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

inorganic particulate matter showed similar trends,
but this was expected because inorganic material
comprises the greatest portion of the total dry weight.
The total particulate organic matter showed a
different trend; on the average more POM was in
the All American Canal and less in the river below
the Imperial Dam. The >25-µm POM was signifi-
cantly lower at All-American Canal than at Imperial
Dam, which indicates that the majority of >25-µm
POM did not drift downstream into the All American
Canal but probably was broken down further into
a smaller size-fraction. The relative proportion of fine
and coarse material suggest that the desilting and
sluicing operations effect a stirring and breakup of
material, thereupon possibly loosening POM from
sediment or grinding POM into smaller fragments.
For both stations below Imperial Dam desilting/
sluicing works, a greater proportion of POM was in
the <25-µm size fraction.
Generally, phase 2 data did not show the same trend
as phase 1 regarding conditions downstream of
Imperial Dam. Where POM increased between
Imperial Dam and Yuma in phase 1, POM decreased
between these stations during phase 2. Phase 2 data
presented here suggest activities in and around
Imperial Dam, to some extent, account for the
decreased POM between Imperial Dam and Yuma,
but it is not possible to determine whether water
diversions, desilting works, or sluicing operations
functioned independently or together to effect these
changes.
Storm Events
Rainstorms, which fortuitously occurred simultane-
ously during on-site sampling, allowed usual
observations of changes in the river environment.
One such storm event occurred over a three-day
period during the August 1988 field sampling; it
affected stations from Havasu Delta downstream to
Yuma. (The two upstream stations were sampled
before the storm.) River water became diluted and
resulted in decreased specific conductance at
Laguna and Yuma. The highest turbidity levels, for
the entire sampling period, were recorded from
Cibola to Yuma immediately following this event.
Similarly, total phosphorus increased dramatically
after the storm, particularly at Imperial Dam where
concentrations reached 0.061 mg/L. Storms defi-
nitely had a direct effect on the increase in the
concentrations of POM. The <25-µm POM was
higher following the storm than that measured
during the June sampling at all the main channel
stations from Cibola to Yuma (table 47A). The <25-µm
POM increased to 5.26 g/m3 at Imperial Dam, which
was the high reading for the entire sampling period.
DISCUSSION
The object of this study was to determine the amount
of organic matter being transported through the
lower Colorado River system and to determine how
dams and structures influence transport. Certainly,
the lower Colorado River system is not a natural
river, but an understanding of how particulate
organic matter flows through a natural river system
helps one to put the recorded data into perspective.
Natural streams receive almost all their biotic energy
in the form of dead organic matter contributed to
the stream channel from outside sources (Petts,
1984 [3]). However, organic matter reflects both
allochthonous (external) sources leached, blown, or
washed into the channel and autochthonous
(internal) sources produced in the channel. Dissolved
organic matter (DOM) is normally more abundant
than POM in the water proper (Birge and Juday,1934
[24]; Fisher and Likens, 1973 [14]), although POM
is considered the main food supply for benthic
invertebrates (Wetzel and Rich, 1973 [25]; Goldman
and Kimmel, 1978 [26]) and provides a trophic
connection between microbial assemblages and
macroconsumers (Kondratieff and Simmons, 1985
[2]).
The River Continuum Concept, a principle introduced
by Vannote et al. (1980[1 ]), has attempted to describe
the various morphological and biological changes
that occur from upstream to downstream along a
natural river. Riverine communities can be separated
into three main groups: (1) headwaters (orders 1-
3), (2) middle-size streams (orders 4-6), and (3) large
rivers (orders >6). Headwater streams are heavily
influenced by riparian vegetation, which is respon-
sible for large scale inputs of allochthonous
nutrients, a major source of new nutrients to the
river system. As stream size increases, allochtho-
nous inputs become less important and autochtho-
nous processing of nutrients-transported from
upstream-becomes more important. This transport
is the basis for all subsequent living processes. The
first affect of this process involves the degradation
of the allochthonous material from coarse POM in
low order streams to progressively finer POM,
ultrafine POM, and eventually to: molecular com-
ponents, amino acids, sugars, etc. while proceeding
downstream.
Webster et al. (1979 [20]) developed a model river
system to evaluate POM transport. The authors
placed an impoundment in a river model and
observed the effects on POM dynamics. They
reported that the model did not reflect accurately
current knowledge of POM behavior in a river/
reservoir system because of:
• Seasonality of allochthonous inputs
• Autochthonous sources of POM
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