a soil bioengineering guide - chapter3 - rivers and streams

8/13/2019 A Soil Bioengineering Guide - Chapter3 - Rivers and Streams

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CHAPTER 3Rivers a n d St rea m s

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Billi ons of years ago, r iv ers w ere formed by runoff flow ing tow ards low elevati ons and

coalescing t o form ri vers that sculpt ed and eroded paths tow ards the low est points on Earth .

Hu ge lak es w ere formed and event ua lly th ey j oined to create our oceans. These processes

contin ue today, albeit, in somew hat subdued fashions (Mount 1995).


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A SOIL BIOENGINEERING GUIDE

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This chapter d i scusses how ba l ance in r iv er sys tems i s

achiev ed by cer t a in fo rces an d h ow these fo rces d ic t a te

the phys i ca l behav io r, s i ze, an d sh ape o f a r iv er. The r i ver

profi le , the chan ne l pa t te rns , an d the loci of deposit i on

an d eros ion a l l refl ec t a r iv er s a t t empts to ba l an ce energy

ex pendi tures , dev e lop a l eas t -w ork des ign , an d process

the d i scharge an d sed imen t suppl ied to i t by it s w a tershed .

B il li ons o f yea rs ago , r iv ers w ere fo rm ed by runo f f fl owing

towa rds l ow e lev a t ions a nd coa lescing to fo rm r ivers tha t

sculp ted an d eroded pa ths tow ards the lowest po in t s on

Ear th . Huge lakes w ere fo rmed a nd ev entua l ly they jo ined

to c rea te our ocean s . These processes cont inue today ,

a lbe it , i n som ewh at subdued fash ions (Mount 1995).

Only a por t ion o f precip it a t i on en ds up as runo f f . More

o f ten p l ant s in tercept it a nd ev apo t ran spi ra t i on occurs o r

precip it a t i on i s infi l t ra ted in to the ground to m o i s ten s o il

an d feed aqui fe rs . Mois ture no t in tercepted , ev apora tedf rom pl ant s an d so i l, o r infi l t ra ted becomes runo f f an d i s

dr iv en by grav it y tow ards th e sea .

C a p t u re d f r o m m o u n t a i n t o ps , h i ll s, a n d s lo p in g w a t e r -

sheds , w ater i s energy tha t runs th e r iv ers eng ine and

grav it y i s the dr iv ing fo rce behind th e w ater , acce lera t ing

i t a t a r a te o f 32.2 fee t per second tow ard th e center o f the

Ear th . G iv en th i s acce lera t i on , one could ea s i ly env i s ion

tha t a r iv er should be const an t l y increas ing in v e loci t y as

i t fl ow s dow n h i ll .Ye t in any g iv en reach o f s t ream ov er

an y short period of t im e, current velocity var ies l it t le . The

fac t tha t w a ter does no t cont inuously acce lera te refl ec t s

the ba l ance be tw een th e fo rces tha t d r i v e wa ter (grav it y ),

keep i t in m o t ion (m omen tum ), an d t ry to s top i t f rom

m oving (friction ) (Ibid.).

Distributing the WorkRegard less o f the v ar ia t i on in s ize , sha pe , an d complex

composi t ion , r iv ers are th e pred ict ab le produc t o f

in terac t ion be tween phys i ca l , chemica l , an d b io log i ca l

processes.

