How much water is contained within Earth’s hydrosphere?

326 mili²

 How much of this water is located in the oceans?

97.2% 

What is the hydrologic cycle?

-unending circulation of water, describes what happens as water evaporates form the ocean, plants, and soil, moves through the atmosphere, and eventually falls as precipitation.

Precipitation that falls onto the ocean has completed its cycle and is ready to begin another. 

                  Is the hydrologic cycle balanced? 

How so?

-YES.

-Hydrologic cycle is balanced

320,000 mm³=precip oceans/year

284,000km³=precip into oceans/year

  36,000km³= carried to ocean as runoff

What are the possibilities for water when it hits the Earth’s surface?

-soaks into the ground= infiltration

-flows over surface as runoff, or immediately evaporates.

 What is runoff?

-single most important erosional agent

 -begin as broad, thin sheets across hilltops known as sheet flow.

  -this thin, unconfined flow eventually develops threads of current that form tiny channels called rills.

rills->gullies->brook/creeks->strams->rivers

(small)---------------------------------------(big)

What’s the difference between a river and a stream?

-stream- water flowing in a channel, regardless of size

-rivers- channels carrying substantial amounts of water and have numerous tributaries

What is a drainage basin?

-Every stream drains an area called a drainage basin.

-Each drainage basin is bounded by an imaginary line called a divide

                  What is a divide?

-something that clearly visible as a sharp ridge in some mountainous areas, but can be rather difficult to see in low areas

-the outlet where the stream exits its drainage basin, is the lowest elevation point within the basin.

-can range greatly in size

                  -a small ridge separating two gullies

                  -a continental divide splitting an entire continent

Where is the lowest point of elevation within a drainage basin?

-the outlet where the stream exits its drainage basin

 What is the largest drainage basin in North America?

-The MS River = (3,000,000 km²)

-it collects and carries 40% of all surface water flow in the US.

-Drainage basins (and divides) exist for all streams no matter their size.

 What are the 3 zones within a river?

                  1) Zone of Sediment Production (erosion)

  2) Zone of Transportation

3) Zone of deposition

What is the headwater region of a stream?

-  The beginning, where most of the water and sediment originate

 -sediment carried by a stream are weathered materials from the upland regions.

 -it was transported to the stream by mass wasting, erosion, etc.

      -additional sediment is derived from bank erosion and bed scouring.

  2) Zone of Transportation

-where sediment is transported along the trunk stream

What is a trunk stream? 

- the main river channel which tributaries which tributaries drain into.

     -when trunk streams are in balance, sediment eroded from the banks equals the amount deposited somewhere else within the channel.

3) Zone of deposition

-when a river reaches a body of water, its velocity decreases.

      -the energy needed to transport sediment is reduced.

                  What are the possibilities for sediment deposited in this zone?

(zone of deposition)

1-accumulates at the mouth (end) to form a delta,

 2-is reconfigured by wave action to form coastal features

 3-is moved far offshore by ocean currents-sediment that reaches the ocean is primarily fine-grained

-sediment that reaches the ocean is primarily fine-grained.

-sediment that reaches the ocean is primarily fine-grained.

   -clay, silt, fine sand-sized particles

-velocities required to transport larger particles is n

existent towards the end of a stream.

                  -this is why you rarely see cobbles, gravels, and boulders along the beach and offshore.

 Where do you find streams exhibiting laminar flow?

- Water travels down slop under the influence of gravity.

   - In low velocity streams, water moves in straight- line paths called Laminar flows.

    What is a turbulent flow?

-Where the water moves in an erratic fashion characterized by a series of horizontal and vertical swirling motions. 

 Fast vs. Slow Velocities:

- Velocity varies within a stream and is also dependent upon channel width.

- Faster velocities = less frictional resistance.

- Slower velocities = More frictional resistance.

- Velocity determines the amount of “work” a stream does.

-“Work”- a streams ability to erode and transport sediment.

          What does stream velocity determine?

-the amount of “work” a stream does.

-“work”- a stream’s ability to erode and transport sediment.

How does a stream transport sediment?

- All streams transport some amount of material.

- During transport, a stream will sort the sediment.

1) Dissolve Load

-consists of soil-based compounds that are in solution.

-as water percolates through the ground, it acquires soluble compounds

-the mineral-rich water eventually reaches a stream

  Velocity’s effect on this load?

-NO effect on streams ability to carry its dissolved load.

     2) Suspended Load

the largest part of a stream’s load is in suspension -“Muddy Mississippi”

-consists of very fine sand, silt, and clay

-The Yellow River (China) carries an amount of sediment equal to the weight of the water carrying it.

   -“Too thick to drink, too thin to cultivate”

     3) Bed Load

- coarse sediment is transported along the channel bottom as bed load.

- Bed load particles move by rolling, sliding, and saltation.

      What is saltation?

-jumping/skipping of grains along the channel bottom.

-saltare: to leap

Capacity

-maximum load of solid particles a stream can tranport per unit of time

-higer velocities= higher capacities

-bigger channels= higher capacities 

Competence

-stream’s ability to carry particles based on particle size, not quantity

-higher velocities= higher comprtency

-channel size does not matter

Deposition of sediment by streams

-deposition occurs when stream velocity decreases

  -slower streams=lower competency

-when a stream slows, sediment will settle

1 -usually, larger particles settle first

2 -then, medium particles

3 -finally, the smaller particles

 What is alluvium?

