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

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PHASE 2, 1988
® PARK MOABI
® HAVASU DELTA
® DR3
® IMPERIAL DAM
JAN MAH MAY JUN AUG MEAN
Figure 11.-Concentration of total inorganic matter
above and below backwater complexes during phase
2 (Jan. to Aug. 1988).
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PHASE 2, 1988
® PARK MOABI
® HAVASU DELTA
® DR3
® IMPERIAL DAM
JAN MAR MAY JUN AUG MEAN
Figure 12.-Concentration of total POM above and below
backwater complexes during phase 2 (Jan. to Aug.
1988).
backwater complexes than above. For March and
June, just the opposite was true, more POM was
in the river below the backwater marshes. The most
enlightening data occurred during August 1988. On
this sampling date (figs. 11 and 12), the highest
values were recorded for both organic and inorganic
particulate matter, which were directly related to a
storm event that affected the stations below Palo
Verde Diversion Dam. Further review of the
particulate matter composition (from the Aug. 1988
sample) provides insight into just how backwaters
function in the lower Colorado River system. During
August 1988, concentration of total inorganic matter
above (DR3) and below (Imperial Dam) the Imperial
Division backwater complex was 60.05 and 58.54
g/m3, respectively (fig.11). These concentrations are
well above mean values recorded at these stations.
For the August 1988 sampling date, at DR3, total
inorganic matter (60.05 g/m3) comprised about 50
percent each of <25-µm size-fraction and >25-µm
size-fraction; below the Imperial Division backwater
complex (at Imperial Dam) total inorganic matter
(49.80 g/m3) was 85 percent of the <25-µm size-
fraction. The concentration of POM for the August
sample was similarly revealing (fig.12). Only 40
percent of the POM above the Imperial Division
backwater complex at DR3 comprised the <25-µm
size-fraction, whereas at Imperial Dam, 88 percent
of the POM comprised the <25-µm size-fraction.
From these observations, the inference is that either
backwater complexes functioned to filter out the
large particles (both organic and inorganic) or that
rainstorms flushed large amounts of fine materials
from the backwaters, or both. In either case, data
suggest that a backwater has the capacity to filter
and to provide a buffer for the river system during
storm events.
At DR3, chlorophyll a concentration was 2.94 mg/m3
following the August 1988 storm, while at Imperial
Dam chlorophyll a concentration was 4.20 mg/m3,
an increase of about 30 percent. It is doubtful the
algal community had responded so quickly to the
storm event by increasing production. It is more likely
the higher chlorophyll a reflects movement of
phytoplankton from backwaters into the main river
channel. Figure 13 shows that, for Topock Gorge
Division (Park Moabi versus Havasu Delta) and
Imperial Division (DR3 versus Imperial Dam), a
generally higher total chlorophyll a concentration
occurs at the downstream stations (below the
backwater complexes). However, during May 1988,
higher chlorophyll a concentrations occurred above
the backwater complexes at Park Moabi and DR3
than at Havasu Delta and Imperial Dam. Why this
occurred is not known, but the fact it did occur-
during the same time period for both the upstream
and downstream backwater complexes-suggests
4,5
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2.0
1.5
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PHASE 2, 1988
= PARK MOABI
® HAVASU DELTA
® DR-3
IMPERIAL DAM
JAN
r
MAR
Figure 13.-Concentration of chlorophyll a above and
below backwater complexes during phase 2 (Jan. to
Aug. 1988).
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MAY JUN AUG MEAN