WSP01181

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 10/1/1994
• Title: Experimental High Discharge Release From Glen Canyon Dam Through Grand Canyon NP - Draft Work Plan
• Water Supply Pro - Doc Type: EIS

.,,,", \

".

.~

determine the effects of high discharges on the ecological processes within backwaters (eddy
return channels), and to determine the effect of flooding on entrainment and drift of channel bed
and detrital organic matter. This organic matter, found in normal drift in small amounts, may be
a critical component of the food base in the heterotrophic downstream reaches of the Canyon.
Emphasis will be given to downstream transport (flux) and communition (recycling) of
Iimnetic particulate organic matter (POM) from Lake Powell and benthic POM from the
autotrophic Glen Canyon Darn - Paria segment in relation to loading of terrestrial POM by the
flood wave. Emphasis on lower trophic levels of the riverine food web is based on the relatively
low magnitude of the experimental flood relative to historic flows prior to closure of Glen
Canyon Darn.

Justification
Regulation of flow by Glen Canyon Dam has dramatically changed the aquatic food web
of the river downstream from the dam. Cold, clear and nutrient-laden water releases from Lake
Powell (Stanford and Ward 1991) stimulate production of benthic algae which arc fed upon by a
simple consumer assemblage, dominated by midge larvae and freshwater scud (Blinn et al
1992). Midges and scud are the primary forage for fishes in the river. Bioproduction is highest
in the river segment from the dam to the confluence of the Paria River, but biomass of algae and
invertebrates declines dramatically downstream because the food web is impacted by high
concentrations of suspended sediment that enters the river from the Paria River, Little Colorado
River and other tnbutaries. Suspended sediment, coupled with fluctuating flows for hydropower
production, prevent significant production of benthic algae through severe light limitation,
reduction of solid substrate, and constant dewatering of near-shore environments where
autotrophic production in unregulated rivers normally mRy;mi7P'S. As a consequence, the food
web of the river is far less productive downstream of the clear water segment (dam to Paria);
drift and benthos samples often contain few, if any, macroinvertebrates (Usher and Blinn 1990,
Blinn et aI. 1993). Indeed, everywhere downstream from the Paria River confluence, the
Colorado River may best be characterized as being much like a glacial river, cold, turbid and
unproductive.
As the Colorado River changes from an autotrophic system near the dam to a
heterotrophic system downstream the composition of organic-matter drift changes (Blinn et al.,
1994). The upper reaches are dominated by Cladophora and its epiphytes. The lower are
characterized by drift from tributaries and eddies, displaced stages of substrate-dependent
invertebrates and live and dead young stages of fish species.
The quantity and composition of drift may depend on discharge and associated river
stage. Studies during interim flows demonstrated that there was at least a two-fold increase in
drift during fluctuating discharges that attained high ebb stages of only 368mJ/s (13,000 cfs) as
compared to steady discharges of 227 m3/s (8,000 fills) (Blinn et al. 1993). High steady
discharges may be expected to produce considerably more drift, at least initially. Time of year
of high discharges may also affect the food base and its representation in drift because of
different life stages of various aquatic organisms. For example, larval forms of some chironomid
species may be common at a given time at one location, but later or elsewhere scarce following
downstream drift during pupation (Blinn et aI1993).

18