Fluvial Geomorphology of the South River:

Implications for Mercury Cycling

Jim PizzutoDept. of Geological Sciences

University of Delaware

Outline

• Geomorphic characteristics of the South River

• South River’s Sediment Budget • Cycling of mercury by the South River: key

processes• South River’s history of sediment and

mercury cycling• Conclusion

Geomorphology of the South River

Geologic Setting

AlluvialFan

Deposits

Carbonate Bedrock Terrace

Deposits

Sediments Deposited by S.R.

The South River flows through a narrow alluvial valley bordered by carbonate bedrock, terrace, and alluvial fan deposits

The South River – A single-thread, sinuous channel with islands

Islands

Bends of the South River Look Different From Typical MEANDERS

A typical meandering river

Narinesingh, 2009

Nonmeandering (but sinuous) Planform: Caused by frequent

exposures of bedrock (?)

• Bedrock is difficult for the river to erode• Impedes the development of bends

Bedrock exposures in the Channel of the

South River, Waynesboro – Port

Republic

Bed Material of the South River

• Mostly gravel-sized– Cobbles predominate

• Other areas consist of:– Sand and gravel– Mud and sand – Bedrock

The Longitudinal Water Surface ProfilePools and Riffles, Bedrock Drops, and Breached Dams

1230

1235

1240

1245

1250

1255

2 2.5 3 3.5 4 4.5 5 5.5 6

Relative River Mile

Elev

atio

n (fe

et)

Dooms Dam (breached)

bedrock "drops"

Exposed bedrock in channel boundary

Geomorphic Classifications of South River

Suggests that sediment yield and sediment fluxes are relatively low

Low Size of sediment supply

Bedrock lowers potential rates of change during storm discharges

Bedrock-AlluvialSource of sediment supply, extent of external control

Both bedload and suspended load transport are important

Mixed load Type of load

Not meandering, braided, or anastomosing; implies lateral stability

SinuousPlan Form

Implies that significant movement of the bed material occurs only a few times per year

Gravel-bedBed material

CommentsClassification of S. River

Type of Classification

The Best Summary Classification For South River

• South River is:– a bedrock channel

• with a gravel bed…

• NOT an alluvial channel….• “A bedrock channel cannot substantially

widen, lower, or shift its bed without eroding bedrock”– Turowski et al., 2007

Two Different Parts of the River

800

900

1000

1100

1200

1300

1400

-10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00

Distance Downstream (RRM)

Elev

atio

n (ft

)

Plant

Oxbow

Hopeman Parkw ay

Dooms

Crimora

Forestry Station

Harriston

UPSTREAM SECTIONSlope = 0.0013

DOWNSTREAM SECTIONSlope = 0.0024

Upstream vs Downstream Reaches

• Upstream:– Wider floodplain– Finer-grained deposits– Lower slope– Fewer islands

• Upstream:– Greater retention of Hg associated with deposition

fine-grained particles– Greater ongoing supply of Hg to the channel from

bank erosion

The Sediment Budget of the South River

• Important processes for sediment transport and storage– Mercury is carried along with the sediment

Summary of Important Processes

• Suspended sediment transport by the river• Floodplain deposition

– “Valley flat”– Near the channel margins

• Bank erosion• Deposits within the channel itself

– Behind large woody debris (“logs”)• “Fine-grained Channel Margin” (FGCM) deposits

– On the bed• Fine-grained sediments stored within the pores of larger

gravel-sized grains

Relative Magnitude of Sediment Budget Components

Suspended Sediment Flux

OverbankDeposition

Bank ErosionFluvial Animals

Deposition: channel margin landforms

Cycled through FGCM deposits

Cycled through bed

Relative Magnitude of Sedimentation Processes, South River, VA, 1930-2005 (RRM 2.5-5.43)

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09Su

spen

ded

Sedi

men

tTr

ansp

ort

Floo

dpla

inSe

dim

enta

tion

Rel

ease

d fro

mFi

ne-G

rain

edC

hann

elM

argi

nD

epos

itsM

eas.

And

UnM

eas.

Fluv

ial B

ank

Eros

ion

Poin

t Bar

HR

ADSe

dim

enta

tion

Trib

utar

yC

onflu

ence

Sedi

men

tatio

n

Rel

ease

d fro

mSt

orag

e in

the

Bed

Bank

Ero

sion

Cau

sed

byAn

imal

s

Process

Mas

s of

Sed

imen

t, kg

Some Quantitative Estimates…

Specific Components of the Sediment Budget

• Bank erosion• Fine-grained channel margin deposits• Floodplain deposition near the channel

margins• The channel bed

– Dating of sediment in storage• Overview

Bank Erosion Rates

Modal rate is 0.05 m/yr (2 INCHES OF BANK RETREAT PER YEAR!!)

