rivers unit

mleziva rahhh (pg 69-74)

3 stages of river development

young → mature → old
or
upper course → middle course → lower course

when a river goes around a bend (meander), which side gets eroded?

outer

when a river goes around a bend (meander), which side gets deposits?

inner

abrasion (river version)

rocks/pebbles in the river scrape and grind against the riverbed (bottom of river) and river banks

aka corrasion
→ widens + deepens the river channel

attrition

rocks within the river wear away at each other when they bump into each other and/or river bed

→ this rounds and smooths the rocks
→ slow process

what do u call the bend in a river

meander

what kind of valley do rivers form

v-shaped valley

explain what an interlocked spur is and how it is formed

• these features form as a result of river erosion, usually in the upper course/young stage

• the river doesn’t have much eroding power → forced to pick the path of least resistance

  • causes river to wind around hard bedrock

• forms ridges of land that protrude from opposite sides of the valley in an alternating order to create a zig-zag pattern

what is the name of the pile of sediments deposited by a river on the inner bank of a meander

slip-off slope or point bar

must know both terms

True or False: The mature stage of a river is faster than the young stage.

True.

The young stage looks faster bc it has white rapids + more waterfalls, however, the avg speed is slower bc there is more friction generated by the large boulders at the bottom of the river

The mature stage is faster bc the riverbed is smooth by this point → less friction, although it looks calm

what is alluvium

• finely sorted and layered/stratified sediment deposited by water

• SUPER fertile

• mix of gravel, sands, silt clay

how is alluvium deposited

• river undercuts its river bank & floods the surrounding plain

• deposits a thick mass of finely sorted and layered sediment called alluvium

what is braiding

when a river splits off into multiple streams

→ forms islands

→ can be temporary or permanent

→ happens when banks are soft + easily eroded

estuary

• partially-enclosed area where the mouth of a river mixes with high ocean tides

  • mix of freshwater and saltwater

• doesn’t form a large delta bc tides sweep sediment away before it can build up

• latin meaning: “tide” or “surging”

define point bar

aka slip off slope

the pile of sediment deposited on the inner bank of a meander/river bend

True or False: Estuaries create a substantial delta

False.

estuaries have tides sweeping in from the ocean very often → the sediment deposited by the river never gets to build up, but just gets sweeped into the ocean

→ however a small delta can form

True or False: The mature stage of a river is slower than the young stage.

False.

The young stage looks faster bc it has white rapids + more waterfalls, however, the avg speed is slower bc there is more friction generated by the large boulders at the bottom of the river

The mature stage is faster bc the riverbed is smooth by this point → less friction, although it looks calm.

river terrace

a piece of floodplain that has been raised above the rest of the floodplain

• there is an escarpment (steep slope) btwn the higher and lower parts

• caused by isostatic rebound from glaciers?? not sure

  • or is it lateral erosion of the river??

hydrologic cycle

just another name for water cycle LOL

precipitation → evaporation → condensation

divide

give 3 possibilities for what happens after precipitation reaches ground

1. it evaporates

   • does not do any gradation

2. it lands on porous rock and sinks inside

   • potentially weather by solution/carbonation

   • if this porous rock is over impervious rock, it can form an aquifer or even a spring if the water table reaches the surface

3. it lands on impervious rock and slides down a slope until it finds a stream/river

   • called runoff

3 main sources of water for rivers

• glaciers

• springs

• lakes

what does the strength of a river’s erosion power depend on?

volume + velocity (V & V)

explain the 3 processes of gradation and how these link to rivers

Weathering: rivers widen and deepen riverbeds + ?

Transportation: rivers carry sediment across long distances

Deposition: rivers deposit sediment at the old stage or along the inner bank of a meander

river or stream

a channel of flowing water that follows a definite course

head or source

highest point of a river

must be the first place where the river is at the surface

mouth

lowest point of a river

where river either leads into a sea/lake or joins another river

levee

• high ridges of sediment (alluvium) on the banks of rivers

  • usually covered with vegetation bc the alluvium is so fertile

• naturally deposited by river when it floods

• characteristic of mature & old stages

dike

extra high ridges of sediment on the banks of rivers

man-made

→ eg to reduce impact of tsunamis

another name for watershed

divide

watershed

the ridge that separates 2 drainage basins; water flows evenly down both slopes of this ridge

not to be confused with arete

aka divide

river valley

the elongated depression in which a river flows

v-shaped bc the river doesn’t have as much vertical eroding power as a heavy glacier

river bed

the bottom of the river

aka part of the river valley that is actually covered by the river

river bank

the sides of a river valley

downstream

direction of river flow → towards river mouth

corrasion

type of erosion where sediment in the river wears away at rock surfaces that they pass over

→ synonym of abrasion

not to be confused with corrosion

is corrasion weathering or erosion

erosion bc rivers move

what are the 4 methods of erosional transportation in a river

• saltation

• suspension

• solution

• traction

SSST

saltation

French of jump: sauter

Spanish of jump: saltar

happens bc river current is inconsistent so sometimes the rocks are propelled by current and sometimes they fall to bottom

literally the rocks be bouncing/jumping along the riverbed

hopping and skipping along

suspension

rock sediments are carried by river current (not dissolved)

• must be in middle of river, not the bottom

solution

dissolved rock sediments are carried by river current

traction

heavy rocks roll along riverbed

young stage/upper course

Characteristics

• steep gradient

• medium water discharge??

• medium velocity

• narrow

• low sediment load

Types of erosion:

• headward

• vertical

  • makes riverbed deeper

Landforms:

• potholes

• rapids

• waterfalls

• interlocked spurs (why not v-shaped valleys?)

