representative photographs - new hampshire · 19\appendices\appendices.docx a-8 appendix a –...

$: Representative Photographs - New Hampshire · 19\Appendices\Appendices.docx A-8 Appendix A – Representative Photographs Suncook River – Confluence Area Looking downstream at cross-section$
J:\51892.00\reports\Draft_Final_Report_May_19\Appendices\Appendices.docx A-1 Appendix A – Representative Photographs

Appendix A

Representative Photographs


Suncook River – Old Primary Channel

Looking upstream at cross-section 1

Looking downstream at cross-section 4


Suncook River – Old Primary Channel


Looking at left bank at cross-section 3


Suncook River – Avulsion – New Channel

Looking upstream at cross-section 1 above the avulsion

Looking downstream at cross-section 2 above avulsion



Looking at right bank and at abandoned channel at cross-section 3




Looking at left bank at cross-section 4



Suncook River – Old Side Channel

Looking upstream at side channel cross-section

Looking downstream at side channel cross-section


Suncook River – Confluence Area

Looking downstream at cross-section 2 above confluence with abandoned channel

Looking at right bank at the confluence of the abandoned and new channel



Looking at sand deposit on right bank below confluence

Looking at left bank and at floodplain below Round Pond



Looking downstream at cross-section 8

Looking at right bank between cross-section 3 and 4

J:\51892.00\reports\Draft_Final_Report_May_19\Appendices\Appendices.docx B-1 Appendix B – Hydrological and Geomorphic Data

Appendix B

Hydrological and Geomorphic Data (PARISH Geomorphic)

B-2 Appendix B – Hydrological and Geomorphic Data

Figure C-1: Key to the locations of detailed field sites on the Suncook River, Epsom, NH.


New Channel

�4

�2

0

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14

16

18

0 20 40 60 80 100 120 140 160 180

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th�(ft)

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New�Channel�� Cross�section�1

Channel�Bed

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25

0 50 100 150 200

Dep

th�(ft)

Width�(ft)New�Channel�� Cross�section�2

Channel�Bed


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th�(ft)


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th�(ft)

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th�(ft)


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th�(ft)


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Series1


FLUVIAL GEOMORPHOLOGY SUMMARYSuncook RiverSite Location: New Channel Number of cross-sections: 10 Date of Survey: July 9th, 2007

Controlling Factors

Upstream Drainage Area: 407 km2 Geology / Soils: alluvial

Modifying Factors

Surrounding Land Use: forest/gravel pit General Riparian Vegetation: forest Existing Channel Disturbances: dams on the old primary and secondary channels Woody Debris: major

Cross-Sectional Characteristics

Range AverageBankfull Width (ft) 141.66 - 233.70 187.77 Bankfull Depth (ft) 0.58 - 17.38 1.00 Width / Depth 12.34 - 28.34 19.06 Entrenchment (ft) 151.50 - 325.96 22.06 Entrenchment Ratio 1.00 - 2.01 1.28 Manning's n 0.035

Bank Characteristics

Range Average Bank Height (ft) 6.07 - 36.09 11.16 Bank Angle (degrees) 0 - 63.3 27.49 Root Depth (inches) 0.0 - 15.75 2.4 Root Density (1=Low - 5=High) 1 - 4 2.1 Protected by vegetation (%) 0 - 100 21.8 Amount of undercut (inches) 0.0 - 11.81 0.85 Banks with undercuts (%) 21%

Bank Materials Torvane values (kg/cm2)S,Cl 1.46 S, Si, Cl,P 0.81 S,Si,Cl 2.00 Cl 2.50 S 0.63

SubstratParticle

Hydraul

EmbeddSub-pav

ParticlePebble C

Field ObXs 1 Boulbanks butXs 2 Largbank) Xs 6 left bbank, nonXs 8 fast

Xs 10 end

te CharacterisShape (inch

ic Roughnes

dedness (%) vement

Sizes (cm) Counts

bservations der riffle, rivet channel doee gravel/cobb

bank highly vene near watert moving wated of pit area, s

B-9 Ap

sticshes+A91)

