Six zones can be described for Turnagain Arm: (0) an outer, mostly nondepositional zone where the arm branches from Cook Inlet, (1) linear sandbars and wide (or large) tidal channels in the outer estuarine funnel, (2) laterally extensive silt and sand flats with narrowing tidal channels, (3) fluvio-tidal point bars in the fluvio-estuarine transition, (4) gravelly, braided to meandering fluvial channels and associated floodplains, and (5) glacio-lacustrine systems at the source of several rivers at drain into Turnagain Arm. Parts of the zonation (especially zones 3 and 4) can also be observed tangential to the main estuary in several marginal valleys that empty into the Arm, albeit at a significantly smaller scale because of the steep gradient along the margins of the estuary.
Except for embayments and river mouth, the outer portions of Cook Inlet are primarily erosional, with depositional facies developed in the inner reaches. In addition to the high tidal range, there is a high sediment supply brought in by glacial-meltwater rivers and reworked by tidal processes. Within the estuarine zone, very high localized rates of sedimentation occur on the tidal bars and flats as well as fluvio-tidal point bars. Previous work had suggested that the high rates of sedimentation were a result of quake-induced subsidence following a large earthquake in 1964. It can be documented, however, that very high short-time rates of sediment accumulation are still occurring within this estuary and are controlled purely by tidal processes.
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