Base level and gradient W5L1

Question 1

Processes that regionally change sea level

Answer 1

mountain forming
global warming → expansion
ice caps will rebound, land that was previously covered with ice “rebounds” up→ relative sea level fall

Question 2

Base Level is

Answer 2

the lowest point to which a stream can erode

Question 3

Base level rise

Answer 3

sediment deposited on flooded coast

Question 4

base level fall

Answer 4

river cuts down to a lower profile

Question 5

deepest eroder

Answer 5

ice

Question 6

temporary base level

Answer 6

An abrupt change of gradient forms temporary/local base levels eg lakes or ledges of resistant rock
eventually removed by changes in velocity & gradient
increase veolcity by creating steeper gradient by uplift (sea level change)

Question 7

Lakes are

Answer 7

Large ponded bodies of water = dissipation of river energy, ie loss of competence & capacity

Question 8

Lakes near shore vs deep

Answer 8

near-shore: gravels & sands
deeper water: well-sorted muds
fine-bedded shales
bottom of lake anoxic

Question 9

Fossils in lakes

Answer 9

fossils of aquatic plants & animals
type of leaves givean idea about climate and plate tectonic
archives of regional climate

Question 10

Laminated lake shale

Answer 10

Laminated Lake shale: Over time, sediment fills the lake and the temporary base level is removed
can be used as climate signal, understand past climate high frequency climates

Question 11

Lakes last as

Answer 11

transient bodies, will not last very long

Question 12

Waterfalls

Answer 12

A concentration of river energy = downstream increase of stream velocity & sediment load – ie capacity & competence
free-falling water scours a plunge pool
undercutting + collapse of overlying rocks
over time, the concentration of energy removes the temporary base level
return to smooth stream profile

Question 13

Permanent streams

Answer 13

flow all year
recharged from water table
banks stabilised by vegetation

Question 14

Ephemeral streams

Answer 14

intermittent
low rainfall
high evaporation

Question 15

flashfloods

Answer 15

random rain over large catchment areas → water runs off quickly, channels → runs off in same canal, ex. deserts
large amounts of sediment transported
channels scoured
sediment dumped, not well sorted but subangular to rounded
dumpsites: alluvial fan

Question 16

Alluvial Fans

Answer 16

Upland change of gradient – alluvial fans
Arid environments –
during flashfloods canyon has high competence & capacity
high loads of coarse sediment
at canyon exit competence & capacity drop: sediment deposited
forms fan shape
form excellent reservoirs

Question 17

proxiamal vs distal

Answer 17

proxiamal: close to source, distal: furthest from source

Question 18

Upland change of gradienr

Answer 18

braided streams

Question 19

Wet environments (braided streams)

Answer 19

large clasts dropped and stranded during low season
coarse sediment is unstable: river can’t cut a deep channel
forms multiple channels across a broad valley

Question 20

Braided streams

Answer 20

bedload > suspended load
low competence & capacity
rapid formation and erosion of sand and gravel bars

Question 21

pre cambrian

Answer 21

no vegetation so large . plains of mazterials

Question 22

Lowland change of gradient

Answer 22

meandering streams

Question 23

Meandering streams

Answer 23

channel full of water and suspended sediment
low competence and high capacity
suspended load > bedload

Question 24

low gradient menas

Answer 24

tream exploits slight changes in topography
they have the lowest gradients as rivers so cut through and energy increases slightly and then cuts somewhere else so creates meandering patterns

Question 25

Thalweg influence

Answer 25

thalweg hugs outer curve = maximum flow
scouring & deepening
thalweg controls meandering motions bcs flows like corkscrew
get inner bank, outer bank, erosional feature and depositional feature

Question 26

meandering because

Answer 26

meanders swing back & forth in response to thalweg position & friction

Question 27

inner curve flow

Answer 27

minimum flow on inner curve

deposition & shallowing = point bars

Question 28

creates spiral flow

Answer 28

meander migration

more sinuous with time

Question 29

Fludial Sediments sorted

Answer 29

Fluvial sediments (alluvium) are sorted: –
channels, mid-channel bars & thalwegs = pebbles
point bars = sands
levees = sand & pebbles 
floodplains = silts & clays

Question 30

Meanderingb streams and flood plains

Answer 30

flood plain sporadically immersed
width is related to overall stream length (ie potential size of flood)
sediment-laden stream overtops its banks, loses energy & dumps sediment
coarse grains close to channel – levee
finer grains away from channel

Question 31

Floods

Answer 31

competence and capacity increase with velocity
water leaves channel, floodplain inundated
currents and debris scour floodplain
away from thalweg, water slows & sediment is deposited

Question 32

flood plain vs. leevee

Answer 32

flood plain: silt and clay

levee: pebbles and cobbles

Question 33

Erosion gradients

Answer 33

Maximum erosion occurs with steep gradients (mountain streams/waterfalls) or floods
chunks break off channel floor & banks
tumble & roll = weathering & erosion of more grains

Question 34

Erosion processes

Answer 34

scouring – particles lifted & moved
abrasion - grains collide
dissolution = mineral ions

Question 35

Drainage Evolution

Answer 35

regional uplift

Question 36

regional uplift

Answer 36

Established streams keep pace with tectonic uplift and cut down on their original course
behave like a mountain streams but look like lowland streams
incised meanders
many tributaries with headward erosion