According to Mount (1995),

One prominent schoo l o f though t a bout r i vers i s based ,

in par t , on the concept o f g rade o r equi l ib r ium . Grade

assum es tha t th e present morpho logy and behav io r of a

r iv er refl ec t s a ba l ance o f th e fo rces tha t opera te through

it a n d u p o n i t . In n a t u ra l s y s t e m s t h a t m o ve e n e rg y a n d

ma t ter, there is a t end ency fo r the sys tem to ar range i t se lf

in a m a n n e r t h a t b o t h r ed u c e s th e a m o u n t o f w o r k a n d

d i s t r ibutes tha t w ork as ev enly as poss ib le .These tw o

tendenc ies are o f ten in confl ict w i th each o th er in r i v ers .The energy an d m at te r tha t fl ow s in to an d through a r iv er

sys tem i s the d i scha rge and sed imen t l oad provided to it

by i t s w a tershed . In tu it i ve ly , the most e ffic ient mean s fo r

a r i v er to route th i s energy and m at te r to the sea w ould

seem to be to d ev elop a per fec t ly s t r a i ght cha nn e l o f

uni fo rm s lope . Howev er, because wa ter acce lera tes under

the infl uence of gravi ty, energy expendi tures (w ork) w ould

no t be eq ual l y d i s t ributed a long the ent i re s t r a i ght

c h a n n e l . In a d d i t io n , s in c e t r ib u t a r ie s a d d w a t e r t o a

r iv er, the am ount o f energy and m at te r in the sys tem

progressively increases dow n slope. Rivers deal w i th this

tendency fo r n onuni fo rmdis t r ibut ion o f w ork by


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RIVERS AND STREAMS

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a d j u s t m e n t s i n p ro fi l e, c h a n n e l c ro s s s e ct io n , a n d c h a n n e l

pa t te rn . The concav e-up long i tud ina l profi les o f r iv ers

an d the i r a l luv ia l fl oodpla ins w i th mea nder ing cha nn e l s

an d a ssoc ia ted r i ffl es a nd poo l s are a l l the produc t o f

the r iv ers a t t em pts to minimize the am ount o f w ork

per fo rm ed and t o spread th a t w ork out as ev enly as

possible . In this ma nn er rivers are self-regulat ing,

ev o lv ing jus t th e r ight pa t te rn an d profi le to ha nd le the

am ount o f d i scha rge an d sed iment de l iv ered to them.

This bala nce is termed grade(no t to be confused w i th

s lope o r grad ient ) an d records a s t a te o f equilibrium

w i th in a r iv er sys tem .

The Shape of RiversMost o f the bus iness o f a r iv er is condu c ted through i t s

chan ne l .The day- to-day t a sk o f ha nd l ing d ischarge , the

year-to-year t ask o f e rod ing , t r anspor t ing , an d deposi t ing

s e d im e n t , a n d t h e l on g -t e rm a d j u s t m e n t s t o w a r d s o m e

equi l ib r ium are a l l dependent on processes tha t occurw i th in o r imm edia te ly ad jacent to a r iv er s m ain cha nne l .

The m orpho logy an d behav io r o f chann e ls ha v e long been

considered a sens i t ive ind icator o f the stat e o f an y r iver

as w e l l as a record o f processes ac t ing w i th in a w atershed .

The ma x imum gra in s i ze tha t a fl ow ca n t r anspor t i s

i t s competence ; the to t a l amoun t o f sed imen t tha t can be

t r anspor ted i s a r i ver s capac i t y . Ra th er tha n rout ing

al l sedimen t through q uickly , r ivers wi l l typical ly store

sed iment w i th in a v ar i ety o f deposit i ona l s it es . Shor t -

te rm s to rage occurs in cha nne l bars and in the chan ne l

bed . In termedia te s to rage occurs w i th in the fl oodpla in .

Long-term stora ge occurs wi th in elevated a l luvial terraces.

Dec lines in capa c i t y and competence l ead t o deposi t ion

w i th in a l l o f these s i t es . Ex cess iv e s t ream power o r

dec l ines in sed imen t supply can ca use a r iv er to

select ively erode m at er ial in th ese si tes.

