3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
While the Knik Arm channel is narrowest at Cairn Point, the channel is also deepest at this
point, with depths reaching 180 feet. This bathymetric depression, hereinafter referred to as
the Cairn Point Trench, essentially defines the axis of Knik Arm through its extensions to the
northeast and southwest.
The Cairn Point Trench is remarkable in that its planform and bathymetric relief have
remained essentially constant for more than 60 years; this is particularly remarkable when
compared with other seabed features of Knik Arm (KABATA 2006c). From this, it is
surmised that the Trench is not likely the result of or even significantly affected by modern
hydrologic processes. Rather, the Trench is more likely a relict of prehistorical scour events
such as the breaching of an ice dam during the period of the Naptowne glacial retreat.
The typical cross section for Knik Arm consists of an entrenched inner passageway, with
shelves of shallow water on both sides. In the northernmost part of the Arm, the shelves
become shoals, which become tidal flats closer to the mouths of the Knik and Matanuska
Rivers. The shelves are usually less than 20 feet below the mean lower low tide water line
(MLLW). North of the Cairn Point Trench, the maximum depth of the thalweg in the inner
trench is about -80 feet with respect to MLLW.
The location of the proposed KAC bridge would be within a mile of the northern limit of the
Cairn Point Trench. Shallow water in this area forms wing-shaped shelves on both sides of
Knik Arm, which at its deepest, reaches to roughly 20 feet below MLLW. The trench in the
center of Knik Arm in this area is typically around 65 feet below MLLW at its deepest.
Tidal flats in the upper part of Knik Arm are defined as the surface within the banks at an
elevation between MLLW and mean high water (MHW). 27 Nonvegetated, these tidal flats
appear mostly in the upper end of Knik Arm, starting at Eagle River and becoming more
predominant further north, as one approaches the mouths of the Knik and Matanuska Rivers.
Below MLLW, such features are characterized as shoals, while above MHW, they are called
“lowlands,” or raised tidal flats, as can be seen between the Knik and Matanuska River
bridges and Palmer on the Glenn Highway.
The dominant climate for Knik Arm, as for all of Southcentral Alaska, is classified as
“maritime.” Summers and winters are milder than what is normally seen in continental
climates of similar latitude, with average temperatures ranging from 64.8°F in July to 19.2°F
in January (Table 3-12). In addition to mild temperatures, the maritime climate of Alaska is
characterized by heightened precipitation and persistent winds.
27 Mean high water: a tidal datum. The average of all high water heights (i.e., the maximum height reached by a
rising tide) observed over the National Tidal Datum Epoch (Tide and Current Glossary, NOAA,
January 2000).
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