Fluvial Suspended Sediment Transfer and Lacustrine Sedimentation of
Recent Flood Turbidites in Proglacial Eklutna Lake, Western Chugach
Mountains, Alaska
Abstract
Suspended sediment delivery and deposition in proglacial lakes is
generally sensitive to a wide range of hydrometeorologic and geomorphic
controls. High discharge conditions are of particular importance in many
glaciolacustrine records, with individual floods potentially recorded as
distinctive turbidites. We used an extensive network of surface sediment
cores and hydroclimatic monitoring data to analyze recent flood
turbidites and associated sediment transfer controls over instrumental
periods at Eklutna Lake, western Chugach Mountains, Alaska. Close to a
decade of fluvial data from primary catchment tributaries show a
dominating influence of discharge on sediment delivery, with various
interconnections with other related hydroclimatic controls. Multivariate
fluvial models highlight and help quantify some complexities in sediment
transfer, including intra-annual variations, meteorological controls,
and the influence of subcatchment glacierization. Sediments deposited in
Eklutna Lake during the last half century are discontinuously varved and
contain multiple distinctive turbidites. Over a 30-year period of
stratigraphic calibration, we correlate the four thickest flood
turbidites (1989, 1995, 2006, 2012) to specific regional storms. The
studied turbidites correlate with late-summer and early-autumn
rainstorms with a magnitude of relatively instantaneous sedimentation 3
to 15 times greater than annual background accumulation. Our network of
sediment core data captured the broad extent and sediment variability
among the study turbidites and background sediment yield. Within-lake
spatial modelling of deposition quantifies variable rates of downlake
thinning and sediment focusing effects, and highlights especially large
differences between the thickest flood turbidites and background
sedimentation. This we primarily relate to strongly contrasting
dispersion processes controlled by inflow current strength and
turbidity. Sediment delivery is of interest for this catchment because
of reservoir and water supply operations. Furthermore, although smaller
floods may not be consistently represented, the lake likely contains a
valuable proxy record of regional flooding proximal to major population
centers of south-central Alaska including Anchorage.