-any sediment that is being deposited by a stream is alluvium

-many different depostitional features are composed of alluvium

-remember alluvial fans??

Make notes about the two types of channels:

1. Bedrock Channels

2. Alluvial Channels

Bedrock Channels

– channels in which streams are actively cutting into the rock

-usually found near the head where steeper slopes exist.

  What are steps and pools?

-Steps and pools typically develop in bedrock channels.

-steps: steep segments where bedrock is exposed

 -rapids, waterfalls

-pools: flat segments where alluvium accumulates

Alluvial Channels

– when beds and banks are composed mainly of alluvium.

Meandering Channels

– an alluvial channel consisting of a series of sweeping bends called meanders.

-can be relatively deep, smooth-sided channels where sediment can be transported

Example?

-The Lower Mississippi River is a meandering channel 

Migration, Cut Banks, and Point Bars

-over time, meanders migrate across their floodplain

-at the outside of the meander, velocities and turbulence is the greatest, resulting in erosion                 

Cut bank

The outside of a meander

Point bar

-debris acquired at the cut bank moves down-stream where it is deposited as a point bar.

   -these form on the inside of a meander where velocities and turbulence are lowest.

Cutoffs and Oxbow Lakes

-due to different migration rates of meanders, a meander upstream can “catch up” to a slower-moving meander

-Eventually, the stream may erode the narrow neck of land separating the meanders.

Cutoff

a new, shorter channel segment.

Oxbow Lake

the abandoned meander is now an Oxbow Lake 

Braided Channels

– complex network of multiply channels threading downstream between islands and gravel bars

-braided= interwoven appearance-tend to be wide and shallow

-tend to be wide and shallow

-Majority of the stream load is bed load.

         -coarse sands and gravels

-where do these occur?

         -where there is a limited water supply.

What is base level?

-the lowest elevation to which a stream can erode its channel.

 -where the mouth enters an ocean, a lake, or a trunk stream.

-first recognized by John Wesley Powell in 1875.

    -he stated that sea level was the ultimate base level.

-changes in base level cause adjustments in the “work” a stream can perform:

   -when a dam is built on a stream, the reservoir that forms raises the base level of the stream.

  -upstream from the reservoir.

What does a stream valley consist of?

-the channel and the surrounding terrain that directs water to the stream.

-valley floors and valley walls

-alluvial channels have narrow, v-shaped valleys.

-in arid areas, narrow valleys have nearly vertical walls.

-slow rate of weathering

Where can you find a stream valley with nearly vertical walls?

-in arid areas

-alluvial channels have wide valley floors.

What is downcutting?

-the dominant process in steams that are steeply-sloped and located in areas of high elevation

  -downcutting- gravity-controlled erosion

-Result: a v-shaped valley with steep sides

  -Ex: Grand Canyon of Yellowstone

          What shape does downcutting create?

V-shaped valley with steep sides

Rapids and waterfalls:

-most prominent features of V-shaped valleys are rapids and waterfalls.

    -occur where the slope increases significantly

-waterfalls occur where streams make vertical drops.

Why is Niagara Falls still a waterfall after 11,000 years?

-supported by a resistant layer of dolostone.

 -underneath the dolostone is shale.

-as water flows over the falls, it erodes the shale, which undercuts the layer at the bottom of the falls.

   -when no more shale is present beneath the dolostone, the dolostone collapses.

-The waterfall retains a vertical cliff while slowly retreating upstream

   -over the past 11,000 years Niagra falls has retreated more than 7 miles upstream.

When do stream valleys begin to widen?

-as stream approaches flatter land, downcutting becomes less dominant.

  -instead, the channel will take on a meandering pattern.

-As a result, the stream valley widens as the river cuts away at one bank and then another.

   -the energy is directed side-to-side.

What is a floodplain?

-continuous lateral erosion caused by meandering streams will produce a broad, flat valley floor covered in alluvium.

 -this is called a floodplain.

-when a river floods, the floodplain is covered with water

What are bars?- 

small-scale channel deposits

What is a delta?

-they form at the mouth where the streamload is dropped

  -decrease in velocity

-as deltas grow out from the mouth, the slope decreases.

                  -eventually, the channel becomes choked with sediment

-The river seeks shorter, higher-sloped path to sea.

      -After numerous shifts, it grows into a triangular shape

                  What is a distributary?

-the main stream channel divides into several smaller ones 

How long has it taken the Mississippi River Delta to form?

-NOLA currently rests where the gulf was once located.

-over 6,000 years, the MS Delta has accumulated.

-its an accumulation of 7 subdeltas.

    Which subdelta are we currently on?

-the “bird’s foot delta” due to its configuration of distributaries.

-it’s about 500 years old.

What is the Mississippi River trying to do?

-for years it has been trying to shift its course to Atchafalaya River.

     -this would shorten its trip to gulf by ~ 160 miles.

What is a natural levee?

Built by years of successive floods.

   -when stream floods, water will flow as a broad sheet.

-because the velocity flows, the coarser sediment is deposited immediately adjacent to the channel.

      What type of stream typically builds natural levees?

-meandering rivers tend to build natural levees.

-they are found parallel to both sides of a stream channel

-levees of the MS River rise 20 feet above stream valley floor.

-areas behind the levees are poorly drained resulting in back swamps.

What is a yazoo tributary?

-streams trying to reach a trunk stream will flow through the back swamp before finding an opening.

   -these are called Yazoo tributaries.