A r iver balan ces an d m inimizes i ts energy expendi tures

through a d jus tment o f i t s chan ne l c ross sec t ion . Along

the ent i re leng th o f a r iv er the shape a nd s i ze o f an

infin i te number o f c ross sec t ions a re in const a nt

v a r ia t i o n , a d a p t in g t o t h e d i s ch a r g e a n d s e d i m e n t l o a d

tha t i s de l iv ered to i t by the cha nn e l reach th a t l iesi m m e d i a t e ly u p s t r e a m . In a g g re g a t e t h e s e a d j u s t m e n t s

produce the d i s t inc t i v e cha nn e l pa t te rns tha t record th e

es t ab l ishm ent o f dyn am ic equi l ib r ium w i th in th e ov era l l

r iver system.


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r iv ers . Apparent l y, ban kfull d i scharges m eet tw o key

cri ter ia for sha ping chan nel cross sect ions: (1) the fl ow s

conta in suffi c ient s t ream power to e rode bank m ater i a l s

an d to t r anspor t an d deposi t la rge v o lum es o f sed im ent ;

an d (2) they occur o f ten enough tha t th e i r ef fec t s a re no t

mut ed by the wea ker, but h igher-f requency, sma l le r-

d i scharge ev ents . The in terac t ion be tw een ba nkful l

d i scharge and i t s chan ne l produces a w ide ran ge o f

chan ne l c ross sec t ions . The cau ses o f these v ar i a t i ons

are nu merous but are usua l ly t i ed d i rec t ly to in terac t ion

b e t w e e n t h e fl o w a n d t h e b a n k [a n d b a n k ] m a t e ri a ls

[an d r ipar i an v ege ta t i on . For ex am ple, ] w i th in any g iven

chan nel reach , the c ross-sec t iona l profi le o f the r iv er

v a r ie s f r om s y m m e t ric t o a s y m m e t ri c . Th i s v a r ia t i o n

is due prima ri ly to th e tend ency of a r iver to develop

m e a n d e r s ra t h e r t h a n a p e rf e ct ly s t ra i g h t ch a n n e l .

Wi thin mean der bends th a t a re t i ght ly curved , the

cross-sec t ion profi le becomes s t rong ly a sym m etr ic . In

the re l a t iv e ly s t r a i ght s t re t ches be tween m ean der bends ,the profi les are more sym m etr ic . (Mount 1995)

R iv ers const ruc t chann e l c ross sec t ions tha t a re bes t

ad apted fo r th e w ide ran ge o f d i scha rges de li vered by

the i r w atersheds . [S tud ies hav e show n] tha t a l though

unusua l ly l a rge d i scharge ev ents a re capab le o f g rea t l y

a f fec t ing r iv er chann e l s and r iv er geomorpho logy, the i r

occurrence is so r are tha t , w hen v iewed in the l ong te rm,

the i r ef fec t s a re usua l ly ma sked by in termedia te , more

f requent d i scharges . The a b i li t y o f in termed ia te fl ow s to

e ro d e , t r a n s p o rt , a n d d e p o si t s ed i m e n t a l lo w s t h e m t o

ev entua l ly un do the e f fec t s o f the l a rger event s a nd to

cont ro l the equi l ib r ium confi gura t ion o f the chan ne l s .The

intermedia te d i scharge tha t a ppears to ex ert the grea tes t

infl uence on th e shape a nd s i ze o f cha nne l c ross sec t ions

an d th us on the ov era l l geomorpho logy o f the r i ver i s

genera l ly known as bankfull stageor bankfu ll di scharge. For

m ost r iv ers , ban kfull s t age occurs w hen d i scha rge fi l ls the

ent i re cha nne l c ross-sec t ion w i thout s i gnifica nt inundat ion

o f the ad jacent fl oodpla in .

Bankful l s t age o r ban kfull d ischarge usua l ly occurs

w i th a f requen cy o f 1.5 to 2 years fo r na tura l , unda m med


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RIVERS AND STREAMS


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Channel PatternR iv er and s t ream chan ne l pa t te rns can be grouped in to

tw o genera l c lasses : s ing le chan ne l and mu l t ichan ne l .

