SPECIAL SUMMMARY
The South Platte Coalition for Urban River Evaluation (SPCURE) which includes Centennial Water and
Sanitation District (CWSD), Littleton/Englewood Wastewater Treatment Plant (L/E WWTP), and Xcel
Energy (Xcel a.k.a. Public Service Company of Colorado) in 2007 commissioned a bioassessment study of
the South Platte River (SPR) as it flows through the urban environment of Denver, Colorado (technically
called Segment 14 by the Colorado Water Quality Commission). The purpose of the bioassessment was
to provide SPCURE with an evaluation of the existing biological integrity of the fish and
macroinvertebrate communities in SPR Segment 14 and develop a database over the 2007-2012 period
of study that can be used to support establishment of appropriate water quality standards.
The bioassessment study consisted of sampling of the fish and benthic macroinvertebrate populations
(primarily insects, worms, crustaceans, and molluscs) with the study beginning in the fall of 2007 and
ending, after six years, in the fall of 2012. The study area included 13 sampling sites on the mainstem of
the SPR from just below Chatfield Reservoir to approximately 0.5 mi. downstream from I-70 for a total
distance of just over 19 miles. The sampling sites were chosen to represent reaches upstream and
downstream from key effluent dischargers (CWSD, L/E WWTP, and Xcel’s Arapahoe and Zuni Power
Plants), major tributaries (Bear Creek and Cherry Creek), and significant stormwater outfalls. The study
was divided into a primary program and a secondary program with the primary program consisting of
the fall sampling of fish at eight sites and benthic macroinvertebrates at all 13 sampling sites. The
secondary program conducted in spring, on the other hand, included sampling of only
macroinvertebrates at 6 of the 13 primary sites. Where appropriate for data comparisons, the study
area was divided into segments located upstream (six sites) and downstream (seven sites) of L/E WWTP
and Arapahoe Plant discharges, hereafter referred to as the upstream sites and downstream sites.
The bioassessment was conducted by Aquatics Associates, Inc. (AAI) from Fort Collins, Colorado. All
components for this 6-year study were completed as contracted and included: 1) five fall surveys of fish
populations at eight sites, 2) whole body concentration analyses for mercury and selenium for selected
fish species collected during the five fall fish surveys; 3) benthic macroinvertebrate collections for six fall
and five spring seasons, as well as three collections using rock basket artificial substrates at the same six
spring sites (the rock basket artificial substrates program was terminated after three years due to
technical complications). Qualitative observations of habitat conditions were also made concurrent with
each sampling event. AAI also analyzed all the fish and macroinvertebrate data results and produced
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two interim (May 2010 and May 2012) and a comprehensive final report for the 2007-2012 study period
(October 2013).
One of the major goals for the macroinvertebrate study was to develop a means to compare the current
and future data to a reference “condition.” Since there are essentially no unimpacted streams in the
South Platte drainage, a simulated, best attainable or Idealized Reference Site condition (IRC) measure
(metric) was developed for Segment 14. This method included the use of eight selected metrics
commonly used to evaluate the health of macroinvertebrate communities. These metrics were also part
of the routine analyses for the macroinvertebrates collected during the SPCURE study. Over the study
period, the highest or best score for each metric at each site was used to develop a maximum
achievable overall score for the study reach. This value was given a score of 100 against which each site
for each year was sampled was be compared. A statistical analysis comparing the upstream sites to the
downstream sites indicated that upstream sites were significantly different (better) than the
downstream sites. The IRC also indicated that while most sites were only moderately impacted, there
was still room for improvement.
The fish community included a total of 25 species with 17 to 22 species collected in any given year. Of
the species collected, 12 were natives with white suckers by far the most abundant followed by
longnose dace, fathead minnows, and creek chubs. Total numbers of fish collected from all sites ranged
from 6,323 individuals in 2010 to 15,137 in 2008. The low number of fish collected in 2010 was likely
due to displacement and loss of habitat caused by the high flows during the late-summer storm events
in 2010, especially at lowest site near I-70 where high flows washed out of large snag and filled in pools
and deep holes. Few fish were also collected in 2012 because of extremely low flow conditions in this
extreme drought year. Non-native (introduced) species were also collected and occasionally abundant
such as common carp, smallmouth and largemouth bass, and rainbow and brown trout. At the most
upstream site, just below Chatfield Reservoir, fish numbers were abnormally high in 2008, 2009, 2011,
and 2012 due to high numbers of young white suckers and were exaggerated in 2010 by high numbers
of recently stocked largemouth bass and brown trout. Otherwise, total numbers of fish collected were
relatively similar every year with the fewest fish usually collected at the first site downstream of L/E
WWTP. No State or federally listed T&E species were collected; however the Iowa darter, a Colorado
Parks and Wildlife (CPW) species of special concern, was collected at the upper three study sites.
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The mercury and selenium concentrations in the whole body tissues were low and of little concern
either for fish consumption (mercury) or as a threat to the health of the fish population (selenium).
While some selenium levels were greater than EPA and various literature criteria, none of the fish
collected showed signs of selenium toxicity.
The macroinvertebrate community was found to be reasonably healthy and stable from year to year
considering the somewhat adverse habitat (shifting silt/sand substrates, widely variable flows) and
effluent stressors in the system. Typical of macroinvertebrate populations, densities varied widely over
the 6-year study period ranging from 1,035 to 31,065 individuals per square meter (N/m2) in the fall
compared to 1,791 to 37,426/m2 in the spring. The fall community was comprised mainly of mayfly,
caddisfly, and dipteran (mostly midges) species that are moderately tolerant of environmental stressors
with tolerant oligochaetes mainly tubificids, and midges appearing in somewhat elevated numbers at a
few sites downstream the effluent discharges of L/E WWTP and Xcel’s Arapahoe Plant. Spring collections
suggested that the river is more stressed over the winter as the pollution-tolerant naidid worms
(oligochaetes) were generally predominant at all sites and often in overwhelmingly high numbers.
Oligochaetes were the dominant group in the spring in most years except 2009 and 2011 when midges
were dominant. Mayflies and caddisflies were also collected but in fewer numbers compared to the fall.
In summary, the aquatic life in urban SPR Segment 14 reflects the effect of multiple stressors including
channelization, fluctuating flows that are primarily due to releases from Chatfield Reservoir, inflows of
effluent from several industrial and municipal WWTP sources, as well as discharges from numerous
storm drains and tributary inflows. Nonetheless, this segment of the river supports reasonably healthy
fish and macroinvertebrate communities.
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