CLIMATIC CONTROL OF SEDIMENT PRODUCTION, STORAGE, AND ROUTING IN THE CHUGACH MOUNTAINS, SOUTHEAST ALASKA

The temporal scale of sediment delivery from glaciated catchments depends on the rates and types of erosion and transport processes, the magnitude and frequency of climate change, and the configuration of drainage basins within a mountain range. Granite Creek (GC) valley, a 600 km² drainage basin on the northern flank of the Chugach Mountains, illustrates the control by climate change and basin topology on sediment delivery from this basin during the Holocene. Advance and retreat of the Tana Glacier (TG) modulates the accumulation and delivery of sediment from the GC valley by damming the outlet of GC during glacial maxima, and releasing stored sediment during glacial minima. Glacial moraines and lake shorelines were used to reconstruct the number of glacial advances and lake-filling events from the mid-Holocene to the present. Lichenometric ages of glacial moraines, combined with optically-stimulated luminescence (OSL) and ¹⁴C ages from within the valley allow estimation of the temporal scale of sediment storage and evacuation in the basin.

Lichen thalli found on TG lateral moraines blocking the outlet of GC indicate initial colonization ~300 yr BP, probably associated with latest Little Ice Age (LIA) cooling. Shorelines associated with the lake resulting from this blockage occur up to 60 m above these moraines. Ice thickness, not moraine height, controls the reservoir potential of the basin. More than 20 m of fluvial-lacustrine sediment is present in the lower 8 km of GC basin, representing at least 2.0 × 10⁸ m³ of stored sediment. Given that shorelines are 110 m above the valley fill surface, lake volume at the maximum lake level was 1.0 × 10⁹ m³ . Preliminary OSL ages of the fill suggest the age of deposition could be as old as 5 ka BP. If all the stored sediment was produced and deposited during the LIA advance, this age represents the deposition of partially bleached sediment. Alternatively, if the mid-Holocene OSL ages are simply burial ages, incomplete evacuation of sediment from the basin is implied, and valley fill represents sediment stored in the basin over multiple Holocene glacial cycles. In this case, delivery from the basin of sediment produced by glacial, fluvial and hillslope processes was lower during the Holocene than at present, and sediment production and delivery to basins may be out of phase on 10³-10⁴ timescales.