NonBasicSpecificComments04

• Title: Comments 4
• Date: 10/23/2003
• SWSI - Doc Type: Comments

Figure 1 ~ Flow regime
is of central Importance
in sustaining the eco-
logical integrity offlow..
ing water systems. The
five components" of the
. flow reglme-magni..
tude. frequency~ dura-
tion, timing. and rate
of change-influence
integrity both directly
and indirectly, through
their effects on other
primary regulators of
integrity. Modification
of flow thus has cas-
cading effects on the
ecological integrity of
r1vers~ After Karr 1 991 ~

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E~Qlogicsr 'Integrity

Wildlife Service for w-ater-depen'd.ent spective on, wat~r management is
spe"cies of sporting, 'commercial, or rieeded' to guide ~ soe iety t s inter ac-
conservation value), making it diffi- tions with rivers~ 1
cult, ifnot impossiblet to manage the
entire river ecosystem (Karr 1991), The natural flo-;..v regime
However. environmental dynamism
is now recognized' as central to sus... The natural flow $f a river varies on
t~in~~g ~nd conserving' native spe... time scales of ho~rs. "dayst.seasons,
. cies diversity find ecological Integ:' . years, and longer ~ Ma-ny years of
rity in rivers and other ecosystems observationfrom*str~amflowgauge
(Holling and Meffe 1996t Hug~es are generallyneeqed to desc,ribe the
1994, Pickett et al. 1992, ~tanford et characteristic patt~rn ofariyer's flow
a1. 1996)t and coordinated actions qttantity, thntng,: and variability--
are therefore necessary to protect th'at 151 its natural flowregtme~ Com..
and restore a river's natural flow panents of a natur:ai flow regim~ can
. variability. be characterize4 ~sing v~rious time
, _. Io.thi:s attlclel we synthesize exlst~ series (e.g. ,'F ouri~r and wavelet)' and
lng scientific knowledge to argue that probabilityanaly&es of, for example I
the natural flow r.egime plays a critical extremely high or low flows" or of
role in s_us~aining native biodiversity the entire range 9f flows expressed
and ecosystem integrity in riversf as average dally)discharge (Dunne
Decades of 0 bservation Of the effects anc;! Leopold-, 19 ~8). In watersheds
of human alteration of natural flow lacking long-term streamflow dataJ
regimes have resulted in a well- analyses can be !extended statist!...
grounded scientific p~rspective on cally ~from gaug~d streams in the
why altering hydrologic variability same geographic area. The frequency
in rivers is ecologically harmful (e.g.. of large-magnitude floods can be es..
, Arthtngton et at 1991;' Castleberry tlmat~d by paleo~ydrologlc studies
et al. 1996~ Hill et a1. 1991. Johnson ofdebri,s left by flQods and by studies
et a1. 1976; Richter et a!. 199 7~. Sparks of histo rical da~age tl? ~i ving trees
1995tStanfordetal.1996,Toth1995, (Hupp and Oster)<amp 1985t Knox
Tyus 1990) . Current pressing demands 19 72) ~ These hlstb:r~cal techniques can
on water use and the continuing' alter- be used to extend ~existing hydrologic
ation of watersheds require scientists records or to' prpvide estimates of
to help develop manageir!ent proto- flood flows for upgauged sites.
eols that can accommoda~e economic River flow reg~mes show regional
uses while protecting ecosystem func- patterns that are tletermined, largely
tions. For humans t~ continue to rely by river size and b~ geographic varia-
on river ecosystem's for sus~ainable tian in climatet ~geo16gy; .topogra-
food production. power productiont phy, and vegetat:!ve cover, For ex':'
waste assimilation. and flood con- ample, some stre*ms in 'r'egtons with
tro1; a new t holistic, ecological per- little seasonality ~n precipitation ex..

770

hibit relatively stable hydrographs
due to high groundwater inputs (Fig-
ure 2a), whereas other streams can
fluctuate greatly at virtually any time
of year (Figure 2b). In regions with
.: seasonal ptecipitatio,n, some streams
are dominated by snowmeltt result-
ing in pronounced, predictable run-
off patterns (Figure 2c) I and others
lack snow accumulation and exhibit
more variable runoff patterns during
the rainy season; with peaks occur-
ring after each substantial storm
event (Figure 2d) ~
Five critical components of the
flow regime regulate ecological pro-
cesses In river ecosyatems: the mag-
nitude. frequency, duration, timing,
and rate 9:f change of hydrologic
conditions (Poff and Ward 1989t
Richter et a1. 1996, Walker et a~.
1995). These c.omponents can be ~sed
to characterize the entire range of
flows and ~pec~flc hydrologic phe..
nOqlena1 such as floqds or low flows, '
that are: critical to the integrity of
river ecosystems~ Furthermore, qy
. defining flow regimes in tJ:les~ termSt
the ecological conseque.nces of par-
ticular human activitles'that modIfy
qne or'more components of the flow
regime ca~ be considered explicitly~

. The magnitud~ of discharge1 at any
given time interval is simply the
amount 9fwater moving past a fixed
location per unit time. Magnitude
can refer either'to absolute or to
relative discharge (e~gq the amount
of water that inundates a floodplain).
Maximum and minimum magnitudes
. of flow vary with climate and water...
shed sIze both within and among
river systems.~
., The frequency of occurrence refers
to how often a flow above a .given
"magnitude recurs ,over some speci-
fied time intervaL Frequency of oc-
currence is Inversely related to flow
magnitude. For example, a IOO-year
flood is equaled or exceeded oil aver-
age c once every 100 years (i.e., a
chance of" 0.0 1 , of occurring in any
given year). The average (median)

~ Discharge (also kn'own as streamflow I flow I
or flow rate) is alWays expressed in dimen-
slons of volume per time. However, a great
variety of units are used to describe flow.
depending on custom and purpose of charac-
terization: Flows can be expressed in near-
Instantaneous terms (e.g.~ ft3/s and rn3/s) or
over long time intervals (e.g., acre..ft/yr).

BioScience Vol. 47 No. 11