
  • Sediment deposition

• Sediment production
  • Zone where most sediment is derived
  • Sediment generated by
    • Bedrock broken into smaller pieces
    • Bank erosion
    • Scouring of the channel bed
• Sediment transport
  • Sediment is transported in trunk streams
• Sediment deposition
  • When a river reaches a large body of water, the energy decreases and the river deposits sediments
  • Typically only fine sediments are deposited in oceans

Drainage Systems

Presentation content:

• Drainage systems are patterns of the interconnected network of streams in an area

Formation of a Water Gap

Presentation content:

• A water gap is a notch where a river cuts through a ridge
• Two possible methods of formation:
  • Antecedent stream
    • Stream existed before the ridge was uplifted
  • Superposed stream
    • Stream eroded into a preexisting structure buried beneath layers of relatively flat lying strata

16.3 Streamflow Characteristics

Discuss streamflow and the factors that cause it to change.

Gravity

Presentation content:

• Water Moves in a River Channel Under the Influence of Gravity
  • Water slowly flowing in a nearly straight path is called laminar flow
    • Water is not standing still, it is moving slowly toward the bottom.
  • Water moving quickly in an erratic fashion (both horizontal and vertical movement) is called turbulent flow
    • Example: running the rapids in the Grand Canyon

Factors Affecting Flow Velocity

Presentation content:

• The slope, or gradient, of the stream
  • A steeper gradient has more gravitational energy to drive channel flow
• Channel shape
  • The wetted perimeter is the area where the river is in contact with the channel
  • The most efficient channel has a small wetted perimeter compared to its cross-sectional area
    • A narrow, deep channel has a small wetted perimeter, less frictional drag, and will flow more efficiently
• Channel size and roughness
  • Water depth affects frictional resistance
    • Maximum flow velocity occurs when a stream is bankfull
  • An increase in channel size will increase the cross-sectional area to wetted perimeter ratio, thus increasing channel efficiency
  • Rough channels (boulders, etc.) create turbulence and decreased velocity
• Discharge
  • Discharge is the volume of water flowing past a certain point in a given unit of time
  • When discharge increases, the width, depth, and flow velocity increase predictably

• Monitoring streamflow
  • The U.S. Geological Survey (USGS) measures flow velocity, discharge, and river stage (height of water surface relative to a fixed point)
  • Stream gaging stations
    • Flow velocity is measured by a current meter at several places across the stream channel. The newest meters use Doppler radar. Older meters use a propeller that spins in response to the moving water.
    • A stilling well, used to determine the stage of a river, is connected to the stream with pipes. When the water level in the stream changes, the water level in the well changes. A float is connected to a recorder. Data can be sent via satellite to various agencies.

Changes Downstream

Presentation content:

• A longitudinal profile is a cross-sectional view of a stream
  • Head or headwater is the source of the stream
  • Mouth is the downstream point where the stream empties into a larger body of water
• Most longitudinal profiles have a concave shape
  • Discharge increases toward the mouth
  • Channel size and velocity also increase toward the mouth
  • Slope decreases downstream
  • Volume increases downstream

Stream Erosion

Presentation content:

• Erosion related to slope, discharge, and bed/bank sediments

  • Sand-sized particles are easily eroded
  • Silt/clay-sized particles and gravels are harder to erode
    • Channels with cohesive silty bottoms are typically narrower than sandy channels

• Streams cut channels by quarrying, abrasion, and corrosion

• Quarrying involves removing large blocks from the channel bed

  • Aided by fracturing of bedrock

  

• By scraping, bumping, and rubbing, abrasion both erodes sediments and polishes them while cutting a bedrock channel

  • Potholes form when fast moving, swirling sediment in eddies abrades a hole by acting like a drill into the streambed

• Corrosion (rocks gradually dissolving in flowing water) can occur in limestone bedrock channels

16.4 The Work of Running Water

Outline the ways in which streams erode, transport, and deposit sediment.

