8132

• UCREFRP Catalog Number: 8132
• Author: Lieberman, D. and T. Burke.
• Title: Limnology and Drift of Particulate Organic Matter Through the Lower Colorado River.
• USFW Year: 1991.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

INTRODUCTION
Particulate organic matter (POM) supports the base
of the food web in many aquatic systems and is one
of many factors that determines the nature of biotic
communities in a riverine ecosystem (Vannote et al.,
1980 [1 ]).' The POM consists of both living and dead
material, such as plankton in impoundments or small
plant fragments, brought into the river from local
agricultural fields. The POM provides a trophic
connection between microbial assemblages and
macroconsumers (Kondratieff and Simmons, 1985
[21). Impoundments created by dams on the main-
stem of rivers trap sediment and organic material
drifting into them (Petts, 1984 [31).
The lower Colorado River from Laughlin, Nevada to
the international border (between the United States
and Mexico) has six mainstem dams and is one of
the most regulated river systems in North America.
Impoundments like Lake Mohave and Lake Havasu
(on the Colorado River) block downstream sediment
transport (Lara and Sanders, 1970 [4]; Prentki et al.,
1981 [5]). While some investigators tend to stress
the negative aspects of such alterations in free-
flowing rivers (Ward and Stanford, 1979 [6]; Baxter,
1977 [7]; Krenkel et al., 1979 [81), other researchers
report that sediment trapping by upstream dams has
allowed for an increase of fishes, albeit exotics, to
inhabit the lower Colorado River (Dill, 1944 [9];
Minckley, 1979 [10]; Burke, 1986 [11]). Burke
reported that sediment load reduction near Parker
Dam resulted in:
• Increased diversity of substrates for colonization
by invertebrates
• Decreased turbidity with corresponding
increased light penetration and phytoplankton
production
• A general reduction in the deleterious effects of
suspended sediments upon aquatic organisms and
their eggs
Minckley (1979 [10]; 1982 [121) reported that the
lower Colorado River system had a detritus-based
trophic economy. He described the food web as being
based upon autochthonous material. Minckley
credited the movement of plankton and seston from
reservoirs as an important source of this material.
The purpose of this investigation was to conduct a
detailed analysis of the lower Colorado River to
determine the amount of organic matter being trans-
ported through the system, and to determine how
dams and structures affect the transport of organic
material. The major question was: "If the Colorado
1 Numbers in brackets refer to the bibliography.
River dams were truly blocking organic transport
through the lower river system, why is there still
a detrital-based trophic economy, and why are there
more detritivorous fishes in the lower river reaches?"
It appears, from a fishery standpoint, the river
functions as a free-flowing river continuum despite
the six mainstem dams.
The two objectives of this study were: (1) in phase
1 to determine the quantity, composition, and
distribution of the particulate organic matter in the
river; and (2) in phase 2, to determine the origin
or sources of the POM. The information is required
to better understand how reservoirs, backwaters,
and irrigation diversions and return flows impact the
overall trophic economy of the lower Colorado River.
Management Implications
Drift of POM (particulate organic matter) through the
lower Colorado River is examined in detail to
investigate the distribution and source of this organic
matter. The lower Colorado River is a regulated river
system with reservoirs, diversion dams, riprapped
banks, and agricultural drains. Study results have
shown that despite these constructed civil structures
and alterations to the natural environment, POM
concentrations increased from upstream to down-
stream-not unlike an unregulated river. Fishery
managers and project planners who are involved in
channeling, riprapping, or otherwise altering a river
channel may use this study to better understand the
distribution and source of POM that supports
detritivorous fishes in an impounded river.
METHODS
Study Area
As shown on figure 1, the lower Colorado. River
meanders for 410 kilometers through the Mohave
Desert of Nevada, Arizona, and California, and
extends into the most arid portion of the Sonoran
Desert. The area receives less than 120 millimeters
of rainfall per year; during summer, air temperatures
often exceed 40 °C. Much of the sparse rainfall
comes in heavy rainstorms; erosion is evident in the
poor quality soil. Eventually, floodplain deposits of
fine sand and silt reach the river by way of desert
washes. Natural vegetation is greatly modified by
agricultural practices and stream bank alterations.
The Colorado River has only two tributaries: the Bill
Williams River (near Lake Havasu City, Ariz.) and
the Gila River (near Yuma, Ariz.). Six major dams
are within the study area.
1. Davis Dam formed Lake Mohave and began
storage in January 1950; it is the uppermost dam