XYZ

ss (cm) MaximuMedian Minimum

Cl 22 Si 1Vfs 10 Fs 11 Ms 3.5Cs 16 Vcs 4.2

D10 D50 D90

er appears to es not appear ble mid chann

egetated on tor on left side. er, gravel barsignificant wo

ppendix B – H

Range1.18 - 27.160.39 - 20.080.39 - 14.96

m 0 - 51.180 - 13.78

m 0 - 1.970 - 100

1.969E-050.11 inches1.66 inches

begin to unrato be over w

nel bar diverti

op of bank, ve

r in middle of oody debris (fa

Hydrological a

Average6 4.18 3.36 2.2

11.5 3.1 0.4 52.2

P 8.80.3937 "0.59" 1.0.7874"1.18" 8.61.57" 1.1.97" 6B dr. 0.5

inchesss

avel at this poidened ng flow to left

egetation not

riverallen trees >7

and Geomorp

e

" 2.5 52.7

6565

5

int, bank eros

t Bank, riffle f

near the wat

75' tall) fast m

phic Data

sion above th

flows towards

ter. Fair rootin

moving water

is point on bo

s east bank (le

ng density at t

oth

eft

top of


Old Primary Channel

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2

3

4

0 10 20 30 40 50 60 70 80 90 100

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Channel�bedBankfull

�1

0

1

2

3

4

5

0 10 20 30 40 50 60 70 80 90

Dep

th�(ft)

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Old�Primary�Channel�� Cross�section�2

Channel�Bed

Bankfull


�1

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 80 90 100

Dep

th�(ft)

Width�(ft)Old�Primary��Channel�� Cross�section�3

Channel�BedBankfull

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 10 20 30 40 50 60 70 80 90 100

Dep

th�(ft)

Width�(ft)Old��Primary�Channel�� Cross�section�4

Channel�Bed

Bankfull


0

2

4

6

8

10

12

0 20 40 60 80 100 120 140 160 180

Dep

th�(m

)

Width�(m)Old��Primary�Channel�� Cross�section�5


0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120 140 160 180 200

Dep

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Width�(ft)Old�Primary�Channel�� Cross�section�6



FLUVIAL GEOMORPHOLOGY SUMMARYSuncook Old Primary ChannelSite Location: Old Primary channel west of Bear Island Length surveyed: NANumber of cross-sections: 6Date of Survey: 10-Jul-07 Controlling Factors

Upstream Drainage Area: 407 km2 Geology / Soils: bedrock (predominant)

Modifying Factors

Surrounding Land Use: forest/scrub/farm General Riparian Vegetation: forest/scrub Existing Channel Disturbances: Bridge upstream of avulsion, dams on both old channels

Woody Debris:


Range Average Bankfull Width (ft) 78.29 - 0.00 91.14 Bankfull Depth (ft) 2.12 - 3.67 2.86 Width / Depth 22.21 - 41.25 32.56 Entrenchment (m) 121.76 - 162.38 135.15 Entrenchment Ratio 1.32 - 1.75 1.51 Manning's n 0.035


Range Average

Bank Height (inches) 3.281 - 6.562 0.541 Bank Angle (degrees) 7.5 - 50 22.625 Root Depth (inches) 0.8 - 3.979 2.1 Root Density (1=Low - 5=High) 2 - 3 2.5 Protected by vegetation (%) 60 - 100 82.5 Amount of undercut (inches) 0.0 - 59.055 5.052 Banks with undercuts (%) Bank Materials Torvane values (kg/cm2) S, Org 3.19 Cbl, Org, Bedrock 4.00 Org 2.00 S, Cl, Org 3.00


Substrate Characteristics Particle Shape (inches) Range Average X 0.394 - 13.386 5.2 Y 0.394 - 10.630 3.8 Z 0.394 - 7.087 2.5

Hydraulic Roughness (inches) Maximum 9.843 - 37.0078 20.9 Median 1.969 - 9.8425 5.0 Minimum 0 - 1.969 0.8 Embeddedness (%) 20 - 50 26.0 Sub-pavement Cl 0 P 16.6666667 Si 16.666667 0.3937 " 0 Vfs 0 0.59" 0 Fs 16.666667 0.7874" 0

