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 – 100year 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 finegrained (clay plugs), but not always.
From reineck and Singh, 1980
Overbank Flood Deposits (1997)
Deposits are Contaminated with Hg
1:12,000
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
Overbank Flood Deposits (1997)
Deposits are Contaminated with Hg
1:12,000
50
Overbank Dep.
in Vegetated
40 Areas
Fine Grained
Overbank Dep.
Coarse Grained
Overbank Deposits
100
80
60
20
40
10
20
0
0
H g
30
0
100
50
200
150
300
250
400
350
Distance from Channel (m)
Hg Concentration
From Miller et al., 1999
% <63 um
500
450
u m
(CR-17)
P e rc e n t < 6 3
(p p
C o n c e n tr a tio n
Hg Concentrations in Overbank Deposits
Lahontan
Reservoir
Virginia
City
11
Dayton
3
7C 7D
7B
7
6
5
9
Table
Mtn.
10
12
15
14
13Mineral
Canyon
95
Fort
Churchill
16
17
Gaging
Station
4
Pyramid
Lake
2B
Carson
City
18
2
1B
Carson
Playa
Stillwater
Wildlife
Refuge
1
Reno
395
0
Gaging
station
Fallon
Lahontan
Reservoir Carson
Lake
Lake
Tahoe
Carson
City
0 1 2 3 4 5
0
1
2
3
Miles
4
Km
Carson River
Watershed Boundary
P r e -1 9 9
1 9 9 7 /H g
Change in Hg Concentrations
at Surface of Valley Floor
2
1.5
1
Lahontan
Reservoir
H g
0.5
Canyon
Reach
0
20
30
40
50
60
70
Distance Downstream from 395 (km)
80
90