CLIMATE, SEDIMENT SUPPLY, AND STREAM POWER: EPISODES OF ENHANCED DEPOSITION ON ALLUVIAL FANS OF THE LOST RIVER RANGE, IDAHO

Recent advances in the application of luminescence dating to fluvial sediments have improved the ability to determine the timing of deposition on alluvial fans. This has provided an opportunity to clarify insights about the relationships among climate, climatically controlled factors (e.g precipitation, vegetation density, temperature), and hydrologic and geomorphic responses (e.g. weathering rates, frost action, glaciation, stream power). To investigate these relationships in the Northern Rockies, optically stimulated luminescence (OSL) has been used to determine the timing of deposition on the ~2-5 km radius fans of the Lost River Range (LRR), Idaho.

Preliminary ages indicate that deposition on alluvial fans of the LRR was enhanced ~12-14 ka and ~40-50 ka. The ~12-14 ka episode is largely synchronous through the range, regardless of degree of glaciation in contributing basins (which varies from ~0-80%). Early MIS 3 deposits are not as widespread but are found on 2 of 5 fans and a terrace of the mainstem Big Lost River. Synchronous intervals of deposition suggest climatic factors drive deposition on these fans, and that major episodes of deposition follow global LGM (and MIS 4?) glacial maximum.

Deposition on alluvial fans of this region following global glacial maximum may be enhanced when 1) reduced effective moisture decreases hillslope vegetation density, freeing accumulated regolith for transport to fans, or 2) changes in stream power alter the balance between sediment supply and transport capacity. Reductions in snowpack and peak spring flows during interglacials could potentially enhance deposition if stream power had previously overwhelmed sediment supplies, flushing sediment beyond fans to the mainstem channel. Conversely, an increase in stream power could also enhance deposition if transport capacity was previously inadequate to mobilize available sediment. Greater stream power following global glacial maximum could result from increased frequency of stochastic storm events (such as rain on snow), or renewed moisture delivery with cessation of moisture starving winds generated by the Laurentide Ice Sheet.