Ms 16.666667 1.18" 0Cs 16.666667 1.57" 0Vcs 16.666667 1.97" 0

Bdr. 0 Particle Sizes (inches

Pebble Counts D10 0.196 inchesD50 6.93 inches D90 24.22 inches

Field Observations xs 3 muskrat in pool, large erratic, pool bottom made up of cobble and boulders xs 4 cobble boulder bottomxs 6 leaves in bottom of stream very silty

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Cla

yS

iltV

FS FS MS

CS

VC

Spe

bble

sFG M

GM

GC

GC

GV

CG

VC

G SC

SC LC LC SB

SB

MB LB

VLB

VLB

Cum

ulat

ive

Perc

ent

Tota

l Per

cent

Particle Size (cm)

Substrate Size Distribution -

Total %Cumulative %


Old Side Channel

Side�Channel��Cross�section�1

0

1

2

3

4

5

6

0 10 20 30 40 50 60

Width�(ft)

Depth�(f



FLUVIAL GEOMORPHOLOGY SUMMARYOld Side Channel

Site Location: Old Side Channel Number of cross-sections: 1Date of Survey: July 9th, 2007

Controlling Factors

Upstream Drainage Area: 407 km2 Geology / Soils: alluvial

Modifying Factors

Surrounding Land Use: forest/gravel pit General Riparian Vegetation: forest Existing Channel Disturbances: dams on the old primary and secondary channels

Woody Debris: minor


Range AverageBankfull Width (ft) 55.4Bankfull Depth (ft) 4.81Width / Depth 11.51Entrenchment (ft) 105.0Entrenchment Ratio 1.9Manning's n .035


Bank Height (ft) 1.15 Bank Angle (degrees) 53 Root Depth (inches) 23.5 Root Density (1=Low - 5=High) 3 Protected by vegetation (%) 100 Amount of undercut (inches) 7 Banks with undercuts (%) 21

Bank Materials Torvane values (kg/cm2)S,Cl 2.77

Substrate Characteristics

Particle Shape (cm) Range Average X 5-24 15.3 Y 2-16 8.0 Z 4-10 7.7

Hydraulic Roughness (cm) Maximum 84 Median 36 Minimum 5


Embeddedness (%) 15 Sub-pavement cl 0 P 0 si 0 0.3937 " 0 vfs 0 0.59" 0 fs 0 0.7874" 0

ms 0 1.18" 0cs 0 1.57" 0vcs 0 1.97" 0

Bdr. 100 Particle Sizes (cm)

Pebble CountsD10 .049 D50 4.79 D90 102.39


0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

< .0

002

.000

2-.0

009

.001

-.004

9.0

05-.0

19

.02-

.049

.05-

.19

.2-.4

9

.50-

.59

.60-

.79

.80-

1.19

1.2-

1.59

1.6-

2.39

2.4-

3.19

3.2-

4.79

4.8-

6.39

6.4-

9.59

9.6-

12.7

9

12.8

-19.

1919

.2-2

5.59

25.6

-38.

39

38.4

-51.

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-102

.39

102.

4-20

4.79

204.

8-40

9.59

> 40

9.6

BED

RO

CK

Particle Size (cm)

Tota

l Per

ce

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Cum

ulat

ive

Perc

Total %

Cumulative %


Confluence Area

0

2

4

6

8

10

12

0 20 40 60 80 100 120

Dep

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Width�(ft)

Confluence�Area� Cross�section�1

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6

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3

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5

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7

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7

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FLUVIAL GEOMORPHOLOGY SUMMARY Suncook River

Site Location: Confluence of old and new channels Number of cross-sections: 9 Date of Survey: July 9 2007 Controlling FactorsUpstream Drainage Area: 407 km2 Geology / Soils: alluvial Modifying Factors

Surrounding Land Use: forest General Riparian Vegetation: forest Existing Channel Disturbances: Dams on Old Primary and Side channels Woody Debris: Major


