meandering stream. floodplain definitions topographic: first flat surface found along the river...
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Meandering Stream
Floodplain Definitions
• Topographic: first flat surface found along the river above the bankfull channel;
• Geomorphic: landform composed of unconsolidated sediments transported and deposited by the related stream (created by the river and composed of alluvial materials);
• Hydrologic: surface subject to periodic flooding by the stream and exerts a role in the hydrologic characteristics of the drainage basin (example – 100-year floodplain).
Topographic Features
• Levees: Ridge that runs parallel to the river; due to enhanced deposition of sediment along the channel margins and a decrease in sediment size away from the channel;
• Splays: fan-shaped deposits associated with a breach in a levee; Splays are generally composed of coarser sediments than the levees;
• Oxbows and oxbow lakes: created by meander cutoffs; Slowly fill in with sediment; commonly fine-grained (clay plugs), but not always.
From reineck and Singh, 1980
1:12,000
Overbank Flood Deposits (1997)Deposits are Contaminated with Hg
Overbank Flood Deposits
Primary Processes Involved In Floodplain Formation
• Lateral Migration: lateral movement of the channel across the floodplain; consists of erosion of outer bank and deposition on the point bar. Produces fining upward deposits of semi-uniform thickness.
Lateral Accretion of Floodplain Deposits by a Meandering Channel
Primary Processes Involved In Floodplain Formation
• Lateral Migration: lateral movement of the channel across the floodplain; consists of erosion of outer bank and deposition on the point bar. Produces fining upward deposits of semi-uniform thickness.
• Vertical Accretion: deposition of sediment over the top of the floodplain during flood events; Produces horizontally bedded layers with varying degrees of lateral continuity.
Vertical Accretion
Common Stratigraphic Facies
• Lateral accretion deposits: produced by lateral accretion.
• Vertical accretion deposits: produced by vertical accretion.
• Channel Fill Deposits: sediments deposited during aggradation of an active channel or a paleochannel (e.g., an oxbow); variety of types.
• Channel lag deposits: found at base of the channel; formed by extensive reworking of the materials (and winnowing of the fines) during floods; associated with the thalweg;
• Colluvium: material shed from the surrounding hillslopes under the influence of gravity with may be interbedded with alluvial sediments; commonly more angular and coarser grained than floodplain materials.
From reineck and Singh, 1980
From Leeder, 1982
Qa3
From Miller et al., 1998
From Miller et al., 1998
From Macklin, 1996
From Macklin, 1996
1:12,000
Overbank Flood Deposits (1997)Deposits are Contaminated with Hg
0
10
20
30
40
50
Hg
Co
nc
en
tra
tio
n (
pp
m)
0
20
40
60
80
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Pe
rc
en
t <
63
um
050
100150
200250
300350
400450
500
Distance from Channel (m)
Hg Concentration% <63 um
Hg Concentrations in Overbank Deposits(CR-17)
Overbank Dep.in VegetatedAreas
Coarse GrainedOverbank Deposits
Fine GrainedOverbank Dep.
From Miller et al., 1999
Reservoir
0
1
1B2
2B
3 4
5
67
7B
7C 7D9 10
11
1213 14
15
16 1718
Gagingstation
GagingStation
MineralCanyon
Dayton
CarsonCity
VirginiaCity
TableMtn.
Canyon
(Brunswick)
FortChurchill
395
0
0 1 2 3 4
1 2 3 4 5 Miles
Km
95
Six Mile
Canyon Fan
Gold Canyon
Six M
ile Canyon
Fork
Reno Fallon
CarsonCity
Carson River
Watershed Boundary
Carson Lake
Carson Playa Stillwater
WildlifeRefuge
Lahontan Reservoir
LakePyramid
SIERRA NEVADA
NevadaCalifornia
ForkE
ast
Truckee R
.
Car
son
R.
Tru
ckee
R
.Wes
tF
ork
C
arso
n R. .
Lake Tahoe
Lahontan
0
0.5
1
1.5
2
Hg
19
97
/Hg
Pre
-19
97
20 30 40 50 60 70 80 90
Distance Downstream from 395 (km)
Change in Hg Concentrationsat Surface of Valley Floor
Canyon Reach
LahontanReservoir