3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
High turbidity and active vertical mixing within Knik Arm limit primary productivity;
however, allochthonous 50 carbon contributions (factored into the primary productivity of
streams and rivers) appear to provide sufficient energy inputs to Knik Arm to sustain growth
and development of juvenile salmon migrating through Knik Arm. For example, substantial
numbers of invertebrates have been found within the waters of Upper Cook Inlet and Knik
Arm (FHWA and ADOT&PF 1983a; Moulton 1997; Pentec 2005; KABATA 2005b) despite
decreasing euphotic zone depth and marine primary productivity with progression up Cook
Inlet (Larrance et al. 1977; Speckman et al. 2005). This suggests that an alternative source of
carbon input, other than from pelagic primary productivity, is available. Moreover, stomach
assays of juvenile salmon within mid-channel waters found that the majority of these fishes
had full bellies and were feeding primarily on terrestrial winged insects (FHWA and
ADOT&PF 1983; Moulton 1997; Pentec 2005), further stressing the potential importance of
allochthonous sources of energy for the subsistence of juvenile salmon within Knik Arm.
Eulachon (hooligan) likely move through Knik Arm in early spring to reach spawning
grounds within freshwater streams and rivers. While smelt habitat is not documented within
the Study Area or in any streams or rivers emptying into Knik Arm (ADF&G 2005b),
eulachon have been observed within Knik Arm waters. KABATA caught gravid eulachon in
April 2005 and post-spawn eulachon were subsequently caught in May. The lack of juvenile
eulachon captured in sampling efforts may suggest that they move quickly through Knik Arm
after hatching in mid-summer. No eulachon were observed during Knik Arm sampling
during July through November of 2004 or 2005 (KABATA 2005b).
The unconsolidated silt, sand, and cobble bottom areas of Knik Arm are expected to provide
suitable habitat for groundfish species such as Pacific cod, sculpin, and pollock. Prey species
such as epibenthic crustaceans and pelagic small fish are available within Knik Arm.
However, recent trawl and beach seine sampling within Knik Arm produced only three
Pacific staghorn sculpin (Leptocottus armatus), three walleye pollock, and no Pacific cod
(KABATA 2005b).
The Knik Arm portion of the Study Area includes one anadromous stream, Sixmile Creek
(247-50-10090) (Figure 3.56). During low tide, Sixmile Creek flows within a confined
intertidal channel within Knik Arm. The lower section of the creek and Sixmile Lake
includes spawning and rearing habitat for Dolly Varden char and sockeye and coho salmon,
and spawning habitat for pink salmon (ADF&G 2005b). However, because the intertidal
channel is very shallow (less than 6 inches), it is unlikely that migrating fish use it during low
tide. Once the tide is high and the intertidal channel is indistinguishable from the rest of Knik
Arm, fish likely move into Sixmile Creek. 51
50 Allochthonous refers to something that originates outside of or is not originally produced within a system. In
this instance allochthonous carbon sources include things such as organic detritus and vegetative mats that are
transported into the estuarine system by the rivers and streams that discharge into Knik Arm.
51 Personal communication, Jim Glaspell, Environmental and Planning Group Manager, URS Corporation,
e-mail message to Robin Reich, November 27, 2005
12/18/07 3-199