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1982.
M.Sc., 214 pp.
THE GEOMORPHOLOGY AND SEDIMENTOLOGY OF THE LOWER REACHES OF THE
ATTAWAPISKAT RIVER, JAMES BAY, ONTARIO
KING,
W.A.
 The
modern environments, processes and sediments of the lower reaches
of the Attawapiskat River have been analysed to elucidate the
initiation, maintenance and disruption the river's anastomosing channel
pattern. The relatively young (8000 years) subarctic river is entrenching into
the emerging Hudson Bay Lowland. The stable multi-channel character is
initiated in the intertidal and shallow subtidal zones. Several embryonic
channels are formed by divergence
of the river plume around highs in the substratum and around local modern
coarse grained deposits ice-rafting. These channels are subsequently deepened
by river flow and tidal currents. As the sea regresses, the channel banks are
preferentially colonized and stabilized by vegetation. They undergo very little
lateral migration and no new secondary channels are formed. The river has a
subarctic niva1 hydrologic regime. The lower reaches are influenced by mixed,
semi-diurnal tides (2.36 m spring tide), which penetrate up to 7.5 km inland
along the main channel. Side channels are flood tide dominated with brackish
(13.2 %) water penetrating up to 20 km inland. The river carries very little
bedload but a sizable (94 mg/l) suspended sediment load, most of which is
flushed into James Bay. Alluvial deposits are present as thin (2-4 m), narrow
(150 m) levees; as occasional sandy bars which form in sheltered areas of
islands and shoals; and as thin (5 cm) silty veneers on the floors of
secondary channels. Near the
coast, side channels are kept open by scouring during floods and by tidal
currents. However, farther inland (30 km) the more rapidly en- trenching main
channel captures their flow, and the river becomes an irregularly meandering
stream.
Pleistocene tills overlain by a
Holocene regressive sequence are ex- posed in the riverbanks. The regressive
sequence is composed of a lower subtidal, fossiliferous clayey-silt unit
overlain sharply by tidal flat deposits of extremely variable lithology and
texture. The intertidal deposits are overlain gradationally by silty marsh
deposits with thin (0.l cm)
discontinuous organic-rich laminae plus lenticular sandy storm beds. The marsh
grades upward into levee deposits characterized by
thicker (0.2 cm) and more regular organic-rich laminae alternating with
thin (1.5 cm) sandy silt beds. In rare instances, the till substratum is
overlain by a totally alluvial sequence (2.5 m thick) characterized by ripple
marked sandy bar deposits. These sediments grade upward into levee sediments
of vegetated islands. Ice-rafted sediments commonly occur in all of the
sedimentary environments. Other
anastomosing rivers reported in the literature reveal basic differences from
the Attawapiskat River, such as crevassing and thick alluvial deposits.
However, strong similarities exist which may be useful in interpreting
deposits of ancient anastomosing streams. These similarities include high and
abrupt lateral variability in texture, a predominance of overbank
sedimentation, limited or negligible point bar deposits, and a lack of
numerous in-channel bars.
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