The fl ow in single chan nel r ivers i s restr ic ted to a discrete ,

s inuous chann e l . The d i f ferences be tw een s ing le and

mu l t ichan ne l r iv ers refl ec t cont ras t ing w atershed

ondi t i ons .

When ex am ining s ing le cha nne l r iv ers in m ap v iew

the grea tes t v ar i a t i on a ppears to be in the w ay th ey snake

across th e l and scape . Thi s sn ake l ike proper ty i s t e rm ed

c h a n n e l sinuosity.The s inuosi ty o f a r iver i s var iably

defi ned but i s genera l ly a refl ec t ion o f the cha nn e l leng th

required to cover a given poin t- to-point or s t raight- l ine

d i s t ance . [Other na tura l infl uen ces on s inuosi t y inc lude

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geo logy, f au l t l ines , an d topography . (S t ream Corr idor

Resto ra t i on inc ludes m ore in fo rma t ion on th i s topic . See

the Bibliograp hy for m ore det ai ls . )]

[As sh ow n in the draw ing], the i rregular course of a r iver

usual l y occupies a por t ion o f a v a l ley, t e rmed th e meander

belt. A l ine draw n dow n th e center o f th i s mean der be lt i s

re fer red to a s th e meander belt axis. In la rge v a l leys the

mea nder be l t ax i s does no t a lways para l l el the v a l ley

w al l s and th us t ends to be longer than the v a l l ey i t se lf .

There are two poss ib le ax i a l mea surements w i th in th e

river cha nn el : th e thalweg, w hich i s the deepest por t ion o f

t h e c h a n n e l, a n d t h e channel axis, w hich is equid i s t an t

f rom the chann e l w a l l s .


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D o w n s t r e a m a s w e l l a s l a t e ra l e x p a n s io n o f m e a n d e r s

w i ll ev entua l l y cause m eand er chann e l s to in tersec t ,

t emporar i ly rees t ab l ish ing a shor ter an d s t r a i ghter

chan ne l and cut t ing o f f the chan ne l tha t o r ig ina l ly ma de

u p t h e l a rg e m e a n d e r b e n d . W h er e bo t h e n d s o f t h e s e

a b a n d o n e d m e a n d e r c h a n n e l s a r e cu t o f f f ro m t h e n e w l y

es t ab l ished chan ne l s , they fo rm horseshoe-sha ped lakes ,

k n ow n a s oxbow lak es.These lakes becom e the s i tes o f accu-

mu la t ion o f fine ov erban k sed iments a nd v as t q uan t i t ies

o f o rgan ic ma ter ia l such as pea t . When the l akes fi l l w i th

s e d im e n t , t h e y c om m o n ly b e co m e s w a m p s o r w e t l a n d s .

I f one end o f th e aba ndon ed m ean der cont inues to rece iv e

fl ow d i rec t ly f rom the cha nn e l, sloughsare fo rmed . These ,

too , ev entua l ly fi l l w i th sed iment but a re less l ike ly to

accum ula te la rge qua nt i t i es o f pea t . A second an d perhaps

m o r e c om m o n c a u s e o f a b a n d o n m e n t o f m e a n d e r s is

assoc i a ted w i th the deve lopmen t o f cu to f f s . Dur ing

fl ood ing ev ents , fl ow across the ins ide o f the po in t bar can

es t ab l ish chutes o r chan ne l s . In tense scour ing o f thesec h u t e s ca n l ea d t o t h e e s t a b lis h m e n t o f a n e w c h a n n e l

a c r o ss t h e p o in t b a r a n d a b a n d o n m e n t o f t h e m e a n d e r.

The tend ency for deposi t ion during overbank flood ing

ev ents to be concent ra ted n ear the r i ver can , in the l ong

term, lead to a bui ldup o f the mea nder be lt i t se lf . Dur ing

v ery l a rge fl oods , a r iv er w i l l occas iona l ly aba ndon i t s

m eand er bel t ent i rel y and es t a b li sh a new chan ne l in th e

surround ing lower-lying a reas o f t he va l ley. Term ed

avulsion, l a rge-sca le chan ne l abandon m ent i s a comm on

fea ture o f m ost la rge and in termedia te r iv er sys tem s .