Transport of Sediment by Streams

Presentation content:

• All streams transport some sediment
• Sediment load is transported in three ways:
  • Dissolved load (in solution)
  • Suspended load (in suspension)
  • Bed load (sliding, skipping, or rolling along the bottom)

Notes from the lecture:

• River does not need a high velocity to have a stream flow

Dissolved load

Presentation content:

• Most of the dissolved load is brought to a stream via groundwater
• Dissolved load is not affected by stream velocity
• Dissolved minerals precipitate when water chemistry changes
• When organisms create hard parts
• When water enters an inland “sea” where the evaporation rate is high

Suspended load

Presentation content:

• The largest part of a stream’s load is carried in suspension
  • Usually only fine sand, silt, and clay are carried this way
• Amount of material carried in suspension is controlled by stream velocity and settling velocity of sediments
  • Settling velocity is the speed at which a particle falls through a still liquid

Bed load

Presentation content:

• Coarse sands, gravel, and boulders move along the stream bed by saltation (skipping or jumping)
• Larger particles slide or roll along the bottom
• Less rapid and more localized than transport via suspended load
• Coarse gravels may only be moved during times of high flow while boulders move only during exceptional floods

Capacity and competency

Presentation content:

• Capacity and competence
  • Describes a stream’s ability to carry solid particles
  • Capacity is the maximum load of solid particles a stream can carry per unit time
    • The greater the discharge, the greater the capacity
  • Competence is the maximum particle size a stream can transport
    • Streams with a faster velocity have a higher competence
• Deposition of Sediment by a Stream
  • Deposition occurs when a stream’s velocity is less than the settling velocity
  • Particles of the same size are deposited at the same time in a process called sorting
    • Larger particles settle out first
  • Sediments deposited by streams are called alluvium

Notes from the lecture:

• Velocity influences a lot the capacity and competence
• Rivers are very effective in naturally sorting out grained sizes (finer sediments)
• As you move down-gradient from the Headwaters you get more sorted material
• Alluvial and river systems are ideal because of this natural sorting
  • Alluvium = sediments being generated at the Headwaters
  • Fluvial is sediment generated at the down-gradient part from the Headwaters
• Even in deep water that sorts out different particles
  • Sand mixed with marbles -> not effective allowance for flowing

Bedrock Channels

Presentation content:

• Bedrock channels are cut into the underlying strata
  • Typically form in the headwater region where streams have a steep slope
  • Energetic flow tends to transport coarse particles that actively abrade the bedrock channel
  • Steps and pools are common features of bedrock channels
  • Channel pattern is controlled by the underlying geology

Alluvial Channels

Presentation content:

• Alluvial channels form in sediment previously deposited in the valley
  • Typically associated with a floodplain
  • Channels can change shape as material is eroded and transported
  • Channel shape is affected by the average size of sediment, gradient, and discharge
  • Channel patterns reflect the stream’s ability to transport load at a uniform rate while expending the least amount of energy

Notes from the lecture:

• Donwcutting, eroding, and transporting existing sediments
• Basically recycling the materials
• The system is trying to distribute the energy

Meandering Channels

Presentation content:

• Streams transport suspended sediment in broad, sweeping bends called meanders
• Relatively deep, smooth channels, primarily transporting mud
• Meandering channels evolve over time
  • The outside of a meander (cutbank) is a zone of active erosion
  • The inside of a meander (point bar) is a zone of deposition
  • Through time, the bends in a channel can also migrate and eventually join together
    • A meander that has been cut off from joined bends is called a cutoff oxbow lake

Notes from the lecture:

• Decreasing energy on the other side -> you actually have deposition

• Eventually the river system will cut and create a shortcut for water to flow, making other parts of the channel to stop receiving flow

16.5 Stream Channels

Compare and contrast bedrock and alluvial stream channels. Distinguish between two types of alluvial channels.

Braided channels

Presentation content:

• A braided channel is a complex network of converging and diverging channels that thread among numerous islands or gravel bars
• A large portion of the load is coarse material
  • Bank material is easily eroded and reworked
• Stream has a highly variable discharge
  • Commonly form at the toe of a glacier

Notes from the lecture:

• Caothic network of channels
• Tend to have a more turbulent flow in general

Stream valley

Presentation content:

• A stream valley is the channel and the surrounding terrain that directs water to the stream
  • Alluvial channels have wide valley floors
  • Bedrock channels have narrow V-shaped valleys
    • In arid climates, narrow valleys have nearly vertical walls—called slot canyons

Notes from the lecture:

• Most material down here is alluvial sediments
• There is not significant stream flow

Base Level and Graded Streams

Presentation content:

