7853

• UCREFRP Catalog Number: 7853
• Author: Auble, G. T., J. M. Friedman and M. L. Scott
• Title: Relating Riparian Vegetation To Present And Future Streamflows
• USFW Year: 1994
• USFW - Doc Type: Ecological Applications
• Copyright Material: NO

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Ecolo$ical Applica/ions. 4(3), 1994, pp,544--554

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RELATING RIPARIAN VEGETATION TO PRESENT AND
FUTURE STREAMFLOWSl

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GREGOR T. AUBLE, JONATHAN M. FRIEDMAN, AND MICHAEL L. SCOTT
National Biological Survey, Fort Collins, Colorado 80525-3400 USA

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Abstract. The intense demand for river water in arid regions is resulting in widespread
changes in riparian vegetation. We present a direct gradient method to predict the vegetation
change resulting from a proposed upstream dam or diversion. Our method begins with the
definition of vegetative cover types, based on a census of the existing vegetation in a set
of 1 x 2 m plots. A hydraulic model determines the discharge necessary to inundate each
plot. We use the hydrologic record, as defined by a flow duration curve, to determine the
inundation duration for each plot, This allows us to position cover types along a gradient
of inundation duration. A change in river management results in a new flow duration curve,
which is used to redistribute the cover types among the plots. Changes in vegetation are
expressed in terms of the area occupied by each cover type.
We applied this approach to riparian vegetation of the Black Canyon of the Gunnison
National Monument along the Gunnison River in Colorado. We used TWINS PAN to
cluster plots according to species occurrence. This analysis defined three vegetative cover
types that were distinct in terms of inundation duration. Quantitative changes in the extent
of cover types were estimated for three hypothetical flow regimes: two diversion alternatives
with different minimum flows and a moving average modification of historical flows. Our
results suggest that (I) it is possible to cause substantial changes in riparian vegetation
without changing mean annual flow, and (2) riparian vegetation is especially sensitive to
changes in minimum and maximum flows.
Principal advantages of this method are simplicity and reliance on relatively standard
elements of plant community ecology and hydrologic engineering, Limitations include use
of a single environmental gradient, restrictive assumptions about changes in channel ge-
ometry, representation of vegetation as quasi-equilibrium cover types, and the need for
model validation,

Key words: bottomland vegetation; dam; discharge; diversion; environmental impact;jlow duration;
Gunnison; hydraulic model; riparian vegetation; TWINSPAN; vegetation change.

4

INTRODUCTION

Throughout the arid and semi-arid regions of west-
ern North America, riparian ecosystems are a con-
spicuous feature ofthe landscape. Riparian ecosystems
are spatially and temporally dynamic and are shaped
by fluvial as well as upland geomorphic processes. In
spite of their limited areal extent in the West, these
systems provide critical physical and biological link-
ages between terrestrial and aquatic environments
(Gregory et al. 1991) and support many vertebrate spe-
cies (Brinson et al. 1981).
Western riparian ecosystems have long been influ-
enced by human activities; widespread modification
began in the mid to late 1800s with diversion of stream-
flow primarily for inigated agriculture (Wilkinson 1988,
Knopf and Scott 1990). Subsequent trans-basin diver-
sion projects and instream dams for water storage and
flood control further altered riparian ecosystems (John-
son et al. 1976, Bradley and Smith 1986, Rood and
Mahoney 1990). With increasing municipal, industrial,

j

, Manuscript received 30 July 1992; revised 15 July 1993;
accepted 16 July 1993.

and recreational demands for western water and grow-
ing recognition of the resource values associated with
riparian ecosystems (Lamb and Lord 1992), arid-re-
gion rivers will be subject to further conflicts, and man-
agers will require quantitative predictions of ecosystem
responses to altered stream discharge.
Several approaches have been used to relate stream
discharge to riparian vegetation at individual sites.
Stromberg and Patten (1990, 1991) used a site-specific
regression between discharge and cottonwood growth
to establish a flow standard for minimum flow. Bovee
et al. (1978) and O'Keeffe and Davies (1991) estimated
evapotranspiration from phreatophytic riparian vege-
tation in order to include this "consumptive use" of
water in a multiple-use evaluation of streamflow.
Stromberg (1993) related abundance of riparian vege-
tation to growing-season flow for a number of streams
within a large watershed. Johnson (1992) developed a
compartmental simulation model of changes in ripar-
ian cover types from water development along the Mis-
souri River. Pearlstine et al. (1985) used an individual-
based dynamic simulation to assess changes in woody
riparian vegetation associated with hydrologic modi-