Range Average Bankfull Width (ft) 61.13 - 132.32 102.44 Bankfull Depth (ft) 1.20 - 5.79 3.14 Width / Depth 14.14 - 71.43 38.81 Entrenchment (ft) 0.00 - 132.32 92.19 Entrenchment Ratio 1.00 - 1.00 1.00 Manning's n 0.035


Range Average Bank Height (ft) 0.34 - 9.84 5.66 Bank Angle (degrees) 1 - 90 21.34 Root Depth (inches) 0.00 - 11.81 3.43 Root Density (1=Low - 5=High) 1 - 4 2.2 Protected by vegetation (%) 0 - 90 45.7 Amount of undercut (inches) 0.00 - 1.18 0.11 Banks with undercuts (%) 21%

Bank Materials Torvane values (kg/cm2) VFS, Cl 0.88 Cl, S, Org 2.30 Cl,S,Cbl, Org 1.07 VFS,FS,MS,Cl 0.66 Cl, FS 0.54 Substrate Characteristics Particle Shape (cm) Range Average

X 0.39 - 5.51 0.89 Y 0.39 - 2.76 0.61 Z 0.2 - 2.76 0.51

Hydraulic Roughness (cm) Maximum 11.81 - 35.83 9.38


Median 15.75 - 15.75 6.20 Minimum 0 - 0 0.00 Embeddedness (%) 1 - 100 154.09 Sub-pavement Cl 3.125 P 11.25 Si 0 0.3937 " 0 Vfs 0 0.59" 0 Fs 10 0.7874" 1.25

Ms 23.125 1.18" 8.125 Cs 37.5 1.57" 3.875 Vcs 0 1.97" 1.75

Bdr. 0 Particle Sizes (cm)

Pebble Counts D10 0.00 inches D50 0.23 inches D90 1.81 inches

Field Observations

xs 1 right banks natural bank is exposed with only a .5 m amount of sand on top, Terrestrial plants under water on right bank xs 3 old channel one pool like here, saw 6" catfish xs 4 LB no growth on all of the gravel part of the bank, normal topsoil appears about 2 vertical m up from in RB growth of plants in water, poison ivy growing through sand xs 6 no roughness indicators, except 1 lump of clay. LB is a deposit burying tree roots

xs 7 valley wall is bank of rand pond, beyond the pond edge next to river, valley wall is over 100m

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Cla

yS

iltV

FS FS MS

CS

VC

Spe

bble

sFG M

GM

GC

GC

GV

CG

VC

G SC

SC LC LC SB

SB

MB LB

VLB

VLB

Cum

ulat

ive

Perc

ent

Tota

l Per

cent

Particle Size (cm)


Total %


Short Falls Area

Short�Falls�Area��Cross�section�1

�1

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120 140 160

Width�(ft)

Depth�(f


Short Falls Area - Cross-section 2

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600

Width (ft)

Dep

th (f

Cross-Section 2

Bankfull


Reference Reach

�1

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80 90

Dep

th�(ft)

Width�(ft)Reference�Reach�� Cross�section�1

Channel�bed

Bankfull

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100

Dep

th�(ft)


Channel�Bed

Bankfull


�4

�2

0

2

4

6

8

10

12

0 20 40 60 80 100 120

Dep

th�(ft)

Width�(ft)

Reference�Reach�� Cross�section�3

Channel�bed

Bankfull

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80 90 100

Dep

th�(ft)


Channel�bed

Bankfull


0

2

4

6

8

10

12

0 20 40 60 80 100 120

Dep

th�(ft)

Width�(ft)

Reference�Reach�� Cross�section�5

Channel�bed

Bankfull


FLUVIAL GEOMORPHOLOGY SUMMARY

Suncook Reference Reach Site Location: Reference Length surveyed: 0 Ft Number of cross-sections: 5Date of Survey:10-Jul-07 Modifying Factors

Surrounding Land Use: forest/agricultural General Riparian Vegetation: forest Existing Channel Disturbances:

Woody Debris:


Range Average Bankfull Width (ft) 77.17 - 110.78 91.87 Bankfull Depth (ft) 2.25 - 8.95 4.74 Width / Depth 12.38 - 34.27 23.30 Entrenchment (ft) 116.54 - 150.15 136.49 Entrenchment Ratio 1.36 - 1.66 1.49 Manning's n