• The base level is the lowest point to which a stream can erode

  • Ultimate base level is sea level
  • Local or temporary base level includes lakes, resistant layers of rock, and large rivers
  • All limit a stream’s ability to downcut its channel

• Changing conditions causes readjustment of stream activities

  • Raising base level causes deposition
  • Lowering base level causes erosion

• A graded stream only transports sediment

  • Has the necessary slope and other channel characteristics to maintain the minimum velocity required to transport the sediment supplied to it
  • No net erosion or deposition of sediment

• Consider displacement by a fault along a graded stream:

  • Raises a layer of resistant rock
  • Forms a waterfall—concentrates energy here
  • Serves as a temporary base level
  • Called a knickpoint

Notes from the lecture:

• Headwaters mountains are being weathered and eroded
  • No erosion is going to happen below sea level
• Eventually were this material is deposited is the ultimate base level
• As you move upward from this base level, anywhere on the river system is going to try to erode
• Base level can be shifted building natural or anthropogenic features

• Over geologic time, river is going to downcut and keep eroding back
• Eventually it will completely flat out the topography

Valley Deepening

Presentation content:

• A steep gradient and channel far above base level leads to downcutting of the channel
  • Lowering of the streambed toward base level
  • V-shaped valleys with steep sides are the result of severe downcutting
  • Rapids and waterfalls are prominent features in V-shaped valleys
    • Occur where the stream’s gradient increases significantly

Notes from the lecture:

• River system is going to continue to downcut

• Over geologic time, this river system has basically cut back several times,
• The river itself is weathering and eroding faster than the ground area
• Gravity does the heavilifting

16.6 Shaping Stream Valleys

Describe valleys created by streams, including V-shaped, broad valleys with floodplains, and valleys that display incised meanders or stream terraces.

Valley Widening

Presentation content:

• As a stream approaches a graded condition, the shape changes to a meandering pattern
  • Downcutting is less dominant
  • More energy is directed laterally (side to side)
• Continuous erosion from moving meanders produces a floodplain (flat valley floor)
• The floodplain will be inundated when the stream overflows its banks
  • Erosional floodplain (floodplain is being formed)
  • Depositional floodplain (produced by major fluctuations in base level or climate conditions)

Notes from the lecture:

• AS this system erodes, it continues to widen that valley
• Channels have basically cut-off
• Why is this a zone for a lot of floods
  • Minerals in the soil help floods develop

Incised Meanders and Streams Terraces

Presentation content:

• Incised meanders are meanders flowing through steep, narrow bedrock valleys
  • Meanders first develop on a floodplain
  • Base level drops causing the meanders to start downcutting
  • Once the river has adjusted to the new base level, it will produce a new floodplain below the old one
    • The flat remnants of the old flood plain are called terraces

Notes from the lecture:

• Over geologic time this peninsula of bedrock will get eroded away
• A shift in natural base lave, downcuts can happen very aggressively
  • Why was there a great shift in the base level?
    • Example: Grand Canyon
    • One thought is that there was a dam that mass-wasted and down-cutted the system

• As base level changes over time, it will continue to down-cut
• It widens and exposes terraces

16.7 Depositional Landforms

List the major depositional landforms associated with streams and describe the formation of these features.

Deltas

Presentation content:

• Deltas form when sediment-charged streams reach a temporary or ultimate base level and enter a relatively still body of water

  • The stream’s forward velocity decreases, lowering its carrying capacity
  • Sediments are deposited by the dying current and produce three types of beds
    • Foreset beds
    • Topset beds
    • Bottomset beds

• Size of sediment varies in the delta

  • Coarse sediments are deposited close to the river mouth (foreset beds)
  • Fine sediments are deposited at the outer edge of the delta (bottomset beds)

• As a delta grows outward, the stream’s gradient continually decreases

  • The channel becomes choked with sediment
  • River seeks shorter, steeper routes to base level
  • The main channel divides into several smaller channels in the delta called distributaries

Notes from the lecture:

• Extending from the continental coastal plain and goes into the ocean
• As soon as sediments enter the ocean, they need more essential accommodation so they develop distributary channels, they start building outward further out in the ocean, because essentially the water has to go out somewhere
• Eventually a channel will be abandoned and system will develop another channel

• As accomodation of sediments becomes limited it build outward to the ocean
• Distributed through distributary channels