RIVERS AND STREAMS

The urge to m ean der involves interact ion betw een

s e d im e n t s t h a t m a k e u p t h e be d a n d b a n k m a t e ri a l[including r ipar ian vegetat ion] an d the osci l lat ory na ture

o f fl ow w i th in a chan ne l .The long itud ina l bed profi le

o f m ost r iv ers a nd s t ream s i s broken in to a ser i es o f

a l t e rna t ing segments o f h igh a nd low grad ient . These

s e gm e n t s f o rm t h e r ifflesa n d pools, respect i ve ly , tha t a re

ev ident a t l ow w ater l ev e ls . Ri ffl es are the topographic

high po int s on a bed profi le .They a re t yp ica l ly spa ced fi v e

t o s e ve n c h a n n e l w i d th s a p a r t a n d a r e c o m p o s ed o f t h e

coarses t bed load th a t i s be ing t r an spor ted by the r i v er.

Poo ls , the hav en o f most r iv er fi sh , are deep-w ater areas

b e t w e e n t h e r i ffl e s . D u r in g h i g h fl o w e ve n t s , t h e p o o ls a r e

usua l ly scoured , l eav ing a coarse grav e l l ag o r chan ne larm or and deposi t ing m ater i a l on the r iffl es .


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To da te , the m ost w ide ly prac t i ced approach in t he Fores t

Serv i ce to r iv er morpho logy , fo rm, an d s t ruc ture is the

Rosgen c l ass i fica t i on sys tem . I t can be used to pred ict the

ex pected s t ream type and fl oodpla in fea tures o f a spec ific

s t ream ba sed on a h ierarchica l ana l ys i s sys tem.

According to Harrelson , Raw l ings, an d Potyondy (1994),

S t ream c l ass i fica t i on prov ides w ays to l ook a t s t ream

chan nel s , to g roup those tha t a re s imi lar , o r to i dent i fy

fea tures tha t a re d if fe rent . As w e ex pec t s t reams o f

s imi lar t ypes to ac t in s imi lar w ays , c lass i fi ca t i on o f fe rs a

pow erful tool for select ing stream s for com parison . This

c lass i fi ca t i on a l lows :

predict ion of a r iver s beha vior from i ts a ppea ran ce,

compa r ison o f s i t e-spec ific d a t a f rom a g iven reach to

da t a f rom o ther reaches o f s im i lar charac ter , an d a

consi s tent a nd reproduc ible sys tem o f t echnica l

comm unica t ion fo r r iv er s tud ies ac ross a r ange o f

discipl ines.

Rosgen s c l ass i fica t i on schem e in i t ia l l y so r t s s t ream s

into the m ajo r, broad s t ream types (A-G) a t a l andsca pe

lev e l . At th i s lev el , the sys tem c lass i fies s t ream s f rom

h e a d w a t e r s t o lo w l a n d s w i th s t r e a m t y p e :

Ah ea dw a t er Bin term ed ia t e

C & Em ea n d erin g Dbra id ed

Fen t ren ch ed Ggu lly.

The Rosgen sys tem breaks s t ream types in to sub types

b a s e d o n s lo p e r a n g e s a n d d o m in a n t c h a n n e l m a t e r ia l

par t i c le s izes . Subtypes are ass i gned num bers co r re-sponding to the m edian par t i c le d i ameter o f chann e l

mater i a l s :

1 = bed rock 2 = bou ld er

3 = cobble 4 = gra vel

5 = sa n d 6 = s ilt/cla y.