Range Average Bank Height (ft) 3.28 - 6.56 4.51 Bank Angle (degrees) 7.5 - 50 22.625 Root Depth (inches) 0.8 - 3.94 2.1 Root Density (1=Low - 5=High) 2 - 3 2.5 Protected by vegetation (%) 60 - 100 82.5 Amount of undercut (inches) 0.0 - 59.06 5.052 Banks with undercuts (%) 21%

Bank Materials Torvane values (kg/cm2) S, Org 3.19 Cbl, Org, Bedrock 4.00 Org 2.00 S, Cl, Org 3.00 0 Substrate Characteristics Particle Shape (inches) Range Average X 0.39 - 13.39 5.2 Y 0.39 - 10.63 3.8 Z 0.39 - 7.09 2.5

Hydraulic Roughness (inches) Maximum 9.84 - 37.01 20.9

J:\51892.00\reports\Dra19\Appendices\Append

EmbeddSub-pav

Particle

aft_Final_Report_May_ices.docx

MedMini

dedness (%) vement

Cl 0Si 16Vfs 0Fs 1Ms 1Cs 1Vcs

Sizes (cm)

D10 D50 D90

B-30 Ap

dianmum

6.6706.67

16.67 16.67 16.67

ppendix B – H

1.97 - 90 - 1.9720 - 50

P0.39370.59"0.7874"1.18"1.57"1.97"Bdr.

Pebbleinchesinchesinches

Hydrological a

9.84 5.07 0.8

26.0

16.6666" 0

0" 0

0000

Counts

and Geomorp

6667

phic Data

J:\51892.00\reports\Draft_Final_Report_May_19\Appendices\Appendices.docx C-1 Appendix C – Geomorphic Analysis

Appendix C

Geomorphic Analysis


Bankfull Discharge Estimates

Old Primary Channel, Bankfull Discharge Estimates

0

200

400

600

800

1000

1200

1400

1600

1800

XS-Ref-1 XS-Ref-2 XS-1 XS-2 XS-3 XS-4 XS-5 XS-6 XS-7 XS-8 XS-9 XS-10

Cross-Section

cfs

)

BrownlieManning'sCowanStricklerL,W, and MLimerinosBrayGriffithsEffective QSELECTED BANKFULL DISHARGE = 2-yearRegional Curve1.25 -yearAverage

Old Side Channel, Bankfull Discharge Estimates

0

200

400

600

800

1000

1200

1400


Cross-Section

cfs

)



Confluence Area, Bankfull Discharge Estimates

0

1000

2000

3000

4000

5000

6000

7000

8000

9000


Cross-Section

cfs

)


Short Falls Area, Bankfull Discharge Estimates

0

500

1000

1500

2000

2500

3000

3500


Cross-Section

cfs

)



Reference Reach, Bankfull Discharge Estimates

0

1000

2000

3000

4000

5000

6000


Cross-Section

cfs



Hydraulic Geometry

Old Primary Channel- Hydraulic Geometry

0102030405060708090

100110120130140150160170180190200210220230240250260270280290300310320330340350360

XS-REF-1 XS-REF-2 XS-1 XS-2 XS-3 XS-4 XS-5 XS-6 XS-7 XS-8 XS-9 XS-10 XS-tmplt-1 XS-tmplt-2

Wet Perim (ft)Bankfull Area (sft)Banfull Width (ft)W/D Ratio

Old Primary Channel - Hydraulic Geometry

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


Bankfull Depth (ft)Hydralic Bankfull Depth (ft)Hydraulic Radius (ft)


Old Side Channel - Hydraulic Geometry

0102030405060708090

100110120130140150160170180190200



Old Side Channel - Hydraulic Geometry

0

1

2

3

4

5

6




Confluence - Hydraulic Geometry

0102030405060708090100110120130140150160170180190200210220230240250260270280290300310320330340350360370380390400410420430440450460470480490500510520



