Cook Inlet physical habitat

Cook Inlet basic map
Figure 2. Major streams and rivers flowing into Cook Inlet. Source: Map created by NMFS 2013.

Cook Inlet is a semi-enclosed tidal estuary located in southcentral Alaska (Figure 2). The Inlet is approximately 370 kilometers (km) (230 miles [mi]) in length and extends in a northeast/southwest orientation from Knik and Turnagain Arms in the north to the southernmost reaches of Kamishak Bay in the south (Figure 2). Cook Inlet covers 20,000 km2 (12,427 mi2) and has 1,350 km (839 mi) of coastline (Rugh et al. 2000). The Cook Inlet watershed includes approximately 98,000 km2 (60,894 mi2). The Susitna River occupies the largest drainage basin (50,800 km2, 31,566 mi2), followed by the Matanuska (5,670 km2, 3,523mi2), Knik, Chakachatna, and Kenai rivers (each exceeding 2,500 km2, 1,553 mi2).

The bathymetry of Cook Inlet is varied and consists of shoals, canyons, and mudflats (Figure 3). Cook Inlet is generally shallow, with most waters less than 73 meters (m) (240 feet [ft]) deep. However deeper waters exist along the channels and at the entrance to the Inlet near the Barren Islands, where depths range from 183–366 m (600–1200 ft; Mulherin et al. 2001). During low tides, large areas along the shoreline are exposed as mudflats in Knik Arm, Turnagain Arm, Chickaloon Bay, Redoubt Bay, Trading Bay, Kachemak Bay, and the Susitna River Delta. In other areas of Cook Inlet, bottom sediments consist of cobbles, pebbles, sand, and clay, with occasional patches of boulders or coal seams. Areas with stronger currents associated with constrictions in Cook Inlet’s width tend to have coarser bottom sediments.

Cook Inlet from MODIS
Figure 3. Glacial input into Cook Inlet. Source: MODIS true color image, acquired 2 September 2002; Okkonen 2005. Return

The physical oceanography of Cook Inlet is characterized by a net inflow along the eastern boundary and a net outflow along the western boundary (Burbank 1977). A major inflow is the Alaska Coastal Current, a current driven by wind and water densities that flows along the southern coast of Alaska and passes through Kennedy Entrance (Figure 4). Upon entering lower Cook Inlet, the Alaska Coastal Current turns west just north of Anchor Point, mixing with western boundary outflow (Burbank 1977, Muench et al. 1978). A significant component of the water along the western boundary originates from Turnagain and Knik Arms, the Susitna River, and numerous other glacial streams. In the lower Inlet, this outflow is typically more turbid than the water further east due to the heavy glacial runoff from these drainages (Figure 3). These sources deposit considerable sediment into Cook Inlet, creating a highly turbid, low visibility environment, particularly in the northern portion of the Inlet. Seasonal stream discharges and sediment transports typically peak in July to August. In the upper Inlet, summer surface temperatures are about 10 degrees Celsius (oC) (50 degrees Fahrenheit [oF]) with salinities less than 20 parts per thousand (ppt). During summer in the lower Inlet, a relatively warm (10oC, 50oF), low salinity (less than 29 ppt), surface layer forms along the west side and a cooler (9oC, 48oF), higher salinity (greater than 30 ppt) layer forms along the east side.

Cook Inlet bathymetry
Figure 4. Cook Inlet bathymetry and locations of major tide rips. Source: Burbank 1977. Return

Cook Inlet experiences some of the greatest tidal fluctuations in the world (Mulherin et al. 2001). The difference between high and low tide levels may reach 12 m (39 ft). These large tidal ranges, combined with broad tidal flats, can result in currents reaching 6.2 meters per second (20.3 feet per second), sometimes causing significant changes to shorelines (Moore et al. 2000). Three distinct convergence zones, known as tide rips, have been identified in the Inlet (Figure 4). The east rip is typically located 2–3 km (1.2–1.9 mi.) offshore of the eastern shore. The west and mid-channel rips are located just east of Kalgin Island, and are associated with a 50–80 m (164–262 ft) deep channel running north to south (Figure 4).

In winter, ice covers much of upper Cook Inlet. Rivers begin to freeze in October and November and waters of upper Cook Inlet contain persistent ice by December. Large amounts of freshwater entering Knik and Turnagain Arms contribute to relatively high ice concentrations in the upper Inlet. South of the Forelands, small floes of open pack ice are typical. Maximum ice extent is typically reached in late January. Inlet circulation and winter winds tend to move the ice south down the west side of the Inlet. Ice breakup in the Inlet typically begins between March and May.

The physical environment of Cook Inlet is shifting towards increasingly long ice-free seasons as Alaska undergoes climate change. Alaska has experienced the greatest warming of any region in the United States (U.S.) (Karl et al. 2009) and Cook Inlet’s reduction in duration of seasonal sea ice is consistent with other portions of the state. Alaska’s regional warming is part of a larger Arctic-wide warming trend (ACIA 2004; IPCC 2013) that is projected to increase over time.