The a bove provides a sim pl ifi ed descr ipt ion of the

Rosgen sys tem . For more comple te in fo rm at ion about

the c l ass i fica t i on a nd a ssoc ia ted inv ento ry procedures ,

see Rosgen (1994). Ul t im at ely, st ream classifi cat ion helps

to d i s t ingui sh v ar i a t i ons due to s t ream type f rom

variat ions in the sta te or cond i t ion of si tes.

Addi t i ona l in fo rma t ion on Grea t Bas in geom orpho logy

an d p l ant m ater i a l s is in appendix B.

m a n y o f t h e w o r ld s g r ea t e s t r iv e rs d o n o t o c c u p y a

s ing le , s inuous cha nne l . Dur ing mod era te to h igh [fl oods]

these braidedr iv ers es t ab l ish mu l t ip le chann e l s tha t

repeated ly diverge and jo in.

An im portan t cha racter ist ic o f braided r ivers is the

i n s t a b il it y o f t h e i r c h a n n e l s . C h a n n e l a b a n d o n m e n t c a n

occur on t ime sca les v ary ing f rom hours to m onths a nd

can inv olve e ither g radua l o r sudden cha nges . The reason

fo r the dyn am ic na ture o f b ra ided r i ver chann e l s is roo ted

in the i r v ary ing d i scha rge, ov era l l coarse sed iment l oad ,

an d un st ab le bank m ater i a l s . Dur ing r ap id ly r is ing r iv er

sta ge, [r iver turbulence quickly increases in intens i ty

caus ing] the coa rses t ma ter ia l be ing t r anspor ted to

a c c u m u l a t e w it h i n t h e c e n t e r o f t h e c h a n n e l . In c o a r s e

bed load sys tem s, these accumula t ions in i t ia te the

f o rm a t io n , g ro w t h , a n d d o w n s t r ea m m i gr a t io n o f c h a n n e l

bars . As a chan ne l bar grows , i t de fo rms o r sp li t s the fl ow,

increas ing bed sh ear s t resses in chan ne l s o r chutes one i ther s ide o f th e bar . Because the r i v er is bed load

dom inated , the ban k mat er ia l s t end to be re la t i v e ly

coarse-grained a nd erosiona l ly non resistan t (Mount 1995).

Al l r iv ers and s t ream s na tura l ly cha nge course . Knowledge

a n d a w a r e n e s s o f h ow t h i s p ro c es s o c c ur s c a n m a k e a

d i ff e re n c e in m a n a g e m e n t a n d i n d e c id i n g w h a t s h o u ld

be s t ab i li zed and h ow.

Stream Channel M orphologySt ream or r iv er morpho logy i s inc luded to m ake the read er

aw are o f the c lass i fi ca t i on sys tems. These sys tems can beuse fu l in a na l yz ing a s t ream an d reso lv ing a problem ,

especial ly if ones interest i s in cha nn el redesign.

Schum m (1997) relat es th e type of cha nn el to the pre-

dom inan t t ype o f bed load in a cha nne l . For ex am ple,

s t a b le s t r a ig h t c h a n n e l s a n d m e a n d e r in g c h a n n e ls h a v e

cohes ive banks an d pr im ar i ly car ry a suspended sed im ent

load . Bra ided chan ne l s mov e prima r il y bed load sed iment

an d hav e noncohesiv e ban ks .

Montgom ery an d Buffi nt ons (1993) w ork reflects resea rch

done in the Pac i fic Northw est fo r a lluv ia l , co lluv ia l , an d

bedrock chan ne l s .Thei r sys tem i s s imi lar t o Schum ms .

They look a t the chan ne l s responses to sed iment

t h r o u gh o u t t h e w a t e r s h e d .


42


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RIVERS AND STREAMS

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FromAppliedRiverMorphology,usedwithpermissionfromDaveRosgen.

F r o m

A p p l i e d R i v e r M o r p h o l o g y u s e d w i t h p e r m i s s i o n f r o m

D a v e R o s g e n

a soil bioengineering guide - chapter3 - rivers and streams

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