The Mississippi River Delta

Presentation content:

• History and Structure
  • The Mississippi River Delta is actually a series of seven coalescing deltas
  • Present delta, called a bird-foot delta, formed over the past 500 years
  • The river is trying to cut through to the Atchafalaya River
    • The river would abandon its current course through New Orleans and the lowermost 500 km of its channel
    • Engineering structures currently keep the river from migrating

Notes from the lecture:

• It has evolved over 5 big phases
• Created and abandoned network of channels in each phase
  • Once the river runs out of room to grow it migrates and shifts to new places to deposit sediments

Natural Levees

Presentation content:

• Natural levees are raised areas adjacent to the channel formed during flood events
  • Water overtops banks and flows out like a flat sheet, loses velocity instantly and drops coarse material near the banks
  • Fine material is laid out on the valley floor
• Following a flood event, levees prevent water from returning to the stream channel
  • Poorly drained back swamps form in the flood plain
    • A lot of organic deposit over here -> coal swamps
  • Yazoo tributaries flow in the back swamp area before reaching the main stream channel

Very similar to a delta system:

• Culturally rich areas
• Still a lot of coastal swamp areas

Alluvial fans

Presentation content:

• Alluvial fans are fan-shaped deposits of sediments at the base of mountain fronts
  • The stream emerges onto a flat lowland, the gradient drops, and sediment is deposited
  • More prevalent in arid climates
  • Mountain streams carry mostly sand and gravel, thus alluvial fans are composed of the same material
  • Fan shape is produced in much the same way as a delta—the flow divides into distributary channels

Notes from the lecture:

• Similar in geometry than deltas but these are above sea level and probably a bit smaller than with river deltas
• When stream reaches the capacity of the channel a flood happens

16.8 Floods and Flood Control

Summarize the various categories of floods and the common measures of flood control.

Floods

Presentation content:

• A flood occurs when the stream exceeds the capacity of its channel
  • The most common and most destructive geologic hazard
• Common types of floods:
  • Regional floods
  • Flash floods
  • Ice-jam floods
  • Dam-failure floods
• Regional floods
  • Seasonal floods that typically result from spring rains or rapid melting of snow
    • Example: 2011 in the Mississippi River

Notes from the lecture:

• Ice jam flood -> deposited ice starts melting in the spring and allows the river to discharge
  • Ice is creating a temporary dam so water accumulates behind it

Flash floods

Presentation content:

• Occur with little to no warning
• Produce rapid rises in water levels and can have devastating flow velocities
• Mountainous areas are extremely susceptible due to steep slopes
  • Example: August 2011 flash floods in upstate New York and Vermont from Hurricane Irene

Notes from the lecture:

• Nearly instantaneous

Ice-jam and Dam-failure

Presentation content:

• Ice-jam floods
  • Ice forms in rivers creating dams that will break when temperatures rise
    • Common problem with north-flowing rivers in the northern hemisphere
• Dam-failure floods
  • Dams designed to contain small or moderate floods face a larger volume flood event
  • Dams fail and release large amounts of water as a flash flood
    • Example: Johnstown Flood of 1889

Notes from the lecture:

• Da m failing -> they have to open up the spill way, not the greatest answer but at least is able to reduce greater consequences

Flood Recurrent Intervals

Presentation content:

• An estimate of how often a flood of a given size can be expected to occur
  • A “25-year event” would be much smaller but four times more likely to occur than a “100-year flood”
  • Probability based measurement
• “100-year flood” means that there is a 1 percent probability in a given year for a flood of that size
  • Stream gage data must be collected for 20–30 years to make a reasonable calculation

Flood Control

Presentation content:

• Artificial levees
  • Most commonly used stream-containment structures
  • Earth mounds built on river banks to increase the capacity of the channel
    • Not built to withstand, and often fail in floods
  • When exceptional floods threaten, openings are created to divert water out of the channel and into floodways
• Channelization
  • Altering a stream channel to make flow more efficient
  • Can make the stream straighter or deeper
    • Accelerates erosion
• Flood-control dams
  • Built to store floodwater and release it slowly (in a controlled manner)
  • Typically provide water for irrigation and hydroelectric power
• Nonstructural approach
  • Best approach to flood control is to limit development within floodplains of high-risk flood areas