WSP12518

• File Number: 8282.400
• Description: Colorado River Operations and Accounting - Deliveries to Mexico
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 9/1/2001
• Author: Various
• Title: The Journal of Arid Environments - Special Issue-The Lower Colorado River Basin and Delta - Volume 49-Number 1 - September 2001 - RE-Colorado River-Mexican Delta Issues - 09-01-01
• Water Supply Pro - Doc Type: Report/Study

26

J. C.STROMBERG

to fully respond to daily power demands. In the long-term, efforts are needed to
shift to more sustainable energy sources such as solar power and thereby eliminate the
need for peaking power from hydro-power dams.

Physical barriers

Dikes, levees, and other structures separate channels from flood plains on many streams.
To remedy this situation on California's Cosumnes River, the Nature Conservancy has
implemented a large-scale restoration project that entails breaching levees and allowing
flood waters free access to abandoned farm land (Reiner, 2000). Potential for success
was demonstrated by a natural breaching of the levee about 10 years ago, which was
followed by development of a diverse riparian forest. Restoration efforts also have
been made in urban areas to reconnect rivers and flood plains (Riley, 1998; de Waal
et al., 1998). There are opportunities to remove physical barriers and restore some
degree of channel-flood plain connectivity along many desert streams, including the
Colorado and Rio Grande. By allowing periodic flows onto flood plains, one provides
the water, nutrients, sediments, and plant propagules to restore and sustain productive,
diverse riparian forests (Molles et al., 1998). Such efforts may require purchase of
agricultural land for conservation purposes, or restrictions on home development in
flood plains. Benefits accrue, however, in the form of downstream flood reduction (as
the flood waters spread laterally across the flood plain), aquifer recharge, and biodiver-
sity increases.

High concentrations of dissolved solids

Primary causes of the increased salinity of Colorado River water, as for many other
streams, are use of flood plain lands for irrigated agriculture and storage of water in
shallow reservoirs (Briggs & Cornelius, 1998). Water chemistry can be improved to
levels that support a wide range of riparian plant species, including fresh-water depen-
dent Populus and Salix, by reducing amounts of irrigated flood plain lands, increasing
efficiency of irrigation systems, growing low-water-use crops, and/or reducing use
of agricultural fertilizers. Reducing reliance on reservoirs for water storage will result in
less evaporation of water and thus lower salinity levels. Released of flood pulses from
dams can flush salts that have accumulated in flood plain soils.

Flood flow alteration

On many streams in western United States and Canada, recruitment rates of Populus
and Salix species have declined because floods have been suppressed or altered in ways
that do not meet their regeneration requirements. To remedy this situation in Alberta,
Canada, scientists and Environmental Advisory Committees have provided information
that has led to changed operation of some dams (Rood el al., 1995, 1998; Mahoney
& Rood, 1998). St. Mary and Oldman Rivers are managed for delivery of summer
irrigation water and still flood fairly regularly in wet years, Rates of river meandering
and channel realignment are relatively unaltered, and so 'are the processes that create
'nursery bars' for establishment of Populus seedlings. Changes have been made, how-
ever, such that flood waters now recede slowly enough to allow for high survival of the
seedlings. Ecological models call for the stream stage to drop less than four cm per day,
allowing seedling roots to maintain contact with moist soil: Another part of the agree-
ment calls for an increase in summer base flow levels, thereby reducing risk of tree death
from drought,

RESTORATION OF RIPARIAN VEGETATION OF FLOW REGIME

27

. Such operating agreements could be put in place for the many flow-regulated streams
10 the south-western United States, including the Verde, Gila, and Colorado Rivers.
Large flows are released ~rom wa~er-storag.e dams on these streams during occasional
wet ye~rs, but the water !S not released so as to optimize its environmental benefits.
Sometunes, relea~es fortu1tously meet the reg~neration needs of riparian plants. EI Nino
weather patterns 10 1978, 1.983 and 1993 vanously allowed for establishment of Populus
and Salzx trees along portions of the lower Gila and Colorado; reservoirs were filled
to . levels that n~cess1tated large releases during winter and spring (Rea, 1983;
~nggs & Cornelius, 1998; Zamora-Arroyo el aI., 2001). With operating agreements
In plac~, ~am managers could be prepared to release flows during wet years in ways
that muruc natural hydrographs and favor establishment of native species adapted to
natural flow patterns. To keep the trees alive, maintenance flows would have to be
secured.
Often, there ~re ~o~strain~s o~ the degree to which natural flood regimes can be
restore? The ~~I ~dliams R1ver 1~ western Arizona is regulated by Alamo Dam, which
was built tonumnuz~ flood pulses mto the Colorado River. Over the past 25 years, size
and frequency of wmter and summer flood peaks in the Bill Williams River have
de.cre~sed. The U.S. Fish and Wildlife Service, Army Corps of Engineers, and university
sC1e';ltlsts have worked together to develop a flow-release plan. The plan calls for release
of high base flow~ a~d periodic fl<?od flows t~ UJ:t~rove the quality of riparian habitat in
th.e b~low-dam w~ldlife refuge, whde also maIntaInIng flood control and recreational and
wildlife benefits 10 Alamo Lake. However, there are physical constraints and design
features that res~ct the size of fl~od pulses that can be released from the dam (Shafroth
~I al., ~001). ~lthO~t large ~counng floods, dense post-dam vegetation (much of which
IS exotlc Ta~aTlx) w1ll ~emaln ~e-I:'rone. Another anticipated consequence of the lack of
larg~ SCOUrIng floods 1S a decline 10 rates of establishment of pioneering Populus and
Sal~. These rates are predicted to decline despite the release of appropriately timed
spn?g flow~ (~hafroth et al., 1998). Solutions being pursued at the Bill Wiilliim1s
Nanonal Wildlife Refuge are intervention in the form of small-sc81e clearing of Tamizrix
to be followed by Populus-Salix regeneration flows. '
. ~~ons that mimic. fluvial processes, such as flood plain ~learing followed by flood
IITlgatlon, also are bemg undertaken at other sites where natural processes cannot be
fully restored (Friedman el aI., 1995). For example, at Bosque del Apache Wildlife
Refuge" as on much of New Mexico's highly regulated Rio Grande Tamarix has
become dominant. Lowered water tables, increased river salinity, and lack of win-
ter/sI:'ring floods have all contrib~ted t~ decline of Populus forests, while past flood plain
c1eanng and at least one appropnately timed summer flood allowed for influx of T amarix
(Everitt, 1998). To restore native forests, Refuge managers have mimicked the ef-
fects of large floods by using bulldozers, herbicides, and fire to clear extensive stands of
Tamarix at a cost of$750-1300 ha-I (Taylor & McDaniel, 1998; Taylor et aJ.., 1999).
T?e~ then released water onto the bare flood plains in spring with a seasonal timing that
mlnucked the natural flood hydrograph of the Rio Grande. This allowed for establish-
ment of a diverse assemblage of plant species. Flood pulsing followed by seeding of
native species also can be undertaken on agricultural fields, as has been done at places
along the lower Colorado River. Restoration of agricultural fields ultimately may prove
more cost-effective than restoration of Tamarix lands. Although land costs may be
high, water delivery structures already are in place and vegetation clearing costs are
negligible.

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Loss of sediment

Dams and reservoirs impede downstream flows of sediment. Reduced transport of silt
and clay may reduce biodiversity below dam~, given that abundance of herbaceous