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

100

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