• gorges

Uses to humans:

• hydropower (i.e. dams)

• drinking water

• tourism

old stage/lower course

characteristics:

• low velocity

• low gradient

• high discharge

• wide channel width

• high sediment load

landforms:

• yazoo streams

• wide flood plains

• large levees

• meanders

• oxbow lakes

• deltas

types of erosion:

• lateral (river valley is so flat by now that erosion can only happen laterally)

  • this also why this stage is so wide → the only erosion happening is lateral

uses to humans:

• transport

• housing

  • flat + fertile floodplains → easy to build settlements

  • rivers → water supply + recreation

• farming

  • (alluvium)

NOT for drinking water bc this river is full of sediment + farm run-off + pollutants

• a river that’s efficient at transportation should actually be brown

what is gradient

slope

what is water discharge

volume of water flowing through a point in a river per unit of time

eg. 3475 m³/s

headward erosion

lengthens the headwaters or source of a river in the opposite direction from downstream/normal flow

• happens bc gullies and rills on a slope get lengthened as rainwater falls and erodes them

  • the rain deepens the gullies and causes it to “eat back” into the hillside

    • towards top of slope → causes river source to move backwards

• young stage

how is an oxbow lake formed

1. the bend in a meandering river becomes tighter over time due to erosion & deposition on the outer bank and inner banks, respectively

   • the bend is called a neck

2. eventually the neck gets so narrow that the river is able to cut through it and flow in a straighter path bc it is more efficient

3. as river continues to go on the new straighter path, it deposits sediment that eventually blocks off the old meander → forms a horseshoe shaped lake that is competely separated from river

mature or old stage only

what is another name for oxbow lake

cut-off lake

gorge

steep, rock-walled narrow valley

• formed by rapid erosion of a river through hard rock

• young stage

rills

small narrow channels carved by intense water erosion

• when further eroded these turn into gullies

mature stage/middle course

Characteristics:

• medium gradient

• medium channel width

• high velocity

• high discharge

• high sediment load

Landforms:

• slip off slope/point bar

• v-shaped valley (why is this not formed in young???)

• flood plains

• levee

Types of Erosion:

• lateral (undercutting of outer banks)

Uses to Humans:

• transport

• housing

  • flat + fertile floodplains → easy to build settlements

  • rivers → water supply + recreation

• farming

  • (alluvium)

• hydroelectric power (dams)

• fishing

NOT for drinking water bc this river is full of sediment

levee vs slip off slope/point bar

levee is deposited by the river when it floods → anywhere along river bank

slip off slope/point bar is specifically at a meander when the river deposits sediment on its inner bank bc the current slows

yazoo stream

tributary that goes rogue and meanders all over the floodplain

→ eventually rejoins main river by breaking thru the levees

how are potholes formed

young stage

• formed thru abrasion or corrasion

• fast water current hits a small natural hollow/weak point in the bedrock and widens it

  • rocks get trapped in this hollow and swirl around, futher widening it like a drill

corrasion vs corrosion

corrasion: erosion of rock underneath a river as the sediments grind + scrape past it

corrosion: dissolving of soluble minerals in a rock by water (think solution/carbonation weathering)

• this usually happens with limestone in humid areas

pls explain difference btwn potholes and kettle lakes

potholes:

• carved from bedrock

• river erosion (young stage)

• abrasion + swirling of small rocks trapped in a hollow → pothole

kettle lakes:

• formed by meltwater deposits

• glacial deposition (retreating)

• solid ice block remains while meltwater deposits sediment around ice block → kettle lake

discharge

volume of water (in a river)

hydraulic action

erosional force caused by river pushing against side of a river bank

ria

a flooded or drowned v-shaped river valley

• connects to the sea

• long, narrow inlet

• similar to fjord but it is an unglaciated valley

when do deltas form

when a river reaches base level

base level

the lowest point to which a river can erode

• typically an open ocean or lake

• river deposits ALL of its remaining sediments here

  • usually forms a delta

neck of meander

narrow point in the u-shape (just look at the picture)

what is a delta (bonus what type is this picture?)

land that is formed by accumulation of deposited sediment at the mouth of a river where it has reached base level and is depositing all of its sediment

• for delta to form (and grow) the amount of sediment deposited must be greater than amount swept away by ocean tides

eg. richmond and delta

this picture is an arcuate delta (same type as richmond/delta)

types of deltas

arcuate

estuarian

bird’s foot/digitate

cuspate

arcuate delta

• rounded or bow-shaped delta

  • large amount of sediment is distributed by many tributaries

  • sea currents round the delta by dispersing the sediments into a fan shape

  • often has lagoons

    • lagoon: a small isolated body of salt water separated by sediment or other stuff

• eg. Fraser River or Nile River

bird’s foot delta

aka digitate

• many tributaries split up from the main river

  • each tributary deposits sediment along its banks, which forms a lot of new land in btwn the streams

    • resulting delta stretches in various directions and resembles a chicken claw

• eg. Mississippi River

lagoon

a small isolated body of salt water separated from the ocean by sediment or a delta or some other type of ridge

tributary

stream that ADDS volume to a river

• young stage

distributary

stream that SPLITS up river into smaller sections

• old stage

another name for bird’s foot delta

digitate

cuspate delta

???

what’s this

rias

rejuvenated river

rising of land gives rivers fresh power to erode bc of a steeper slope (just like young stage again…good ol days)

• either caused by isostatic rebound (after glacial retreat) or tectonic plate movement

  • google said also could be caused by drop in sea level

• usually forms waterfalls at knick points

  • knick: a sharp change in gradient in the slope of a river channel

knick point

• a sharp change in gradient in the slope of a river channel