310lecture2
Lowlands
and Stream Geomorphology in Louisiana.
I)
Intro
A)
Most
of Louisiana’s lowland area is geologically young, from the Holocene period.
B)
Three
Holocene plains (map 2.3)
1)
Mississippi
and Red River plain
2)
Deltaic
Plain
3)
Chenier
Plain
II)
Mississippi
River Alluvial Plain
A)
Drainage
Basin (map 2.4)
B)
Meandering
Process and Landforms (map 2.5)
1)
Meanders-doubles
the length of the river in La
2)
Sediment
load
3)
Thalweg-deepest
line though river
4)
Cutbank
5)
Pointbar
6)
Meander
belt
7)
Scour
pool
8)
Ridges
and swales
9)
Meander
neck
10) Neck cutoff
11) Towhead island
12) Clay plugs
13) Oxbow lake
14) Natural levees-(diagram 2.7)
a
Formed
when water overflows the banks and the coarser sediments fall out first.
b
During
the 1973 flood, the levees got 21 inches and the backswamps only got .4 inches
c
Nearly
all the cities in floodplains are on the levees
15) Crevasse splay
a
Is a
break in the levee that happens generally during a flood event.
b
Generally
on the cutbank side.
c
Is a
depositional feature and many settlements are put on them
d
Eventually
the river will repair it by itself.
e
La
Place, LA
16) Distributary channels are formed
when a breakthrough becomes more permanent and find a way to the ocean or the
gulf.
C)
Meander
Belts (map 2.8)
a
The
Atchafalaya basin is a backswamp
b
Smaller
streams, both tributaries and distributaries often occupy older meander belts.
2)
Figure
2.8 older meander belts
3)
Pleistocene
Entrenchment
a
During
the ice age, the sea level went down and the Mississippi River dug a deeper
channel, which has been largely filled in by sediment since.
b
Was a
braided stream then and deposited a lot of course sediment.
D)
Macon
Ridge and Bastrop Hills
1)
Are
both older geologically than the surrounding alluvial plains
2)
Are
made of Pleistocene-aged glacial outwash/deposits.
3)
Would
have rockier soils, but would be up out of the floodplain.
E)
Flood
Control
a
Prompted
by the 1927 flood
2)
Levees
(diagram 2.17)
a
40 ft
above surrounding flood plain.
3)
New
Orleans (diagram 2.12)
a
Situated
between the Mississippi River and lake Pontchartrain and has levees on both
sides
b
Highest
elevation is ten feet above sea level and the lowest is 6 feet below sea level.
c
For
many years the city was confined to the highest levees.
d
New
pumps required before the low lying areas in NO could be settled.
e
The
peat soils, when drained can cause subsidence, which is a problem and can be
seen on the streets in the Kenner-Metarie areas.
4)
Sand
boils (diagram 2.15)
5)
Spillways
(map 2.13) (diagram 2.16)
a
Atchafalaya
or Old River Control Structure
b
Bonnet
Carre spillway
III)
Red
River Alluvial Plain
a
Gets
its red color from the Permian beds in OK and Texas
b
It
makes red deposits around Shreveport and along the Bayou Teche, south of
Alexandria a former course.
c
Alexandria
is positioned at a former ‘rapides’.
The rapids were made after the Red River shortened its course by
diverting through the Moncla gap.
d
The
steepened river cut down rapidly through the deposition layers, moving the
nickpoint backwards until it hit resistant rock at Alexandria.
e
The
lower red cut down to the Mississippi Rivers base level, the upper red cut much
less.
f
The
lakes around this area, including Caddo, Bistineau are former tributaries of
the Red that were damned after the course change on the lower river. Aggradation occurred on the upper reaches.
g
The
rapids were removed by the Army Corps of Engineers and the log-jammed raft
lakes were cleared (naturally), but re-dammed for water supply and recreational
purposes.
IV)
The
Deltaic Plain
a
Forms
the coastline of Southeastern Louisiana
b
Six
deltas total: in order of age
(i)
Maringouin
(oldest)
(ii)
Teche
(iii)
St.
Bernard
(iv)
La
Fourche
(v)
Modern-
a birdsfoot delta
·
Is
created by crevasse splays
·
Will
sink and/or be eroded as soon as the stream channel is abandoned.
·
Jetties must be built on the distributary
mouths in order to increase the velocity of the water entering the gulf, so
that shoals do not build up and clog the entrance into the main channel.
·
Mudlumps are blocks of prodelta clays that
are forced up (much like Salt Domes) by the weight of heavier nearby
sedimentation. May pop up over night as
islands or may stay submerged, creating a navigation hazard. The Mississippi River. birdfoot is the only
delta in the world with mudlumps.
(vi)
Atchafalaya-began
creating a delta after Shreve removed the logjam.
(vii)
The
Atchafalaya bay will eventually fill up with sediment, perhaps within the next
50 years.
V)
Chenier
Plain
a
Gets
its name from the swales of sand /shell that rise above the marshy plain.
b
Cheniers
support vegetation, like live oaks.
Chenier means place of Oaks.
c
Probably
formed as a result of longwave re-deposition of Mississippi River sediments
during the Holocene when it was creating the Maringouin and Teche Deltas, then
the Mississippi River moved east and the Beach was built of shells/sand.
d
Others
debate this and put forth alternate ideas.
(i)
Flood/low
flow periods of the Mississippi?
(ii)
More
local/non-Mississippi sediment sources?
e
The
Atchafalaya is adding sediment to the Cheniers now.
f
Maximum
subsidence of the ridges has been somewhat inland creating the string of lakes
(White, Grand, Calcasieu and Sabine).
B)
Causes
of Land Loss
1)
The
deltas are being lost to the sea
2)
LA
has 40% of the nation’s wetlands
3)
50%
of LA’s population is within 50 miles of the coast.
4)
Tourism,
recreation economies threatened by the loss of wetlands
5)
25
sq. MI per year over the last 100? has come down recently to 20-sq. mi. per
year.
6)
75%
of the total loss is due to compaction and subsidence.
7)
Steps
taken
a
Diverting
the Mississippi River outside its levees to restore wetlands
(i)
Caevernon
freshwater diversion project downstream for New Orleans
(ii)
400
acres of freshwater marsh have been created near Breton Sound.
(iii)
Artificial
crevasse splays
(iv)
Siphoning
water over the levees
(v)
Barrier
island reconstruction.
(vi)
Losses
projected to be stopped by 2043.
b
Delta
lobes are always being created or destroyed.
(i)
They
are built by sedimentation
(ii)
They
disappear through subsidence and wave erosion
(iii)
The
entire modern delta is being destroyed, mostly because the delta has reached
the continental shelf.
8)
Why
is the Delta disappearing?
a
Sea
level has risen…four ft. per 100 years
b
Subsidence-due
to the weight of the depositions, especially in the gulf. Has also isostatically lifted the interior
of the state.
c
Compaction-
as the water is squeezed from the newly deposited materials.
d
Reduced
sediment supply.-80% reduction in the amount of sediment load since 1850
e
Improved
agricultural practices
f
Dam
construction on the Arkansas and Missouri rivers.
g
Artificial
levees prohibit much deposition in the backswamp areas, which allows them to
subside too much.
h
Canals-
generally built through wetland areas.
Interfere with the natural operation of the wetland