Mercury Loading From Bank Erosion: Highest in the Upstream ½ of the Study

Area

0.000

0.500

1.000

1.500

2.000

2.500

0-1 2-3 4-5 6-7 8-9 10-11 12-13 14-15 16-17 18-19 20-21 22-23

Relative River Mile

Hg

Load

ing

(kg/

mi/y

r)

Fine-Grained Channel Margin Deposits

Large Woody Debris

Fine-Grained Channel Margin

Deposit

Distribution of Sediment Ages in FGCM Deposits

The residence time of sediment in these deposits is ~ 2 years, but ~10% of the deposits are > 50 years old !

FGCM Deposits Provide A Record of Past Hg Concentrations On Sediment in the South River

Hg releaseperiod

Channel Margin Floodplain Deposits

• Deposits initially identified by shoreline mapping from aerial photographs– 1937-2005

• Hypothesis:– Deposits dating from the period of mercury

release could have very high Hg concentrations

• Denoted as HRADs:– Hg Release Age Deposits

Point Bar HRAD

Channelmigration

“Bench” HRAD

HRAD 1 - Slow Lateral Migration, 1930-2005

Lateral migration

HRAD 1

HRAD 1

0

10

20

30

40

50

60

70

0 100 200 300 400 500

Hg (ppm)

Dept

h (in

)

mud sand gravelDepth (in)

0‐66‐18

18‐30

30‐42

42‐54

54‐66

“Release” age

“Post-release” age

The Channel Bed

• Large areal extent• A potential “reservoir” of Hg-contaminated

fine-grained particles– Stored in between larger gravel grains

• Always in contact with the water column– Potential pathway for Hg to move up the food

web

Pilot Dating Study

0

5

10

15

20

25

0 20 40 60 80 100 120

Age (years)

Dep

th (c

m) Core 3 Pb 210

Core 5 Pb 210

Core 3 Cs 137

Core 5 Cs137

Hg-contaminated particles can be stored in the bed for many decades!!

Fine-Grained Sediment Budget -Summary

• Transport Process– Sediment carried in suspension by the river

• “Cycling” processes – storage and reworking– 1000 yr timescale

• Floodplain deposition• Bank erosion

– 1-100 year timescale• Fine-grained channel margin deposits• River bed

Sediment Budget of the South River – A Historical Perspective

• Based on many studies of stream valleys of the eastern U.S.

Colonial Settlement – early 20th

century• High sediment loads due to land clearing

and poor agricultural practices• Widespread construction of mill dams• Resulting in:

– Accelerated deposition of sediment on flood plains

• Implication:– Enhanced storage of Hg released at the end

of this period on floodplains

Historic Mill Dams and ImpoundmentsIn 1937 ~ 1 Impoundment every 2 mi

1930 1940 1950 1960 1970 1980 1990 2000

Port Republic (23.55)Below Grottoes (22.86)

Below Grand Cav. (20.2)Above Grand Cav. (19.3)

(16.08)1957 only (15.62)

Forestry Sta. (11.62)Above Crimora (9.62)

1957 only (5.62)Dooms (4.9)

1957 only (1.52)N. Park (1.05)

Plant (0)

Last Year Visible GeologyRelative River Mile

All mill dams are gone or breached today!

Mid-20th century-Present• Widespread breaching of mill dams• Decline of agriculture• Regional reforestation• Urban development• Resulting in:

– Floodplain reworking through bank erosion• Implication

– Mercury stored in floodplains during early-mid 20th Centurecontinues to be released to the channel of the South River

• BUT:– Erosional processes are VERY SLOW

• Mercury in floodplain storage will persist for millenia

Conclusions• Mercury was originally released to the South River from 1930-1950

– Mercury was stored in sedimentary deposits on the floodplain andchannel downstream of Waynesboro

– Most storage occurred in the upstream half of the study area• Sediment transport processes are slow along the South River

– Significant deposits of sediment and mercury from the original release period remain today on the floodplain and in the stream channel itself

• Mercury reenters the water column today from:– Bank erosion– Reworking of:

• The channel bed• Fine-grained channel margin deposits

– These processes appear to be most significant in the upstream half of the river

• Many thousands of years will pass before mercury in storage will be removed by through natural transport by the South River