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

August 1994

FLOW REGIME AND RIPARIAN VEGETATION

Water Surface Elevations

5

..-...
E

c
o
+=
~
>
Q)
w

3

2

o
o

20

40

547

Discharge

- --- 190 m3/s
"*' .- 28 m3/s
...... ___ 8.5 m3/s

Inundation Duration

1%
56%
89%

------
Channel

~

Upstream bar,
River right

60

80

100

Width (m)

FIG. 3. Selected water surface elevations at cross section 8. Water elevations are those predicted from the hydraulic model;
inundation durations ofthose discharges under the Reference hydrologic regime are also indicated. The location ofthis cross
section is depicted in Fig. 2.

from 151 to 339 plots/ha. The total area of the five
bars was 0.603 ha.
At each plot we determined the presence or absence
of all vascular plant species using nomenclature fol-
lowing the United States Department of Agriculture
(1982). We visually estimated total vegetative cover
for each plot on a scale of I to 4: 1 = 0-25%; 2 = 26-
50%; 3 = 51-75%, and 4 = 76-100%. Finally, we char-
acterized the dominant substrate as organic matter, silt,
sand, gravel, cobbles, or boulders. Two-way indicator
species analysis (TWINSP AN) (Hill 1979a) was ap-
plied to the species occurrence data to cluster the plots
into cover types. TWINSP AN was restricted to those
60 species that occurred in three or more plots. De-
trended Correspondence Analysis (DCA, Hill 1979b)
was applied to the same data set to illustrate the dis-
tribution of plots among cover types.

Hydrology

The model is based on the inundation duration of
points in the riparian zone. First, the inundating dis-
charge of a point was determined by comparing the
surveyed elevation of the point to the stage-discharge
relation. Then the inundation duration of the point
was determined by comparing the inundating discharge
to a flow duration curve.
Because the plots were scattered randomly across the
bars and not confined to cross sections, it was necessary
to construct a stage-discharge relation for each plot.
The National Park Service established a series of nine
hydraulic cross sections in the Monument, including
four within the study reach (Fig. 2). The Park Service
calibrated a hydraulic simulation model, HEC-2, (Hy-
drologic Engineering Center 1990, Hoggan 1989) to
predict water surface elevations at the cross sections.
The HEC-2 model was calibrated by five sets of field
observations at discharges from 9.5 to 44.9 m3/s and
used to construct stage-discharge relations for each cross

section. Fig. 3 illustrates calculated water surface ele-
vations for several discharges at one of the hydraulic
cross sections.
We used output from the water surface model to
develop stage-discharge relations at each sampled plot
for discharges of 0-283 m3/s at increments of 0.6 m3/s
(20 cfs). For each discharge, the water surface eleva-
tions at cross sections upstream and downstream of a
plot were read from the stage-discharge relations for
these cross sections. The water surface elevation at a
plot was then estimated by a linear interpolation based
on the location of the plot along an idealized channel
edge connecting the two cross sections.
Assuming a static channel geometry, the discharge
necessary to inundate a point remains constant across
different hydrologic regimes. However, each hydrolog-
ic regime has a different flow duration curve and thus
produces a different inundation duration for a given
point. The Reference hydrologic regime is the historical
record of mean daily discharges from 1971 to 1989
obtained from a compilation of United States Geolog-
ical Survey data (Earthlnfo 1991). We describe results
for three hydrologic alternatives: The Diversion alter-
native is a modification of the Reference regime in
which each daily discharge is divided in half, except
that discharge is not allowed to fall below 8.5 m3/s
(300 cfs). This minimum of 8.5 m3/s has been vol-
untarily maintained by water managers since the early
1980s to protect the trout fishery ofthe Gunnison River
(United States Department of the Interior 1990). In
1992, the Colorado Water Conservation Board ob-
tained a senior water right of 8.5 m3/s for in stream
flow maintenance; therefore, any future diversion would
probably have to respect the 8.5 m3/s minimum. The
Diversion-Increased-Minimum alternative is identical
to Diversion, except that the minimum flow is in-
creased to 17.0 m3/s. The Moving-Average alternative
is a 1-yr moving average of the Reference regime. The