Confluance - Hydraulic Geometry

0

1

2

3

4

5

6

7

8




Short Falls - Hydraulic Geometry

0200400600800

1000120014001600180020002200240026002800300032003400360038004000420044004600



Short Falls - Hydraulic Geomoetry

0

5

10

15

20

25

30




Reference Reach - Hydraulic Geometry

0102030405060708090100110120130140150160170180190200210220230240250260270280290300310320330340350360370380390400410420430440450460470480490500510520530



Reference Reach - Hydraulic Geometry

0

1

2

3

4

5

6

7

8




Regional Curves

From: NHDES and NHDOT (2005), Guidelines for Naturalized River Channel Design and Bank Stabilization

J:\51892.00\reports\Draft_Final_Report_May_19\Appendices\Appendices.docx Appendix D – Whittkop (2008)

Appendix D

Whittkop (2008)

Suncook River Long Term Observations

DEPARTMENT OF CHEMISTRY AND GEOLOGY 242 TRAFTON SCIENCE CENTER N · MANKATO, MN 56001

PHONE 507-389-1963 (V) · 800-627-3529 OR 711 (MRS/TTY) · FAX 507-389-5625 An Affirmative Action/Equal Opportunity University.

To: David Wunsch, State Geologist Rick Chormann, Senior Hydrologist, NHGS, NHDES Steve Couture, Rivers Coordinator, NHDES Peter J. Walker, VHB

From: Chad Wittkop Date: February 24, 2008

RE: Suncook River long-term observations

I write to share my long-term observations of the new channel of the Suncook River in Epsom, New Hampshire. This document is intended to serve as an addendum to the Northeastern GSA field guide published in spring 2007, which described the basic geology of the site and outlined the causes of the avulsion. Since assembling the field guide, I have made two more trips to the site—one during the Northeastern GSA field trip in March 2007, and another more extensive examination over several days in late July 2007.

During the more recent visit, a student assistant and I collected new differential GPS data to record changes in the position of the new channel. Additional comparisons were made using digital photos, field relationships, and anecdotal accounts of Epsom residents. I have also recently received 1-foot contours and May 2007 orthophotos courtesy of Mr. Couture and the Town of Epsom, and have begun incorporating these valuable data in my ongoing analysis.

Results of my long term observations are grouped into themes below.

I. Erosion and the New Channel

Differential GPS data were collected in May and June of 2006 in effort to map the position of the new channel. Among the features documented at the time were positions of banks, areas of avulsion-related erosion, and knickpoints. Using GIS, I compared the location of the 2006 features with new GPS data collected in July 2007. This analysis shows:

� Portions of the new channel have migrated laterally a minimum of 150 feet to the south-southeast. Examination of three sets of digital photos taken from the same vantage in May 2006, March 2007, and July 2007 clearly show this erosion, the bulk of which appears to have taken place between March 2007 and July 2007 (Figures 1 & 2).

� Lateral migration of the new channel to the southwest has eroded up to 80 feet of a gravel ridge near the capture point of the old channel.

� Digital elevation models derived from the 1-foot contour data developed by Eastern Topographics allow incision in the new channel to be quantified (Figure 3). At the capture point of the new channel, 14 feet of incision has occurred between the thalweg of the former channel and the water surface of the new channel. In the center of the new channel, 24 feet of incision has occurred between the former wetland surface and the new channel water surface.

� As a result of the incision described above, severe headcutting and associated bank failures have developed upstream of the capture point. Significant bank failures have occurred as far as 2,500 feet upstream from the new channel capture point.

Chad Wittkop February 24, 2008 Suncook Long-Term Observations Page 2

� Headcuts are also progressing upstream in tributaries whose base-levels were altered by the incision. A knickpoint on a small tributary of the Suncook, Leighton Brook, migrated upstream over 300 feet between May 2006 and July 2007. Similar incision displaced granite cribbing on the railroad grade crossing the Little Suncook River, which is only 1000 feet downstream from the Route 4 bridge.

� Incision upstream of the capture point has exposed boulders identified as local bedrock (Rangeley formation; Peter Thompson, personal communication). The presence of bedrock may inhibit further incision, but unrestricted lateral erosion could allow the channel to bypass bedrock knobs and allow a new cycle of incision. The topography of the bedrock surface in this area is unknown, and geophysical surveys and/or drilling would be required to examine this issue in depth.

� Upstream of Route 4, residents have reported subtler changes in the river and a lower water table, which are likely related to the increased gradient and drop in base level associated with the avulsion. Owners and residents of the Epsom Valley campground north of Route 4 report that since the avulsion the river level is lower and has become flashier. In the campground, portions of bank have failed resulting in the loss of several mature white pine trees, and the water level in an oxbow pond has dropped as much as 3 feet.

II. Depositional Features Severe aggradation (channel filling), is evident from the new channel south to Short Falls. In 2006 I estimated as much as 150,000 cubic yards of sediment was introduced to the Suncook as a result of the avulsion. I have not had a chance to refine this estimate, but the significant post-avulsion erosion described above is providing a continued supply of sediment downstream of the avulsion site. Impacts of this deposition include:

� Comparison of 2005 aerial photos and 2007 GPS data reveals a delta building into the north end of Round Pond near the Epsom well house (Figure 4). As much as 125 feet of open water has been filled with delta sediment. Two outlet channels have developed at the southeastern corner of Round Pond, and likely contain significant discharge during flood events.

� Aggradation to bankfull height in portions of the reach between the southern tip of Bear Island and a series of tight meanders above Short Falls. An active cutoff channel is developing in the area, and will likely overtake the previous channel, which is superelevated (main channel bed is higher than the cutoff) relative to the cutoff (Figure 4). If it completes, the cutoff will abandon an additional 1,700 feet of channel for a new course that is approximately 700 feet long.

III. Stratigraphy of the New Channel

Ongoing incision in the new channel reveals a clearer picture of the stratigraphy of Pleistocene and Holocene deposits in the avulsion area. These sediments were initially described in the 2007 field guide, but a more coherent synthesis of this stratigraphy is offered below (Figure 5).

� A silty glacial till was observed in deeply incised-portions of the channel near the capture point. The sediment contained cobble-sized clasts in a silty, dark gray matrix. In New Hampshire, this type of sediment is often described as an “Illinoian” (i.e. old) till but no age relationship is implied here beyond its relative stratigraphic position.

� Glacial lake sediments occur stratigraphically above the till. Additional exposure of deformed, silty glacial lake beds near the capture point suggests an overall coarsening upward sequence in


the sediments exposed there, from silt to fine sand to sand and gravel, with the coarsest sediment occurring in glaciofluvial deposits that form the higher ridges in the valley. The coarsening upward sequence, combined with the presence of deformed lake beds, is suggestive of an increasing proximity to the glacial margin with time, i.e. a local glacial advance or re-advance.

� A Holocene fluvial sequence is well exposed in the new channel, displaying broad, gently dipping lateral accretion surfaces, which are formed by lateral migration of pointbars in a meandering river system. These sandy, post-glacial fluvial sediments can be distinguished from sandy glacial sediments by their unoxidized (white to gray) color, the presence of dark stringers of organic-rich sediments, and by the presence of lateral accretion surfaces. Vivianite-bearing, silty wetland sediments described in the 2007 field guide likely represent vertically accreting channel fill (oxbow) sediments.

� The presence of Holocene fluvial sediments underlying much of the new channel suggests that the Suncook flowed through areas occupied by portions of the new channel in the past, but was unable to completely breach the coarse-grained glacial ridge to initiate an avulsion.

IV. Impoundments and Gradients

The large-scale patterns of erosion and deposition described are consistent with the steeper gradient and increased stream power resulting from the avulsion. The impoundment at Hukins Mill dam acted to maintain a very low gradient in the Suncook upstream of the site. Loss of this impoundment and the increased gradient of the new channel is also consistent with the reports of lower water tables and increased flashiness in the stream above Route 4.

In the absence of an impoundment at Hukins Mill, roadbank fill in the floodplain along U.S. Route 4 may act in part as an impoundment during high flow events, such as the April 2007 flood. This may have the effect of further concentrating flow downstream of Route 4 as the bridge opening acts as a spillway-like constriction. Left unmitigated this situation, particularly when combined with the upstream migration of headcuts from the avulsion site, could compromise the Route 4 bridge.

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